The present invention relates to an apparatus, such as an image setter, and a method in which a sheet of a material comprising a layer containing a photo sensitive substance is irradiated.

A number of different types of apparatuses, such as laser image setters, performing this task have been suggested, but mostly for sheet-like materials in the form of lengths of film which are cut into individual pieces before or after irradiation thereof and from which part of the film material cannot be irradiated and is cut away subsequent to irradi¬ ation.

Part of these are of the so-called internal drum type, in which a sheet-like material is supported inside a concavely curved "drum" in order to obtain a circularly cylindrical shape.

The advantage of this type of image setter is obtained when the irradiation means are positioned and moved along the axis of symmetry of the circularly cylindrical surface and, there- fore, obtain the same distance from the irradiation means to the photo sensitive substance throughout the sheet-like material. This is important when optics are involved as e.g. a beam of light will have a single focus and diverge on both sides thereof. Thus, illuminating a material with this beam at different positions thereof will generate different il¬ luminated spot sizes. This is in no way desired and is typi¬ cally attempted reduced to a minimum.

An internal drum laser image setter of this type may be seen from PCT applications with publication numbers WO 92/14609 and WO 94/00295.

Another problem in image setters, especially when producing so-called positive prints, is that of elements of the image

setter producing shadows on the illuminated sheet. When producing positive prints, all parts of the sheet, which are not to have a colour in the final version, are illuminated during illumination. Thus, in this situation, a shadow will generate a coloured area of the final product.

Those parts which may generate shadows in image setters are typically means for holding the sheet-like material during illumination.

In typical image setters, such as the image setter of PCT application with publication number WO 92/14609, using thin and pliable sheets of material, this problem is solved by holding the sheets by means of vacuum during illumination.

The present invention relates to an internal drum image setter for sheets of materials. Especially for sheets of materials, problems may arise due to the fact that, cf above, preferably the whole of the sheet should be irradiatable. Vacuum would be a possible solution, as this requires no contact on the side of the material pointing towards the optical elements of the instrument.

However, it has been found that vacuum is not always suffi¬ ciently powerful and reliable, especially, when the photosensitive material is not very pliable - such as print¬ ing plates of e.g. aluminum.

In WO 94/00295, this problem is solved by forcing the material in place. However, in order to do so, means are used which make shadows on the sheet, thus not always removing the problem that the irradiated material should be cut to measure after irradiation. In addition, in the solution of this reference, the material is introduced in the drum in a direc- tion along the longitudinal direction of the drum. This, however, is not preferred due to the more complex setup of the instrument.

In a first aspect, the present invention relates to an alter¬ native way of introducing and holding the sheet of material during illumination. In this aspect, the present invention relates to an apparatus for irradiating a sheet of a material comprising a layer containing a photo sensitive substance, said apparatus comprising

a concavely curved support surface means for supporting the sheet and for defining at least part of a substan¬ tially circularly cylindrical surface,

- means for feeding, in a direction at an angle to a longi¬ tudinal axis of the support surface means, the sheet into contact with the support surface means in a position in which the sheet extends peripherally between first and second opposite edge portions,

- irradiating means for irradiating the photo sensitive substance of the sheet,

means for retaining the sheet in a predetermined position in relation to the support surface means, the retaining means comprising means for abutting the opposite edge portions of the sheet and for forcing the edge portions substantially peripherally so as to force the sheet into a substantially circularly cylindrical shape in contact with the support surface means,

where a sliding member is adapted to be moved between a first and second position, in which the sheet in the first position may be fed into its said position and where the sliding member in the second position is adapted to abut one of the first and second opposite edge portions so as to constitute part of the abutting means.

In this manner, a problem which may be seen when using the rollers to immobilize the edge portion, a solution also seen in the art, i.e. that a roller may generate shadows on the

irradiated sheet, may be removed. It is possible to choose a sliding member which abuts the edge portion without generat¬ ing shadows on the sheet.

Referring to the method and apparatus of WO 94/00295, this apparatus supports only a single with of material and is not easily adaptable to very different widths of material. In contrast, the apparatus and method according to the present invention easily adapts to different dimensions in that any length of a sheet along and supportable by the periphery of the support surface may be immobilized and having any width in the axial direction of the support surface (smaller than the width of the support surface means) .

Thus, the apparatus and method of the invention is adaptable to illuminate and immobilize different widths and lengths of material .

Typically, the irradiation is an illumination with infra-red or visible light, However, depending on the type of photo sensitive substance on the sheet, the irradiation may also be performed using ultra-violet light or perhaps X-ray radi- ation. Naturally, the photo sensitive substance and the irradiation means should match so that the photo sensitive substance of the sheet is sensitive to the emitted radiation from the irradiating means.

In this first aspect of the invention, the support surface means define a surface which, when the sheet is abutted and forced in contact therewith, makes the sheet material define the substantially circularly cylindrical shape which is preferred in order to obtain the above-mentioned advantage of the equidistancy to the irradiation means.

In the present aspect, the "opposite edge portions" are those edge portions which define the two peripheral positions in the support surface means, between which the rest of the sheet material is positioned. Depending on the shape of the

sheet and on how it is fed into said position, these two edge portions may vary from being constituted by approximately point-shaped edge portions (as if the sheet had a circular shape) to being consisted by sides of e.g. a rectangular sheet.

Naturally, the abutting means should be adapted to abut the actual shape of the edge portions in question.

In the present context, "forcing" the sheet into a shape in contact with the support surface means means that the abut- ment exerts such a force onto the edge portions so that the sheet material contacts the substantially circularly cylin¬ drical surface defined by the support surface means and assumes a corresponding shape.

When the force is exerted substantially peripherally in relation to the support surface means, the sheet material is automatically forced towards the inner support surface of the support surface means.

At present, it is preferred that the abutting means comprise stationary means peripherally immovable in relation to the support surface means for abutting one of the first and second opposite edge portions and movable means peripherally movable in relation to the support surface means between first and second positions in which it is out of engagement with and in abutment with the other of the opposite edge portions, respective¬ ly.

In this manner, a stationary abutment means is positioned in the support surface means and a movable means abuts the sheet and operates to generate the force exerted on to the edge portions.

Preferably, the sliding member, in its second position, constitute part of the stationary means.

When rendering one edge portion peripherally immovable in the support surface means, and preferably at a well-defined position, the peripheral position of the sheet, when abutted, is relatively well defined. This is preferred so as to be able to irradiate images also close to the edges of the sheet, which would not be possible if the position of these edges was not known.

In fact, it is typically desired to have the irradiated image positioned with a precision of ±0.5 mm in relation to the edges of the sheet, whereby some positioning of thereof should be performed.

The feeding means are preferably adapted to feed the sheet into said position in a first direction along a first path of which a part is defined by the substantially circularly cylindrical surface. This feeding preferably takes place so that the part of the path substantially follows the periphery of the support surface.

When the apparatus according to the invention further com¬ prises means for removing the sheet having been fed into said position, these means are preferably adapted to remove the sheet along a second path, part of which is substantially identical to the part of the first path defined by the sub¬ stantially circular cylindrical surface, in a direction substantially opposite to the first direction.

When transporting the sheet into and out of the support surface means along substantially the same path, part of the transportation means may perform both tasks and need, consequently, only be positioned at one position in order to feed and remove sheets therefrom.

The feeding means may comprise two rollers between which the sheet is fed.

Preferably, one of the two rollers being adapted to contact the layer of the sheet containing the photo-sensitive sub¬ stance is displaceable between a first and a second position where the roller, in the first position, contacts the layer in order to be able to feed the sheet and, in the second position, the roller is moved away from the layer in order to not create shadows on the sheet during irradiation.

It may otherwise be a problem to be able to "deliver" the sheet at a position on the support surface (which normally requires that a pair of rollers is able to contact the sheet) and to still irradiate all parts of the sheet.

In order to retain the typical setup of this type of instru¬ ments, the means for feeding the sheet into contact with the support surface means are typically means for feeding the sheet in a direction substantially perpendicular to a longi¬ tudinal axis of the support surface means.

Naturally, the shape of the abutting means will depend on the actual shape of the edge portions to be abutted. However, it is preferred to have abutting means comprising means for abutting each of the two opposite edge portions at one or more positions. These means may comprise one or more means independently resiliently connected to a main holding means.

The resilient connection is preferred as all feeding of sheet will be performed with a certain precision. Thus, as the edge portion to be abutted may deviate slightly from its ideal position, the abutting means should be adapted to adapt to this deviation.

If the shape of the edge portion to be abutted is e.g. straight, such as the edge of a square-shaped sheet, it may be preferred that these means comprise a main member and a single rigid means resiliently connected thereto and for abutting the edge portion.

A more complex shape of the edge portion may be better abutted using means which comprise several individual abut¬ ting means so as to better adapt to the shape of the edge portion.

The resilient connection between the means for abutting an edge portion at one or more positions and the main holding means may be a spring loaded connection. However, also other suitable resilient connections such as connections comprising rubber, flat springs etc. may be used.

At present, it is preferred that the angular position of the sheet in the support surface means is defined by the sliding member and that a resilient connection is provided at the movable abutment means.

Furthermore, due to the possibility of the sheet translating in a direction parallel to the axis of the support surface means during abutment due to the angular definition of the sliding member, the resilient connection of the movable means should be able to also adapt to this movement.

Using normal springs for the resilient connection, these may adapt to movements in all three dimensions. However, it may be preferred to have a resilient connection only adapting to movement in two dimensions and being more rigid in the third dimension, such as in the radial direction in the support surface means. Means of this type is a flat spring having an S-shape and directed so that its plane is directed radially in the support surface means.

A spring of this type will be able to adapt to both a peri¬ pheral movement - the abutment - and a movement in the direc¬ tion parallel to the axis of the support surface means - angular correction of the sheet. The more rigid direction is in the radial direction, where no movement of the sheet should occur.

In order to be able to control the abutment of the sheet, the abutting means may comprise force measuring means for deter¬ mining the force applied to the abutted edge portions. Pre¬ ferably, the force applied depends on the thickness and stiffness of the sheet material in order not to deform the sheet during abutment. Deforming the sheet will reduce the precision of the irradiated image, as the photo sensitive substance of the sheet material is no longer equidistantly positioned in relation to the irradiating means. It may also be more difficult to use deformed sheets for their final purpose such as during a printing process. In addition, it may not be possible to remove a deformed sheet from the apparatus without further deformation of the sheet or damage to the apparatus.

In order to facilitate the relative movement between the movable abutting means and the support surface means, the abutting means preferably comprise first engaging means engaging with second engaging means constituting part of the support surface means, and a motor for driving the first engaging means. Optionally, the motor may be positioned so at to drive the second engaging means.

In the presently preferred embodiment, the first engaging means comprise a toothed wheel and the second engaging means comprise a toothed portion along a radial part of the support surface means. However, any other suitable means, such as a friction engagement, arrangements utilizing belts, wires, chains, etc. may be used.

When subsequently using an irradiated sheet in e.g. a print¬ ing process, it is of great importance that the irradiated image is correctly positioned during the printing process. This criticality is especially visible when performing mul¬ tiple color prints where e.g. four prints are superposed.

This positioning of the irradiated image is preferably obtained by using registering edge portions precisely defined

in relation to the irradiated image. In this way, the image may be positioned independently of small deviations in the feeding of the sheet prior to irradiation. Thus, the present apparatus preferably further comprises means for providing registering edge portions in the sheet.

It is presently desired by operators that the registering edge portions are positioned with a precision of 1/100 mm in relation to the irradiated image. Often, these edge portions are used in the positioning of the irradiated sheet and not the outer boundaries of the sheet due to the larger precision in the position of the registering edge portions.

If manufacturing of the registering edge portions is per¬ formed while the sheet is present in the support surface means, the position of these edge portions will be well- defined in relation to the irradiated image if both are generated while the sheet is in the same position.

Typically, an irradiated sheet comprises two registering edge portions, whereby it is preferred that the edge portion providing means comprise at least two edge portion providing tools each providing one registering edge portion and being connected to a main member.

Depending on the desired number of registering edge portions and the positions thereof, the at least two edge portion providing tools and the main member may be movable between at least two positions so that each of the at least two edge portion providing tools may provide multiple edge portions in the sheet.

Naturally, it is possible to simply fit the apparatus with a number of fixed edge portion providing tools if the positions of the registering edge portions never change. However, as it may be preferred to be able to irradiate sheets having dif¬ ferent sizes, the positions of the registering edge portions may vary. Typically, two registering edge portions are posi-

tioned at one side of the sheet and symmetrically around the middle thereof. Thus, in order to avoid fitting the apparatus with an excessive number of edge portion providing tools, the tools are preferably movable.

In fact, some registering standards require that the regis¬ tering edge portions are positioned with one of a number of fixed distances. This may be followed providing e.g. three tools on the member in order to be able to provide two edge portions with two different distances therebetween without having to move the member between the production of the two registering edge portions.

In order for the edge portion providing tools to be able to ^' provide registering edge portions at multiple positions of the sheet, the at least two edge portion providing tools and the main member are preferably movable along a substantially circular path, as this is the shape of the preferred edge portion of the sheet to receive the registering edge por¬ tions.

A second aspect of the present invention relates to a tool for providing a registering edge portion in a sheet of a material and typically for use in the above-mentioned appar¬ atus. This tool comprises

a first edge portion defining means having a first edge portion defining the registering edge portion to be provided,

a second edge portion defining means having a surface for contacting the sheet on the opposite side of the layer containing the photo sensitive substance during production of the registering edge portion, receiving means at said surface for receiving the first means and comprising a second edge portion substantially inverse of that of the first means,

where the first edge portion defining means are adapted to move between a first position where a gap is defined between the first means and the contact surface of the second means and - a second position where the first edge portion defining means are introduced in the receiving means in a manner so that the first means to no substantial degree protrude beyond the plane of the contacting surface and so that the first and second edge portions become adjacent during the movement between the first and second positions.

The advantage of a tool of this type is the fact that, subsequently to providing the registering edge portion, the tool cannot generate shadows on the sheet to be irradiated as substantially all of the tool is positioned on the other side of the sheet material.

Usually, the edge portions to be defined in the sheet are indentations or notches at an edge portion of the sheet - and typically the same edge portion thereof.

The first edge portion of the tool is preferably an outer edge portion and the receiving means preferably constitute a hole or an indentation of the second means, wherein the second edge portion constitutes part of the edge of the hole or indentation. Thus, the first means are introduced in the indentation or hole during production of the edge portion in a manner so that substantially all of the first means are received in the hole or indentation.

Preferably, the first and second means are interconnected by an interconnecting body so that this and the first means are introduced in the hole or indentation of the second means when the first means are in the second position. Naturally, in order to avoid shadows on the irradiated sheet, also the interconnecting body is preferably substantially fully intro¬ duced in the receiving means.

In a third aspect, the present invention relates to a method of irradiating a sheet of a material comprising a layer containing a photo sensitive substance, said method compris¬ ing the steps of:

- feeding the sheet into contact with a support surface means defining at least part of a substantially circularly cylindrical support surface in a direction at an angle to a longitudinal axis of the support surface means and in a position in which the sheet extends peri- pherally between first and second opposite edge portions,

irradiating the photo sensitive substance of the sheet while retaining the sheet in a predetermined position in relation to the support surface means by abutting the opposite edge portions of the sheet and forcing the edge portions substantially peripherally so as to force the sheet into a substantially circular cylindrical shape in contact with the support surface means

characterized in that a sliding member, subsequent to the feeding step, is moved between a first and second position, where, in, the first position, the sheet may be fed into its said position and where the sliding member, in the second position, abuts one of the opposite edge portions during irradiation.

Again, it is preferred that the abutting comprises - peripherally immobilizing one of the opposite edge por¬ tions of the sheet in relation to the support surface means and moving a movable means peripherally in relation to the support surface means from a first position where there is no engagement with the sheet to a second position where the movable means abut the other of the opposite edge portions.

As mentioned above, the feeding of the sheet into said posi¬ tion is preferably performed in a first direction along a first path of which a part is defined by the substantially circularly cylindrical surface.

Where the method also comprises removal of the sheet having been fed into said position, this removal is, naturally, preferably performed subsequent to irradiation thereof, and the sheet is preferably removed along a second path, part of which is substantially identical to the part of the first path defined by the substantially circular cylindrical sur¬ face, in a direction substantially opposite to the first direction.

As described above, the force applied to the abutted edge portions is preferably determined by force measuring means. This determined force may then be compared to a predetermined force, and the result of the comparison may be used for controlling the abutment of the edge portions. This predeter¬ mined force will typically depend on the thickness and stiff¬ ness of the sheet and the size of the edge portions to be abutted. Naturally, thin materials may not require abutment with the force required in order to force a thicker material into shape. On the other hand, exerting the force required to force a given material into shape may deform the sheet, if the force is exerted on a too small edge portion. Thus, the material thickness and stiffness should correspond to the force required/desired and the area of the edge portion receiving the force.

Even though it is contemplated that this retainment of the sheet will function using almost every thickness/stiffness of materials, the advantage of using the present invention is largest when relatively stiff, and, thus, usually relatively thick, sheet are irradiated. Using e.g. vacuum retainment of the sheet may not function if the sheet material is too thick or stiff. This problem is overcome by using the apparatus and method of the invention.

Thus, it is preferred that the sheet comprises a sheet-shaped base material having a thickness of 0.1-0.5 mm, preferably 0.1-0.4 mm, more preferably 0.3 mm onto which a photo sensi¬ tive layer has been applied. The stiffness of the sheet preferably corresponds to that of a sheet of fully hardened (deformation hardened) aluminium having a thickness of 0.1- 0.5 mm, preferably 0.1-0.4 mm, more preferably 0.3 mm. The base material may be made of any suitable material such as metal, preferably aluminium, alloys-, preferably steel, plas- tics, preferably polyester, etc. Naturally, a more stiff material may be made thinner compared to that of a more soft material in order to obtain the full advantage of the present abutment.

Again, it is preferred to provide at least one registering edge portion by edge portion providing means while the sheet is retained by the retaining means.

In order to have the edge portion providing means provide the edge portions in the sheet, it is presently preferred to have the sheet fed into said position in a manner so that a part thereof projects beyond the support surface means in a direc¬ tion parallel to a longitudinal axis thereof, so that the edge portion providing means may be positioned so as to provide the at least one registering edge portion in that part of the sheet.

In this situation, where the sheet extends outside the sup¬ port surface means, the edge portion providing means may be movable between a first and a second position, the movement being at an angle to the periphery of the support surface, and the second position being further away from the support surface means than the first position.

If the edge portion providing means are in the second posi¬ tion when the sheet is fed into position and is moved into the first position before the sheet is irradiated, a longi¬ tudinal positioning of the sheet is performed at an angle to

the periphery of the support surface. Thus, this positioning together with the peripheral positioning of the sheet using the abutment means is preferred in order to position the sheet in a well-defined position.

Thus, the edge portion providing means may help in position¬ ing the sheet in the support surface means together with the abutting means. In fact, as the feeding of the sheet will always be performed within certain tolerances, the required tolerances may be reduced due to the positioning performed by the edge portion providing means.

In order to facilitate the positioning by the edge portion providing means, a first predetermined force is preferably applied during abutment to the sheet while moving the edge portion providing means from the second position to the first position, and this force is preferably smaller than a second predetermined force applied during irradiation of the sheet. Translating the sheet while abutting it with an excessive force will increase the risk of deforming the sheet during this process.

Alternatively, the translation of the sheet may be provided by not the edge portion providing means but an other element, such as an element rigidly connected thereto, which performs a similar movement prior to irradiation of the sheet.

Even though the positioning provided by the edge portion providing means or a similar means, the angular positioning of the sheet in the support surface means is preferably determined by the abutment means and more preferably by the stationary part thereof.

A typical sheet is substantially rectangularly shaped and the edge portion providing means abut and provide registering edge portions in an edge of the sheet which is not abutted by the retaining means.

Alternatively, it may be preferred to provide the registering edge portions at the edge portions abutted by the abutting means. In this situation, the edge portion providing means may be incorporated in or at the abutting means.

When it is desired that each of the edge portion providing tools should be able to provide multiple edge portions in the sheet it is preferred that the edge portion providing means are movable substantially along the periphery of the support surface between at least two positions. In this manner, the edge portion providing means may provide substantially any number of registering edge portions at substantially any position in the part of the sheet extending outside the support surface means.

In a fourth aspect, the present invention relates to a method of providing a registering edge portion in a sheet of a material, the method comprising

providing a first edge portion providing means having a first edge portion defining the registering edge portion to be provided,

- providing a second edge portion defining means having a surface for contacting the sheet during production of the registering edge portion, receiving means at said surface for receiving the first means and comprising a second edge portion substantially inverse of that of the first means,

moving the first means into a position where a gap is defined between the contact surface and the first means,

positioning the sheet on said surface so that a part thereof is introduced in said gap,

- moving the first means into a second position where the first edge portion moves through said gap, the first edge

portion defining means being introduced in the receiving means in a manner so that the first means to no substan¬ tial degree protrude beyond the plane of the contacting surface.

The first and second edge portions preferably become adjacent during the movement of the first means from the first to the second position.

In the most preferred embodiment, the first and second edge portions are positioned adjacent to said gap so that these edge portions become adjacent when the first means is intro¬ duced in said receiving means. In this manner, these two edge portions are the ones actually providing the edge portion. When these edge portions are substantially congruent, the best possible indentation of the sheet is obtained without deformation of the adjacent sheet material.

A preferred embodiment of the four aspects of the invention will now be described with reference to the drawing where

Fig. 1 is a cross-section of the preferred embodiment of the apparatus according to the invention,

Fig. 2 is a cross-section of a detail of Fig. 1 illustrating the operation of the stationary abutting means,

Fig. 3 is a cross-section of a detail of Fig. 1 illustrating the operation of the movable abutting means,

Fig. 4 is a block diagrammatic top view of a preferred embo- diment of the irradiation means,

Fig. 5 is a side view of the preferred registering edge portion providing means according to the invention,

Fig. 6 is a top view of the preferred registering edge por¬ tion providing means according to the invention,

Fig. 7 is a side view of the edge portion providing means of Fig. 6,

Fig. 8 is a cross-sectional view of a preferred alignment detector according to the invention,

Fig. 9 is a schematic view of a quadrant detector,

Fig. 10 is a cut-away illustration an alignment setup for the alignment detector of Fig. 7,

Fig. 11 is a cross-sectional view of the alignment setup of Fig. 10 through line A and the center of the outer tube, and

Fig. 12 is a cross-sectional view of the alignment setup of Fig. 10 seen from the end thereof from arrow B.

Fig. 13 illustrates a preferred method of controlling the illuminated spot size by controlling the output power of the laser.

In Fig. 1, a cross-section of the presently preferred inter¬ nal drum laser image setter 2 according to the invention is illustrated. In this instrument, the sheets, typically sheets of aluminum comprising a layer containing a photo sensitive substance and having a total thickness of at least 0.1 mm, such as 0.15 mm or 0.3 mm, are positioned on a tray 4.

When feeding a sheet of the material into the instrument 2, a suction means 6 is moved from its position as shown in Fig. 1 by moving means 8 to a position adjacent to the tray 4 so as to contact a sheet of the sheet. Subsequently, the suction means 6 are translated to their first position in order to have the sheet contacted by a pair rollers 10 so as to start feeding the sheet into the instrument 2.

Subsequently, the system consisting of the pairs of feeding rollers 10, 12, 14, 16 and 18 together with guides 20, 22,

24, 26 and 28 and helping rollers 30, 32, 34, 36, 38, 40, 42 and 44 will translate and guide the sheet into a support sur¬ face means or drum 50 of the instrument 2. It is preferred to contact the sheet in a manner so that the side having the photo sensitive substance predominantly is contacted by rollers in order not to scratch and stress this surface. Feeding of the sheet ends, when the rollers 18 no longer engage the sheet.

Retainment and illumination of the said sheet will be described further below.

When removing the illuminated sheet from the drum 50, the pair of rollers 18 again engage the sheet and together with guides 58, 60 and 62 as well as helping rollers 64, 66, 68 and 70 and removal roller pairs 72 and 74 cooperate in order to remove the sheet from the drum 50 and deliver this to an output tray (not shown) .

In the presently preferred instrument 2, the guide 62 is preferably an endless conveyer belt which, when no material is present between this and the rollers 66, 68 and 70, fol- lows a path defined by rollers 66, 68 and 70, the roller 76 of the pair of rollers 74, roller 78 of the pair of rollers 72, and the helping roller 69. This path is shown in a full line. When a sheet is guided by guide 62, the path thereof is slightly changed due to the presence of the sheet. This changed path is shown in a broken line.

In order not to stress the sheet, a body 75 holding the rollers 74 and 70 is preferably movable by operating means 77 so that the body 75 is in a more vertical position when the sheet is introduced between rollers 72 and moved to the more horizontal position shown in Fig. 1 before the sheet contacts the roller 76 of the pair of rollers 74.

In order to be able to feed the sheet into the drum 50 and especially in order to be able to remove the illuminated

sheet therefrom, the roller 52 of the pair of rollers 18 preferably comprises a number of rollers positioned along a common axis (not shown) . This will allow a comb-shaped part 79 of the drum 50 to extend between the individual rollers and thereby also support the sheet in these positions.

The support provided with the part 79 should, naturally, be sufficient in order to have the sheet retain the preferred shape. At present, the roller 52 consists of a total of four rollers having a length of 40 mm. This has been found to provide a part 79 giving sufficient support for the sheet.

This setup will enable feeding of the sheet into the drum 50 into a position where substantially all of the sheet may be supported by the drum 50. Furthermore, after illumination, the sheet may be contacted by the pair of rollers 18 and be removed from the drum 50.

Retaining a sheet fed into the drum 50 is performed by trans¬ lating rollers 52 and 54 out of engagement with the sheet and away therefrom, as may best be seen from Fig. 2 where refer¬ ence numerals 52' and 54' refer to the retracted positions of the rollers 52 and 54, respectively.

Abutment of the sheet is performed by translating a sliding member 60 between a first position as shown in Fig. 2 in full line and second, abutting position shown in a dashed line and referred to reference numeral 60' in which it is adapted to abut an end portion of the sheet.

At the opposite end portion of the sheet, the means for abutment are shown on Fig. 3, where a movable means 70 is movable along a support surface 56 of the drum 50 from a first position which is substantially at the end of the drum 50 opposite to that at the sliding means 60 to a second position wherein a substantially straight and rigid contact¬ ing means 72 of the movable means 70 contacts and abuts the opposition edge portion of the sheet.

At present, it is preferred that the movable means 70 com¬ prise two toothed wheels 74 positioned at the opposite peri¬ pheral side portions of the drum 50 and engaging with a toothed rim portion 76 on the outer side of the drum 50. The two toothed wheels 74 are interconnected by a common axel and driven by a motor 78.

As the abutted edge portion of the sheet may not be perfectly plane and co-linear with the means 72, the means 72 are preferably resiliently connected, such as through springs, to the rest of the movable means 70 in order to compensate for small deviations in the angle between the sheet and the means 72.

The means 70 are furthermore preferably equipped with force measuring means (not shown) in order to determine the force exerted by the means 70, that is of the springs thereof, onto the edge portion of the sheet in order to keep this below a predetermined maximum force, which would otherwise lead to deformation of the sheet.

Thus, abutment of the sheet comprises, subsequent to feeding the sheet, a translation of the rollers 52 and 54 and the sliding means 60 into positions 52', 54' and 60', respective¬ ly, and subsequent movement of the moving means 70 from a first position to the second position, wherein a predeter¬ mined force is exerted on the two opposite edge portions of the sheet.

When the predetermined desired force is suitably selected, the sheet will now be forced outwards toward the support surface 56 of the drum 50 and, thus, closely resemble the substantially circular cylindrical shape of the inner surface of the drum 50.

Illumination of the sheet is performed, as it is known per se using a carriage moving along a linear track 81 and on which a laser and suitable optical elements are positioned as well

as a rotatable mirror, which directs the laser light onto the photo sensitive substance of the sheet.

An irradiation means 80 of this type is illustrated on Fig. 4, where they comprise a laser 82 positioned on a base plate 84 which is adjustably mounted on the means 80 by means of three adjusting screws 86, 88 and 90, a collimating lens 92, and an acousto-optic modulator 94, lenses 96, 98 and 100, a rotatable mirror 102 and a motor 104 for rotating the rotatable mirror 102.

As is known per se and as may be seen from Fig. 4 the trans¬ mission of the light through the acousto-optic modulator 94 introduces a small angular displacement of the laser beam, whereby the laser 82 and lens 92 are at a small angle to the optical axis of the remainder of the means 80.

As is also known, in order to function optimally, the light entering the modulator 94 should be focused in the middle thereof and have a diameter of at the most 50-100 μm. As the laser beam of the presently used laser is collimated with a beam diameter on the order of 0.7 mm, the lens 92 focuses this beam in the modulator 94.

Even though the lens 92 is positioned in a housing 93 fixed to the means 80 and extending over the base plate 84 in order to be correctly positioned in relation to the laser 82, a position this close to the laser 82 may not be required.

Illumination of the photo sensitive substance is consequently performed by rotating the rotating mirror 102, modulating the laser and translating the means 80 in a manner so that the rotating mirror 102 translates along the symmetry axis of the substantially circular cylindrical surface 56 of the drum 50 in order to have the same focused laser beam on all parts of the illuminated sheet.

In order to be able to precisely position the illuminated sheet during a subsequent process wherein it forms the basis of e.g. a printing process, it is preferred to provide regis¬ tering notches in the sheet, which notches are precisely positioned in relation to the illuminated image.

Providing these notches is preferably performed while the sheet is in the illumination position and, thus, in connec¬ tion with the illumination thereof and most preferably before illumination cf. below.

In the present instrument, the sheet is preferably fed into the drum 50 in a manner so that a part thereof extends beyond a side of the drum 50 in a direction parallel to a longitudi¬ nal axis thereof. Thus, part of the sheet extends beyond the drum 50, whereby this part is free for the engagement of notch providing means 200 which may engage with the sheet without interfering with the drum 50.

As is illustrated in Fig. 5, the notch providing tools 200 comprise a base part 202 and a movable part 204 between which a gab is defined wherein the sheet is introduced before making the notch. By operating a motor 206 which through a worm gear 208 and an eccentric 209 translates the movable means 204 from its first position illustrated in Fig. 5 to its second position, wherein it is submerged in the base means 202, and back, indentation of the sheet is performed.

One center axis of the eccentric 209 is connected to the movable part 204 and the other center axis of the eccentric 209 is connected to the worm gear 208.

The movement of the movable means 204 is monitored by moni¬ toring means 210 in a manner which will be described below.

The base part 202 is fastened to the apparatus along an axis C. Due to the movement of the eccentric 208, the motor 206 may rock during the provision of the edge portion. In fact,

the movement of the motor 206 and eccentric 208 is the move¬ ment detected by the monitoring means 210, as this means is positioned so as to detect movement between the base part 202 and the eccentric 208.

The movement of the motor 206 is, due to the position of the eccentric 208 into and out of the plane of Fig. 5, and is monitored by the means 210, which detects the movement 90° out of phase with the movement of the movable part 204.

Thus, the means 210 is activated when the movable means 204 moves from the first position to the second position - that is, during provision of a registering edge portion - and de¬ activated during the opposite movement. The means 210 shifts its mode when the moving means 204 is in its first and second position.

Using a means 210 in this manner, only a single means is sufficient for monitoring the movement of the moving means 204.

The notch providing means 200 are preferably movable in a direction transverse to the edge to be indented of the sheet so that the indenting or notch providing means 200 are posi¬ tioned away from the sheet and wherein the movable means 204 are in their first position when the sheet is fed into its position. This is illustrated on Fig. 6.

Subsequently, the indenting means 200 are translated into contact with the sheet, initially having a position as shown in a dashed line with the reference numeral 212, and trans¬ lated into its final position shown in a dashed line with reference numeral 214, so that the indenting means 200 take part in the defining of the final position of the sheet prior to illumination thereof.

This translation of the sheet is performed while the retain¬ ing means 60 and 70 retain the sheet. However, it is pre-

ferred that the force exerted by the abutting means thereof is smaller than the predetermined force exerted during illum¬ ination thereof, as this translation of sheet while exerting a relatively large force thereon, may result in deformation of the sheet.

Subsequent to translation of the sheet into its final posi¬ tion, the movable means 204 are translated from its first to its second position, whereby a suitable indentation is made in the sheet.

In order not to deform the sheet during indentation thereof, the stationary means 202 preferably comprise a support sur¬ face 220 on which the sheet may rest during indentation. Preferably, this surface 220, into which the movable means 204 are received subsequent to indentation of the sheet, has an edge 222 fitting the indenting edge 224 of the means 204. In this situation, only the part of the sheet closest to the position of indentation is influenced (and, thus, has a small risk of deformation) by the indentation.

As described above, it is preferred to be able to illuminate all parts of the sheet. Thus, it is desired not to have any kind of shadowing elements positioned over the sheet during illumination thereof. Thus, it is preferred that, subsequent to indentation, no part of the indentation or notch providing means 200 to any substantial degree extend beyond the plane of the sheet positioned on the surface 220.

As may be seen from Figs. 5 and 6, the moving means 204 and the stationary 202 are manufactured so that the moving means 204 may be introduced completely into the stationary means 202 so as to not project above the support surface 220 of the stationery means 202. Thus, no part of the indentation means 200 may provide shadows during illumination of the sheet.

The axial positions of the indentation of the sheet may, naturally, be adapted to the individual needs or requirements

the operator. However, at present, a rectangular indentation and an indentation as illustrated where parts of the outer contour of the movable means 204 comprise an edge portion defining part of a circle are provided. These indentations help in positioning the illuminated sheet in e.g. a printing machine.

A widely used European industry standard requires a distance between the two indentations of 220 or 425 mm and that these indentations be positioned symmetrically about the middle of the sheet.

Thus, it may be required to provide either multiple indenta¬ tion tools 200 or that one or more of these tools be movable so as to be able to each produce multiple indentations.

In Fig. 7, a first position of three indenting tools 200 positioned on a main member 225 are illustrated in a first position in full lines and in a second position in broken lines.

Providing three tools 200 on the member 225 facilitates providing the two notches with the two different distances cf a widely used European industry standard without having to move the means 225 between the indentations of the sheet.

When moving the main member 225 and the tools 200 between these two positions, each indenting means 200 may, alterna¬ tively, produce two indentations in the sheet. This provides the use of also other indenting standards in the present apparatus.

Providing a movable main member 225 and, thus, one or more movable indentation means 200, the instrument will be more versatile and will be able to accommodate different lengths of sheet. This is, naturally, highly preferred when the length of the sheet is often changed.

EXAMPLE 1: Alignment of the laser beam on the carriage

In the presently preferred embodiment of the internal drum plotter of the invention, alignment of the laser beam on the carriage (of Fig. 4) is performed by altering the position and direction of the laser.

It is presently contemplated that a method comprising introducing an alignment detector in the beam at one or more well-defined positions on the carriage will produce sufficient information concerning the direction and position of the laser beam to ensure a proper alignment of the beam. This alignment detector should produce information of the beam direction and location relative to the alignment detector.

The presently preferred alignment detector comprises a hollow tube 300 and an optical grating 302 - preferably a holo¬ graphic grating - positioned so that its perpendicular is at an angle (0) to the axis 304 of the tube 300. The tube 300 is positioned in a manner on the carriage so that the laser beam should preferably follow the axis of the tube 300.

The laser beam is launched on to the grating 302, which will consequently diffract the laser beam in at least a zeroth and a first order beam. If the laser beam coincides with the axis 304 of the tube 300, the diffracted beams will impinge on detectors 306 and 308. If a holographic grating 302 is used, this may be manufactured to only reflect the zeroth and first order so that the amount of disturbing stray light in the tube 300 is reduced.

The detectors 306 and 308 preferably have a certain area so that the diffracted laser beam will impinge thereon even if this does not exactly coincide with the axis 304 of the tube 300.

The function of the presently preferred tube 300 will now be described in more detail:

On Fig. 8, a cross section of the presently preferred cali¬ bration tube 300 is shown. In this tube 300, a tilted grating 302 is positioned which will diffract at least the zeroth and the first order of the incoming laser light in two different directions and, in the case where the laser beam coincides with the symmetry axis 304 of the tube 300, directly onto the center of two quadrant sensors 306 and 308 positioned at the wall of the tube 300. The reason that the grating 302 is tilted is the fact that, otherwise, the zeroth order beam would be emitted in a direction close to the symmetry axis of the tube 300 (and in the situation where the laser light coincides with the symmetry axis 304, the zeroth order will also coincide with this axis) , which makes it more difficult to detect.

If a coordinate system with an origo as shown on Fig. 8 is chosen, D and θ are known. (x ₀ , D, z ₀ ) and (x _1# D, z _x ) are the positions of the zeroth and first diffracted order, where x ₀ , z ₀ , x _x and z _± are measured at the inner wall of the tube 300 at the points of impingement of the laser beams. Knowing one coordinate (y or z) of the position of impact of the laser beam on the grating 302, the other coordinate may be found from

y tanθ = z ⁽ D For the zeroth order beam which impinges a position detector 306 at (x ₀ , D, z ₀ ) the following equation applies:

( z-z ₀ ) tan (θ+α) + y = D (2)

where c. is the angle between the diffracted laser beam and the perpendicular of the grating 302 and, thus, α+θ the angle

between the diffracted beam and the axis 304 of the tube 300, This gives

(y tanθ-z ₀ ) tan(θ+α) +y = D (3)

for the zeroth order beam.

Equation (4) is general for the angles of beams diffracted from gratings, (a is the grating constant denoting the sepa¬ ration between the lines of the grating 302 and α and β are as defined in Fig. 8)

a (sinα+sinβ) =nλ (4)

For the first order (n=l) , this gives

For the first order beam being diffracted at an angle β , resulting from a beam impinging on the grating 302 at (x, y, z) , and impinging on a position detector positioned at (x ₁₍ D, z _χ ) , common geometry gives

Introducing equation (5) gives

( z-z ) tan (Θ+Arctan ( —-sinα) ) +y = D (7)

¹ a

which, using (1) , may be transformed to

(y tanθ-z.) tan(Θ+Arc tan(—-sinα) ) +y = D (8) a

This equation and the corresponding equation (3) for the zeroth order give a system of two equations having two unknown variables, y and α. Determining the y-coordinate of the position of impact of the laser beam on the grating 302 will automatically determine the corresponding z-value.

Furthermore, φ (the angle between the incoming laser beam and the plane of Fig. 8) may be determined from the equation

(9)

( z ₁ -z ₀ ) tanφ = x ₁ -x ₀

Thus, from the positions of impact of the diffracted beams on the detectors 306 and 308, the position of impact (x,y,z) of the laser beam on the grating 302 and the angles of impact (a and φ) may be determined.

z-z x ^X l ⁺ r ^(x ι ^_x o ⁾ (10) ^z -z*

These values are the values required in order to determine the path of the laser beam on the carriage.

Naturally, as described above, it is preferred to use detectors having a certain area in order to be able to also detect laser beams not exactly coinciding with the axis 304 of symmetry of the tube 300. In fact, in order to be able to not only detect the diffracted laser beam, which would nor¬ mally be the situation if typical simple detectors were used, but to also be able to determine in which direction this beam deviates from its desired path, the used detectors are pre¬ ferably position sensitive.

At present, the so-called quadrant sensors are preferred. This type of sensor 306 comprises four quadrants being four individual sensors 320, 322, 324 and 326 positioned as four quarters of a circle (See Fig. 9) .

The advantage of this type of detector is that the actual position of impingement of a laser beam or the like on a detector 306 of this type may be calculated from the output of the four sensors 320, 322, 324 and 326.

The coordinates x and y are now used in a Cartesian coordinate system with origo in the center of a quadrant detector.

The power of a laser beam is gaussian, whereby the intensity of the beam impinging in position x _L ,y _L may be found from

(X-

-2 ly-yj ^{■ '} lΛx, y) =e (11)

Looking at Fig. 9, from a simple circle, where capital x (X) and y (Y) denotes the outer limits of the detector 306 in question:

_γ2 ₊ y2 _=r 2 __ _» χ ₌ / _r 2 - γ2 (12)

Thus, integrating the total intensity, the power detected by the sensors 320, 322, 324 and 326 may be found from:

^ ₃₂ =/ / ι _L ( ^χ , y) dx dy (15)

- ^■

From these four outputs of the sensors, the position (x _L , y _L ) of impact of the laser beam on or near the detector may be determined.

The two positions (one from each detector 306, 308) are introduced in equations (3) and (8) in order to give the position of impact of the incoming laser beam on the grating 302 and the angle under which the laser beam enters the tube 300.

The angle and position of the laser beam may then be cor- rected while monitoring the output of the two detectors 306 and 308 so as to obtain the desired reading from these detectors.

Thus, when positioning the tube 300 in a given position on the carriage 80 of the presently preferred plotter 2, where the x-axis of the tube coordinate system is horizontal, the position and direction of the laser may be corrected in order to have the correct illumination of the sheet.

It may be preferred to perform the alignment of the laser beam in more than one step where the tube 300 is positioned at more than one location on the carriage 80 at different positions further and further away from the laser 82. This

method may be preferred as the above-described tube 300 has a limited "area of sight" whereby a too mis-aligned laser may not be detected by the detectors 306 and 308. When performing a coarse alignment using the tube 300 at a position closer to the laser 82, it is ensured that the laser beam will be detectable at the position further away from the laser 82.

Thus, it may be preferred to first remove the lens 92 and the housing on the carrier 80 (See Fig. 4) and here perform a coarse alignment of the laser beam. Subsequently, the final alignment is performed by removing lenses 96, 98 and 100 and replacing the spinner-motor 104 and mirror 102, which are positioned on a reference surface, with the tube 300. Final¬ ly, the position of the carriage 80 in the apparatus may afterwards be adjusted by launching the laser into the tube 300, while the carriage 80 is mounted on the apparatus 2. The tube should in this case be connected to the support surface means 50 and be centered along the axis thereof.

At present, it is preferred that the inner diameter D of the tube 300 is of the order of 28 mm, the outer diameter on the order of 38 mm, and that the length thereof is on the order of 12 cm. This geometry of the tube 300 limits the maximum first order diffraction angle to 45°. The required grating constant should consequently be higher than 700 nm when using a laser beam with a wavelength of 532 nm.

The precision obtainable using the present alignment sensor may be described by:

If the laser on the carriage 80 is coarsely adjusted so that the laser beam at a distance of 1 m is within 5 cm from its aligned direction, this corresponds to an angular error of under 5 cm/l m = 0.05 rad. Along the length of the tube 300 (12 cm) , this angular error corresponds to under 6 mm. This will also correspond to the maximum displacement of the laser beam spots near the detectors 306 and 308 of the tube 300. Due to the area of the preferred detectors (which may have a

radius of several millimetres if desired) and to the area of the reflected laser beam, a laser beam impinging 6 mm from the center of a detector 306 or 308 may easily be detected and, thus, aligned.

Thus, even this very coarse initial alignment will allow the tube 300 to detect the beam and to allow for this to be properly aligned. As a quadrant detector may detect the position of a laser beam within μm's, the final precision of the alignment having positioned the laser beam within e.g. 2.4 μm will be 2.4 μm/12 cm = 20 μrad.

At present, the typical manner of adjusting the laser 82 on the carriage 80 is to position two small apertures spaced approx. 0.5 m and thereby to adjust the laser to obtain maximum intensity therethrough. This manoeuvre may take hours, and the final precision typically corresponds to 3 cm over 10 m, that is on the order of 3 mrad.

In fact, the present alignment is not only contemplated to be better but also faster, as the instrumentation translating the detected positions of the reflected laser beams may be made to not only inform the operator of these positions but also of the alignment knobs or the like to operate in order to align the laser beam. In its last consequence, this align¬ ment may be fully automated.

Even though the above was illustrated using a quadrant sensor having a circular detector area, the same, naturally, applies to similar detectors, such as quadrant sensors having a quadratic or square detector area.

Also other setups are contemplated to give similar results. These setups differ from the above-mentioned by employing different optical elements for performing the diffraction or other modification of the incoming laser beam. Naturally, this will in some cases require other positions for the detectors .

In a first alternative setup, the grating is replaced by a prism (such as a Melles Griot wedge 02 PRW 009) which will reflect part of the incoming laser beam and transmit the rest thereof. Again, it may be preferred to have the front surface of the prism tilted so that its perpendicular is at an angle to the axis 304 of the tube 300.

Two position sensitive detectors may, thus, be positioned so as to detect the two beams. The angle and position of inci¬ dence of the incoming laser beam on the front surface of the prism may be determined from Snell's law.

In a second alternative setup, two thin, light transmitting gratings are used, where the zero-order beam from the first grating impinges onto the second grating. One first order beam from each of the two gratings are detected in order to generate the information.

If the angle of the incoming beam is not perpendicular to the gratings, the beam will impinge on the two gratings at dif¬ ferent positions. Furthermore, the position and angle of impingement of the beam on the first grating may be deter- mined from the positions of impingement on the tube 300 of the two diffracted beams in much the same way as described above using two equations similar to (8) .

The above method is contemplated to function independently of the grating constants of the two gratings being equal or not.

In a third setup, the laser beam is diffracted by a thin transmission grating. The first and zeroth order of the diffracted beam are projected onto a screen, making two spots. The distance between these spots vary as a function of the angle of the incoming beam, whereby this angle may be determined. From the positions of these two spots relative to the axis of the tube, the position of the beam on the grating may be determined. The grating may be tilted in order to improve the resolution of the determination, as it may be

calculated that a tilt angle of approx. 90° to the axis of the tube 300 is optimal using a grating constant of 1 μm. The detection of the spots may be accomplished using e.g. a CCD- array, a diode array or a PSD (position sensitive device) .

When producing an alignment detector of the above-described type, production imperfections will typically render it necessary to calibrate this instrument before using it to align lasers on e.g. sensitive plotters. If positioning of the laser beam on the detectors is desired to a precision of μm's, the positioning of these detectors should, naturally, be within at least the same precision.

Calibrating the present alignment detector is presently contemplated to be performed by launching a laser beam in a direction along the symmetry axis 304 of the tube 300 and determining the positions of the reflected beams on the two detectors 306, 308. These positions are thereafter the cali¬ brated positions, which the reflected laser beams to be aligned are to obtain.

Aligning the tube 300 after a given laser beam may be obtained using the setup of Figs. 10 and 11, where a laser beam is produced by a laser 330 and directed as the zero- order beam from a grating 332 into the tube 300. Positioning of the tube 300 is performed by way of manipulating a total of twelve positioning rods 340, 342, 344, 346, 348, 350, 352, 354, 356 and 358, of which two are not shown. These position¬ ing rods are connected to an outer tube 336 to which the laser 330 and grating 332 are also connected in order to provide a frame for the calibration of the alignment detector.

It may be preferred to first perform a coarse positioning with the rods 340, 342, 344, 346, 348, 350, 352 and 354 with no engagement to the four other rods, as an adjustment of one of these rods may include the loosening of a fastening device (not shown) introducing a small error. Subsequently, a finer

adjustment using the four rods 356 and 358 (the other two, which are not shown, are similarly positioned at the other end of the tube 300) which are positioned so as to abut the lower side of the tube 300 whereafter the rods 340, 342, 344, 346, 348, 350, 352 and 354 are removed.

As the first-order beams from the grating 332 have well- defined directions in relation to the zero-order beam, these may be used in order to position the tube 300 in relation to the zero-order beam inside the outer tube 336. To this effect, a total of four detectors 360, 362, 364 and 366 are temporarily positioned on the outer side of the tube 300 to detect the first-order beams. Naturally, as the first-order beams and the zero-order beams diverge away from the grating 332, the two detectors 360 and 362 positioned on the front end of the tube 300 closer to the grating 332 are positioned closer to the tube 300 than the detectors 364 and 366 posi¬ tioned at the farther end of the tube 300.

Having firstly positioned one end of the tube 300 correctly, this is rotated 90° so as to have the two other detectors detect the first order beams and in order to be able to position the other end of the tube 300.

By positioning the tube 300 so as to obtain optimum reading from the four detectors 360, 362, 364 and 366, the zero-order beam entering the tube 300 should coextend with the symmetry axis 304 of the tube 300, whereby the calibrated position of the two detectors 306 and 308 of the tube 300 may be deter¬ mined.

A more simple calibration setup for the tube 300 may be one where only the eight rods 340, 342, 344, 346, 348, 350, 352 and 354 are used and where the rods 340, 342, 348 and 352 are replaced by spring-biased elements so that operation of the other four rods will provide sufficient displacement of the tube 300.

In the above suggested calibration setups, the detectors are positioned so as to be able to detect the first order beams, that is, in positions so that the rods are not in the way. However, it may be preferred to have the detectors positioned so as to receive light transmitted through the rods. Thus, it may be preferred that at least part of the rods are shaped so as to allow the laser light to pass. A suitable shape may be a shape comprising a throughbore for the laser light.

The present alignment detector may be used for detecting the position and direction of a laser beam or any other substan¬ tially monochromatic beam relative to a reference direction. Naturally, the wavelength of the laser 330 should be one detectable by the detectors 360, 362, 364 and 366.

The present alignment detector may be used in the adjustment of a ruby laser. This is presently performed using a heat sensitive sheet of paper for detecting the invisible beam. This procedure can be replaced using the above-mentioned third alternative setup, which allows for a large percentage of the energy of the laser beam to be transmitted through the detector. In fact, any laser adjustment may benefit from the present alignment detector, especially the cumbersome adjust¬ ment procedures of power lasers.

Additionally, the alignment detector may be used in the alignment of optics of any kind, such as where optical elements are to be centered around an optical axis.

EXAMPLE 2 : Controlling the spot size of the laser beam on the photo sensitive substance

Usually, the controlling of the spot size of a spot illumi¬ nated by the laser beam on the photo sensitive layer of the sheet is performed by controlling the size of an aperture positioned at a given position in the laser beam.

However, as this method has a number of disadvantages, an alternative method is suggested.

At present, it is preferred to control the spot size of a spot illuminated by the laser beam on the photo sensitive substance by regulating the power of the laser beam according to the below procedure.

A typical photo sensitive layer as used in the art has a certain burn threshold ( ) defining the lowest amount of energy required in order to expose the sheet material . At present, an c- of 1.3 J/m ² is assumed, but this number may, naturally change from material to material.

At present, the procedure will be described with reference to the characteristics of the preferred DMX 620 internal drum laser image setter from ESKOFOT A/S, Denmark and of a typical size and type of sheet. However, these characteristics may easily be changed in order to adapt the procedure to another apparatus.

The present apparatus comprises an internal drum having a diameter of 320 mm. A typical material is a 0.15 mm Aluminum material comprising a photo sensitive layer having the above- mentioned burn threshold and having a length (plate-length) of 720 mm along the direction of the lines illuminated during the rotation of the movable mirror. This constellation gives that the sheet extends 257.8° in the drum.

The angular velocity of the rotating mirror in the apparatus is 20,000 rpm and the spot radius of the laser beam on the photo sensitive layer of the sheet is 16 μm. The manufacturer (Coherent) of the presently preferred laser, Coherent DPSS 532, gives a correction coefficient (M ² -coefficient) of 1.1, which means that the spot size is actually 1.1 times the 16 μm due to the beam of the laser not being ideal. This coeffi¬ cient is described in detail in "Beam Characteristics and Measurement of Propagation Attributes" by M. W. Sasnett and

T. F. Johnston, Jr., SPIE Proceedings, Vol. 1414, 1991. This correction, however, has not been taken into account in the following procedure.

The spot size is defined as the size of the spot where the intensity is e ^"2 (approx. 13.5%) of the maximum intensity of the beam in the center thereof.

Usually, the resolution (res) desired in this type of appa¬ ratus is in the area of 750-3600 dpi (dots per inch) .

Assuming that the laser beam is perfectly Gaussian, the volume of normalized Gaussian rotational body is 7τw ² /2.

The intensity distribution of the laser beam is: r ² (17) f (r) = e ^~2

The desired exposed radius of an illuminated spot (in meters] is

_{R i} res) = - ²⁵ ' ^4'10'3 (18)

2 res

The total energy required in order to expose a dot having the radius of equation 18 - that is to bring the intensity at this point over the burn threshold - is

P ( res) = ^a - ^{π 'w2} (19)

where the intensity of the laser beam at the radius R(res) is α., as required. From the fact that the intensity at the radius R(res) is the maximum intensity of the beam multiplied f(R(res)), α/f(R(res)) is the maximum power in the center of the beam. Equation 19 is derived from an integration of the power of the spot.

The time required for making a single rotation (in seconds) of the mirror is

As the sheet extends a total of 257.8° in the drum, T _L is the time required in order to make a line along the full length thereof along the direction of movement of the mirror

T _L = ²⁵⁷ ' ⁸ -T _r (21)

^L 360 ^roC

Knowing the resolution, the number of spots (N) along the length of the material may be found from

_{Ni res)} = Pla te-length . _{reε (22)}

0.0254

The time available for making one of these spots may be found from

Thus, in order to be able to expose the sheet in the time available for generating the spot, the power from the laser required is

p (res) = ^PtotaJ , ^(reS) (24)

T _spot (res)

From Fig. 13, a curve describing the relation between the resolution desired and the laser output (in W) required is shown.

As the rotation of the rotatable mirror is not altered by a change in resolution, the velocity of the carriage carrying

the laser is. Thus, the difference in the situations of Fig. 13 both having the same output power but different resolu¬ tions are situations having different carriage velocities.

In the art, one may desire that the spot size illuminated is slightly larger than the resolution of the illumination in order to have an overlap between the generated spots. Thus, e.g. a spot size of 1.2 times the resolution may be desired. In order to facilitate this, a lower resolution should be entered in the above calculation in order to give the desired result.

From Fig. 13 it may be seen that a small power variation of the output of the laser will have a large effect on the final resolution, if this is in the area of 1200-1800 dpi, whereby it is desired to have a precise monitoring and control of this output power and a suitable power supply for the laser, as this also has an effect on the power emitted.

Naturally, the precision of the spot size of the instrument depends on both the mechanics of the instrument and on the optics thereof. This precision may be made larger or smaller depending on the apparatus and is often defined in order to provide an instrument honouring this definition.

Knowing this defined or pre-defined uncertainty (Δres/res) , the power stability p/p) may be calculated from

Δ _ A∑es res dp (res) (25) p res p (res) dres

where dpres/dres may be found from Fig. 13 or the pertaining equation 24.

Thus, if Δres/res is defined to a few percent, Δp/p would be on the order of a few tenth of a percent outside the interval of 1200-1800 dpi and even smaller in this interval.

This will give the requirements for the laser and its power supply.

One drawback of the present method compared to the above- mentioned prior art method is the fact that the sharpness of the illuminating spot of the laser light will depend on the power of the laser. Thus, as this power will vary with vary¬ ing spot size, the sharpness thereof will vary.

The reason for this is the sharpness of the Gaussian inten¬ sity distribution of the beam. If the burn threshold of the material is at a position on the Gaussian curve where this is steep, the defined spot will be very sharp. On the other hand, if the Gaussian curve is relatively flat, such as at the top or bottom thereof, the edges of the spot may be less well-defined due to the more flat intensity distribution.

However, this may not have a serious effect on the illumina¬ tion. Some of the widely used photo sensitive emulsions used have a very steep illumination curve which means that if the intensity is above the burn threshold, whereby the illumi¬ nated spot will be fully coloured, and no coloration will take place even if the intensity is quite near the burn threshold - as long as it is below this threshold.

Using emulsions of this type, a less well-defined spot size will only have a very limited effect on the final result.

Thus, simply by altering the output power of the laser and the velocity of the carriage, the resolution of the illumi¬ nated image may be controlled.