Apparatus for illuminating a surface

FIELD OF THE INVENTION

The invention generally relates to the field of illuminating surfaces. More in particular, the invention relates to illuminating surfaces for making prints, for exam- pie by means of a screen printing process.

BACKGROUND OF THE INVENTION

The screen printing process is a printing technique that is used for printing graphic images on, for example, ob- jects, posters and textile. The technique is also used industrially, for example for the production of printed circuit boards and solar cells.

The principle of the screen printing process is as follows. A screen (polyester or steel) is stretched over a holding device (frame) . The screen is then provided with a photosensitive layer, on which a film is subsequently patterned. The film is exposed to ultraviolet (UV) radiation, after which the unexposed parts of the photosensitive layer can be washed away in conformity with the pattern of the film. As a result, open spaces are formed in the film. When subsequently ink or solder paste is applied to the screen, the shape of the stencil can be reproduced on the object to be printed. This process can be repeated with different colours and forms, which are printed beside each other or on top of each other.

An essential step in printing processes in general is the illumination step. As described above, the illumination step in a screen printing process comprises illuminating the photosensitive layer on the screen so as to form the stencil after the developing step.

The illumination apparatuses that are currently used have a number of shortcomings, which adversely affect the quality of the stencil. Such illumination apparatuses mainly consist of a UV light source with a surrounding large

reflecting shade, which shade reflects the UV radiation in the direction of the screen.

A first shortcoming of prior art illumination apparatuses concerns the distribution of the amount of UV radiation that reaches the photosensitive layer on the screen. With the lamps that are used in practice, the amount of UV radiation appears to be distributed quite disproportionately over the screen. As a result, the required exposure time of the photosensitive layer varies according to the po- sition on the photosensitive layer, which is undesirable. In practice illumination apparatuses having comparatively large dimensions are often used, therefore, so as to illuminate the screen as uniformly as possible

Another shortcoming of prior art illumination appa- ratuses is the fact that only a limited edge sharpness of the exposed photosensitive layer can be obtained with such illumination apparatuses.

SUMMARY OF THE INVENTION It is an object of the invention to provide an apparatus for illuminating a screen that obviates or at least reduces the above shortcomings.

Consequently, an apparatus for illuminating a screen is proposed which comprises a holding device for sup- porting a screen and an illumination apparatus for illuminating a screen. The illumination apparatus comprises a lamp for generating ultraviolet radiation for illuminating the screen. The illumination apparatus further comprises a dish-shaped reflector and a reflecting obstruction element for reflecting at least part of the ultraviolet radiation. The dish-shaped reflector is provided with an opening facing towards the holding device, through which the ultraviolet radiation can exit. The lamp is in large part incorporated in the dish-shaped reflector. The reflecting obstruction element is arranged for reflecting the ultraviolet radiation from the lamp at least in part. The obstruction element is disposed between at least a part of the lamp and the holding device. The dish-shaped reflector and the obstruction element are so

configured that, in use of the illumination apparatus, the ultraviolet radiation exits through the opening of the dish- shaped reflector and is substantially uniformly distributed over the screen to be illuminated. Another object of the invention is to provide an illumination apparatus for uniformly illuminating a surface, preferably for use in printing techniques such as screen printing.

Consequently, an illumination apparatus comprising a lamp for generating ultraviolet radiation is proposed. The illumination apparatus also comprises a dish-shaped reflector for reflecting the ultraviolet radiation, which dish-shaped reflector is provided with an opening through which the ultraviolet radiation can exit. The lamp is in large part incorporated in the dish-shaped reflector. The lamp further comprises a reflecting obstruction element arranged for reflecting the ultraviolet radiation from the lamp at least in part. The obstruction element is preferably disposed between at least a part of the lamp and the opening. The dish-shaped reflector and the obstruction element are so configured that, in use of the illumination apparatus, the ultraviolet radiation exits through the opening of the dish-shaped reflector and is substantially uniformly distributed over a surface to be illuminated which is spaced from the opening of the dish- shaped reflector.

It will be understood that the light source that is used may also be a light source other than a UV lamp, if this should be desired for a specific application.

The amount of light from a light source decreases quadratically with the distance to the light source. Placing a light source above a screen without taking further measures will therefore by definition lead to a non-uniform distribution of the amount of light over the screen. This effect is hardly alleviated by placing a reflecting shade around such a light source, as is done in the prior art.

The invention is based on the perception that it is important that the screen is not directly exposed to the radiation of the lamp that is used at the location where the

distance between the illumination apparatus and the screen is smallest (for example in the centre of the screen, if the illumination apparatus is disposed in front of said centre) . By arranging the illumination apparatus so that the amount of UV radiation that exits in the centre of the opening in the dish-shaped reflector is reduced and the amount of UV radiation that exits along the edges of the opening in the dish- shaped reflector is increased in comparison with the prior art illumination apparatuses as a result of the UV radiation being reflected in the illumination apparatus according to the present invention, a more uniform distribution of the UV radiation over the surface to be illuminated is realised. This effect is obtained by the use of the reflecting obstruction element between the lamp and the surface to be illuminated and the configuration of the dish-shaped reflector and the reflecting obstruction element. Shadow effects caused by the reflecting obstruction element can be prevented or at least reduced by positioning the obstruction element close to the lamp. The reflecting obstruction element may in particular be so configured that the UV radiation of the lamp incident on the reflecting obstruction element is mainly reflected in the direction of the dish-shaped reflector and exits through the opening via said dish-shaped reflector. It is of course also possible that a (small) portion of the light exits through the opening via the reflecting obstruction element without being reflected from the dish-shaped reflector.

The dependent claims define embodiments of the pre- sent invention. It will be understood that said embodiments, or aspects thereof, may be combined.

The invention will now be described with reference to the appended figures, which schematically show an embodiment of the invention. It will be apparent that the invention is by no means limited by this embodiment.

BRIEF DESCRIPTION OF THE FIGURES In the figures:

Fig. 1 schematically shows an embodiment of an apparatus for illuminating a screen according to the present invention;

Fig. 2 schematically shows an illumination appara- tus according to the prior art;

Figs. 3A and 3B are a top plan view and a bottom plan view, respectively, of an embodiment of the illumination apparatus according to invention;

Figs. 4A and 4B are sectional views along the lines 4A-4A, 4B-4B, respectively, of the illumination apparatus of Figs. 3A and 3B;

Fig. 5 schematically shows the distribution of the amount of UV radiation over a surface for the apparatuses shown in Fig. 2 and Fig. 3A, respectively; Figs. 6A and 6B schematically show the way in which the edge sharpness of the photosensitive layer is improved by using the illumination apparatus shown in Figs. 3A-3B and Figs. 4A-4B; and

Figs. 7A and 7B are a sectional view and a bottom plan view, respectively, of an alternative embodiment of an illumination apparatus according to the invention.

DETAILED DESCRIPTION OF THE FIGURES

Fig. 1 schematically shows an apparatus 1 for illu- minating a screen 2. The apparatus 1 comprises a holding device 3 for supporting the screen 2 and an illumination apparatus 4 for illuminating the screen 2 with UV radiation.

The illumination apparatus 4 is positioned a distance d from the screen 2 to be illuminated. For a screen 2 measuring 1x1 m, said distance d is usually about 1.5 m.

It is noted that although the screen 2 is shown in a horizontal orientation in Fig. 1, the screen 2 may also have a different orientation, such as a vertical orientation, for being illuminated. Fig. 2 shows a prior art illumination apparatus 100 for illuminating the screen 2. The illumination apparatus 100 comprises a UV light source 101 and a dish-shaped reflector 102 for reflecting the UV radiation in the direction of the

screen 2. The dish-shaped reflector 102 has walls 103. The distance between the walls 103 is usually about 50 cm.

As already indicated in the introduction, a uniform illumination cannot be achieved with the known illumination apparatus 100. This is schematically represented by the characteristic A end Fig. 5. In Fig. 5 the amount of UV radiation is plotted on the vertical axis Q and the position x on the screen shown in Fig. 1 is plotted on the horizontal axis.

Figs. 3A-3B and 4A-4B show an embodiment of an il- lumination apparatus 4 according to the invention which in practice has proved to provide a more uniform distribution of the amount of UV radiation on the screen. This effect is schematically represented by the characteristic B in Fig. 5. When an illumination apparatus 4 according to the invention is used, the difference in the amount of radiation between the centre of the screen and the edges is maximally 5%, whilst the difference of only 3% is within the bounds of the possible.

The illumination apparatus 4 comprises a lamp 5 for generating ultraviolet radiation for illuminating the screen 2, a dish-shaped reflector 6 and a reflecting obstruction element 7. The lamp has a length of 10 cm and a diameter of 2 cm, for example. The power of the lamp 5 is for example 5000 Watt. The lamp 5 is tubular in shape and the reflecting obstruction element 7 extends substantially transversely to the axis of the tubular lamp 5.

The dish-shaped reflector 6 is provided with an opening 8 defined by side walls 9 of the dish-shaped reflec- tor 6, through which the UV radiation can exit. The lamp 5 is incorporated in the dish-shaped reflector 6, and part of the UV radiation is reflected from the dish-shaped reflector 6, exiting the illumination apparatus via the opening 8, as is shown in Fig. 4A. The reflecting obstruction element 7 is disposed near the lamp 5 for reflecting the UV radiation from the lamp 5 at least in part in the direction of the dish-shaped reflector 6, as is indicated in Fig. 4A. The obstruction

element 7 is disposed between at least a part of the lamp 5 and the opening 8, thus preventing the UV radiation being directed incident on the screen 2 from the centre of the opening 8. Furthermore, a small portion of the UV radiation will directly exit through the opening 8 via the reflecting obstruction element 7.

The dish-shaped reflector 6 and the obstruction element 7 are so configured that when the illumination apparatus 4 is used, the UV radiation is substantially uniformly distributed over the screen 2 to be illuminated, as is schematically indicated by characteristic B in Fig. 5. The amount of UV radiation that arrives in the centre and in the corners of the screen 2 is thus substantially uniform, that is, the amount of UV radiation is distributed much more uniformly over the screen 2 than in the situation in which the illumination apparatus 100 according to the prior art is used.

As shown in figure 4A, the obstruction element 7 is a V-shaped obstruction element, the point of which is directed towards the lamp 5. This configuration of the obstruction element 7 makes it possible for the UV radiation from the lamp 5 to be reflected from the walls 7A of the obstruction element 7 and exit the illumination apparatus a long or via the side walls 9 of the dish-shaped reflector 6, through the opening 8, in the direction of the screen 2 or another surface to be illuminated.

The legs of the V-shaped obstruction element 7 corresponding to the walls 7A in Figs. 4A vary in length in the longitudinal direction of the obstruction element 7, having a maximum value near the lamp 5 and a minimum value near the walls 9 of the dish-shaped reflector 6. This achieves on the one hand that the UV radiation from the tubular lamp 5 will be reflected from the V-shaped obstruction element 7 over a significant portion of the axial direction of the lamp instead of exiting directly through the opening 8, and on the other hand that the reflected UV radiation will uniformly exit through the opening 8.

The dish-shaped reflector 6 comprises an upper wall which has several surfaces 10A-10D, whose normals each point

in another direction. Said surfaces 10A-10D are oriented in such a manner that UV radiation from the lamp 5 is at least partially reflected in the direction of the side walls 9 of the dish-shaped reflector 6 (see Fig. 4B) via the surfaces 10A-10D, whether or not as a result of being reflected from the reflecting obstruction element 7, to be subsequently delivered to the surface to be illuminated, such as the screen 2. The lines of intersection C of said surfaces substantially coincide with the longitudinal direction of the lamp and the reflecting obstruction element 7, respectively.

The above features effect a more uniform distribution of the amount of radiation over a surface to be illuminated, as is schematically shown in Fig. 5. This is realised by arranging the reflecting elements 6, 7 of the illumination apparatus 4 so that the radiation from the lamp 5 reaches the surface to be illuminated mainly via the dish- shaped reflector 6 instead of directly from the lamp 5, and by configuring the reflecting elements so that an effective distribution of the radiation over the surface to be illumi- nated takes place.

An additional effect of the above features is that the dimensions of the dish-shaped reflector 6 can be significantly smaller than those of the corresponding reflector 102 of the illumination apparatus 100 according to the prior art shown in Fig. 2. As a result, the edge sharpness of a stencil to be formed in the photosensitive layer 20 can be significantly improved by using a film 21. This effect is schematically shown in Fig. 6A (prior art) and Fig. 6B (illumination apparatus according to the present invention) . It has been found that the edge sharpness can be improved by a factor of about 3.

The dimensions between the walls 9 of the dish- shaped reflector 6 of the illumination apparatus according to one embodiment of the invention are about 14x16 cm near the opening 8. The depth of the dish-shaped reflector 6 is about 8 cm. These dimensions are mainly determined by the dimensions of the lamp 5. If smaller lamps with sufficient power

are available, it will be advantageous to reduce the dimensions of the illumination apparatus further.

Figs. 7A and 7B show an alternative embodiment of an illumination apparatus 4 according to the invention. Iden- tical or corresponding parts of the illumination apparatus are indicated by the same numerals.

The illumination apparatus 4 according to the invention shown in Figs. 7A and 7B is different from the previously discussed embodiment as regards the construction and configuration of the reflecting obstruction element 7.

Although the reflecting obstruction element 7 is V- shaped also in the embodiment shown in Figs. 7A and 7B, the angle of the legs 7A is more acute. The exact magnitude of the angle can be determined by experiment. In addition to that, the reflecting obstruction element 7 is provided with a reflector opening 30. The reflector opening 30 is located in the centre of the lamp 5 and substantially corresponds to the centre of the opening 8 of the dish-shaped reflector 6. As a result, a part of the lamp 5 is visible near said centre. The reflector opening 30 acts as a Fresnel lens on the UV light. As a result of the presence of the reflector opening 30, the tubular lamp 5 can intersect the reflecting obstruction element.

It has been found that the light of loss in the em- bodiment of the illumination apparatus according to the invention as shown in Figs. 7A and 7B is limited in comparison with the previously discussed embodiment, whilst the uniformity of the illumination is still suitable for screen printing.