The invention relates to the optical scanning of transparent image media for the purpose of converting images into digital data for subsequent processing and more particularly, to a means of illuminating and constraining strips of transparent media such that it will not degrade the scanning process through deformation of the media. BACKGROUND OF THE INVENTION

There are a number of methods for simultaneously constraining and illuminating strip transparency media. The most common are the rigid frame types of film gates commonly used in motion picture projectors and similar devices. A more sophisticated version of this rigid constraint approach is the glass platen type of configuration wherein the transparency material is forced to conform to a single flat optically transparent surface (typically glass) or alternatively sandwiched between two such surfaces.

Desirably, the film gate should be such that physical contact between the film surface and a friction imparting element is prevented, in order to avoid scratching the film, which could possibly cause a loss of information contained in the image frame. To this end, improvements in film gate structures have led to the development of fluid gates, to which a curtain, or stream, of air is generated in the area of the gate so as to provide a medium by way of which the film may be frictionlessly supported. An example of such an air pressure gate is described in US Patent No. 3,457,007 to Conroy, and consists of a differential pressure gate for a motion picture projector and has

a plurality of peripheral ducts which pass through the walls of the gate aperture. Air pressure is introduced through these ducts in an effort to apply a substantially uniform pressure throughout an air chamber disposed on one side of the photographic film.

US Patent 4,411,503 to Bailey et al is directed to an air platen bearing. The fluid bearing platen for the film gate structure contains a pair of cavities to which a pressurized fluid, such as air, is applied against opposing surfaces of the photographic film. Between each of the respective cavities, and on one side of the film, there is a thermal dynamically non-throttling material, such as porous stainless steel, through which the pressurized fluid from the cavity passes, where it is directed against opposite sides of the film.

All of the above-mentioned air bearing film gates require an external source of air pressure which can be both cumbersome and costly. Another problem that can be encountered with film gates having a rigid transparent platen have the propensity to develop optical interference patterns between transparency materials and optically flat platens unless indexing matching fluids (typically oils) are used. In addition, it is difficult to maintain true flatness of a transparency over the scanned area which can be detrimental to the scanned image quality in systems with limited depth-of-focus.

SUMMARY OF THE INVENTION A rigid transparent cylinder is rotated about its axis of symmetry at a relatively high rate of speed by means of a motor. Around a portion of

the circumference of the rotating cylinder is wrapped a flexible imaging medium such as photographic film to which is applied longitudinal tension. The film is held in an equilibrium position conforming to but slightly removed from the surface of the cylinder by aerodynamic forces and the applied tension, thus effecting a compliant surface air bearing. Illumination is provided through means of a stationary lamp and diffuser placed within the transparent drum whereby the light emanating from the lamp passes through the drum and air gap. An image of the illuminated transparency is formed upon the scanning element by means of an optical lens system. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a side elevational view in schematic form of the projection gate, optical system and film path, the film gap has been exaggerated for the purpose of clarity; Figure 2 is a front elevational view of

Figure 1.

Figure 3 is a perspective view of the film passing through the projection gate wherein it is supported by the air bearing of the present invention, once again, the film gap has been exaggerated;

Figure 4 is a graph showing gap size as a function of the longitudinal coordinate of the film in the gate; Figure 5 is a graph showing Hoop Stress as a function of rotational speed of the cylinder; Figure 6 is a side view of a modified cylinder having tapered pockets about its outer circumference for illustrative purposes, the film gap has been reduced to zero in places, in an effort

to show the ramped surfaces to scale; and

Figure 7 is a front elevational view of Figure 6.

Modes of Carrying Out the Invention Because web handling apparatus are well known, the present description will be directed in particular to elements forming part of, or cooperating more directly with, the present invention, and elements not specifically shown or described herein are understood to be selectable from those known in the art.

Referring now to the drawings, and to Figures 1 and 2 in particular, the present invention is illustrated and will be described in connection with a film transport generally designated 10 that comprises a supply reel 12 for the film 14. The supply reel is mounted on a spindle 16 that is normally driven by a tendency or friction drive in the takeup (counter-clockwise) direction as indicated by the arrow for urging reel 12 in a direction for taking up film onto that reel. The tendency or friction drive means allows the reel to be rotated in an unwinding (clockwise) direction during removal of film from reel 12. The film travels along a film path between reel 12 and reel 18 with reel 18 being mounted on spindle 19. Along the film path is a film gate shown generally at 20; adjacent thereto are two substantially identical and symmetrically mounted capstan drive means 22 and 24. The film gate will be discussed in detail below.

Film 14 can be advanced through the gate 20 either from supply reel 12 to takeup reel 18 or in the reverse direction. Because the transport mechanism of this invention is capable of operation

for advancing film in either direction with equal facility, the terms "supply reel" and "takeup reel" are used as a matter of convenience, and each may be used with equal accuracy for designating either reel. As shown diagrammatically in Figure 1, spindles 16, 19, are driven by motors 26, 28, respectively.

The capstan drive means 22 comprises a rotatable film-engaging drive member shown as pulley 30 mounted on spindle 32 that is driven from motor 34. In addition, an encoder 36 which is incorporated on shaft 32 and is used to accurately control the speed and direction of film movement.

Pulley 30 is located along the film path so that the film is wrapped at least partially around the pulley, the extent of the wrap being substantially 180°. Capstan 24 is substantially identical to capstan 22 having a spindle 38 which is driven by motor 40 with no encoder, only a single encoder is needed in the film path to accurately control the speed and direction of film on this transport. The capstans each act as a web tension amplifier and therefore may be used as feedback or servocontrol mechanisms for controlling tension forces in the film at film gate 20 even when relatively large static and dynamic loads and load variations are encountered during operation of the transpor .

Float rollers 42 and 44 are provided along the film path between each reel 12, 18 and each capstan 22, 24 respectively. Each roller 42 and 44 is rotatably supported at one end of an arm 46 and 48 respectively that is movably connected at its other end to a support plate 50. Arms 46 and 48 can flex upwardly from the position shown in solid lines

toward the position shown in dotted lines in response to an increase in tension forces encountered by that portion of the film between capstans 22 and 24 and corresponding reels 16 and 18. Arms 46 and 48 are preferably formed of a spring material or is spring biased (as viewed in Figure 1) so that they tend to return to their solid line position when permitted to do so by film tension. Operation of the film transport shown in

Figures 1 and 2 will now be described. Assuming that capstan 22 is pulling film from supply reel 12 from the right side of film gate 20, the tension force in the low-tension side of the film between film gate 20 and capstan 22 is increased and this increase is amplified (to some extent) by the resulting movement of roller 44 against the bias of spring arm 48. This increases the tension in the portion of the film wrapped around pulley 30, thereby increasing the driving force applied by the pulley to the film. Capstan pulley 30 then pulls film from the loop of film around the roller 42, thereby causing deflection of roller 42 from its solid line position toward its dotted line position against the biasing force of arm 46. The increased driving force of capstan 22 coupled with the increased tension force applied to the film by virtue of the deflection of arm 42 causes film 14 to be withdrawn from supply reel 12 against the tendency or friction drive applied through spindle 16. This increases the length of film between supply reel 12 and film gate 20, and permits roller 42 to return toward its solid line position under the influence of spring arm 46 that supports it. This, in turn, reduces the radial pressure of the

film against capstan pulley 30 to reduce the driving force applied by capstan 30 to film 14. The various forces acting on film 14 to produce automatically an equilibrium situation or condition wherein the driving force applied to the film by capstan 22 substantially equals the opposing forces acting on the length of film 14 between film gate 20 and supply reel 12.

The tension force in the film segment between film gate 20 and takeup reel 18 is reduced, and there is a corresponding reduction in the driving force applied by capstan 24 to the film portion wrapped around capstan 24 and between capstan 24 and reel 18. The tendency drive applied through spindle 19 to reel 18 then becomes effective to pull additional film around capstan 24 and onto takeup reel 18 in the direction shown in Figure 1. Here, again, the forces acting on the segment of film between film gate 20 and takeup reel 18 soon produce an equilibrium situation or condition wherein the driving force applied to film 14 by capstan 24 is effectively opposed by the force applied to film 14 through the tendency drive acting on spindle 19. Because the film-handling elements of the transport as described hereinbefore are substantially symmetrical with respect to film gate 20, reverse movement of the film from reel 18 to reel 12 is the same as described hereinbefore except, of course, that the film is driven in the opposite direction. Also, because capstans 22 and 24 are always driven in a direction to feed film toward gate 20 and because they are effective to maintain a very low tension in those segments of the film between gate 20 and each capstan 22, 24. Also,

the portion of film 14 in film gate 20 is substantially isolated from the tendency drives and external forces that otherwise tend to move film in the gate, a steadier projected image is obtained when all forces are in balance. In addition, the low-tension forces encountered permits the film transport to be switched instantaneously from forward to reverse modes of operation without film damage even at very high rates of film movement. A drive of the type previously described comprises a web tension amplifier and such may be used in a feedback or servocontrol type mechanism for automatically maintaining a low-tension loop of film adjacent to a film gate, for controlling tension of the film at the gate even when relatively large static and dynamic tension forces and tension force variations are encountered. Use of the optional encoder 36 on capstan 22 helps provide the necessary feedback to motor 34 that drives capstan 22. Thus, it can be seen that web tension force differences that are encountered can be controlled by adjusting one or a combination of several variable factors that affect operation, including, inter alia, adjusting the actual tension force applied to a whole strand adjacent the capstan, changing the radial force between the web capstan at some point within the wrap angle of the web about the capstan, modifying the extent of the wrap angle by increasing or decreasing the extent of the contact between the web and capstan drive pulley 30, and by making adjustments or changes that vary the effective coefficient of friction between the capstan and the web. Thus, it can be seen that accordingly, the encoder 36 is used to control capstans 22 and 24 and thus the tension of the film

in film gate 20.

Figures 1 and 2 illustrate the use of two capstan drive motors 34 and 40 located along the film path with the film being trained or wrapped around a portion of each pulley. The portion of the film between each reel 12 and 18 and the next adjacent member may be trained around suitable guide pulleys 52 and 54. While only two guide pulleys 52 and 54 have been shown, additional pulleys can be provided in series or cascaded if required during high speed film transport.

Turning our attention to the inventive film gate 20 of the present invention, which comprises a rigid transparent cylinder 56 rotated about its polar axis of symmetry at a relatively high rate of speed by means of a directly coupled low cost DC motor 58. Around a portion of the circumference of rotating cylinder 56 is wrapped a flexible engaging medium such as photographic film 14 to which is applied longitudinal tension by means of the film transport described above, another suitable transport system is disclosed in US Patent No. 4,752,124, filed in the name of Ueda et al on 6/21/88. Rotating transparent cylinder 56 is made of a thermoset carbonate plastic or glass which has a hub 38 formed on one end with a spindle 57 mounted thereto, aligned with the axis of the cylinder A-A, for supporting cylinder 56 with the other end attached to motor 58. Motor 58 rotates cylinder 56 at a constant rotational speed of approximately 5,000 rpm. Care must be taken to ensure that the surface of cylinder 56 is true and concentric with the axis of rotation of motor 58. As mentioned earlier, the strip of photographic film 14 is

wrapped around a portion of the circumference of the rotating cylinder 56. Longitudinal tension is maintained by film transport 10 described earlier; such that film 14 is held a small distance above the outer surface 59 of cylinder 56 by balancing the applied film tension with the aerodynamic forces developed by the air bearing formed around the circumference of rotating cylinder 56. The cylinder is located in a first position when the cylinder is at rest, as indicated by the dotted lines. After threading of the film, the cylinder is brought to its proper rotational speed and then moved toward the film to its second position, shown in solid lines in Figure 1. This is done to prevent film from rubbing against the surface of the cylinder before the cylinder has reached its operational speed. Illumination is provided through means of a stationary incandescent lamp 60 placed behind or preferably within cylinder 56. The inner surface 62 of cylinder 56 is grit blasted to create an optically diffuse surface, the uniformity of which is enhanced by virtue of the high rotational speed of cylinder 56. The illuminated film image is projected upon the scanning element 64 by means of a conventional lens system 66. The optical encoder 36 in the film transport 10 is used as a feedback sensor as part of a constant velocity film transport to continuously transport the film strip through the imaging area of film gate 20 effecting a second direction of scan longitudinal to the film. Alternatively, such film movement can be accomplished by use of a step-and-repeat drive. The aerodynamic generation of the air bearing is described by Reynold's equation as:

where X is the coordinate parallel to the axis of rotation of the cylinder, h is air bearing gap thickness, p is the pressure developed within the air bearing, S is the longitudinal direction of the transparency strip, U is the relative surface velocity of the cylinder and the film transparency, μ is the viscosity of the air, and T is the web tension per unit width.

Assuming a sufficiently flexible film transparency of sufficient width and invariant characteristics with respect to time, and combining with the equilibrium equations results in what is known as the "foil bearing" equation:

The solution of this equation in fact indicates an undulation in the shape of the film transparency strip in the entrance and exit regions of the bearing area as illustrated in the graph shown in Figure 4. However, in the central region of the bearing,

It can be noted from the equation that the gap generated in the air bearing is linearly proportional to the radius of the cylinder and is less sensitive to variations in μ, U, and T the viscosity of the air, the relative surface velocity of the cylinder and the film, and the foil tension

per unit width respectively. Thus, these parameters can be used to control and maintain a given gap.

The following 5 operating parameters affect the operation of this film gate are: (1) Cylinder radius and speed

(2) Film transparency flexural rigidity

(3) Wrap angle of the film

(4) Film transparency strip tension

(5) Air viscosity The operation of each of these parameters and how they affect the film gate will be discussed individually. Cylinder Radius and Speed

A primary concern that determines the upper bound as to the size of the cylinder that can be used practically is the rotational velocity which determines the induced centrifugal loading of the cylinder.

The parameters that represent the lower bounds upon size and speed relate to such factors as the minimum radius of curvature that can be induced in a film web without imparting damage to the web. In addition, the forces required to bend a web around too small a radius would exceed the forces capable of being generated by the air bearing to maintain the desired gap between the film and cylinder. This is because minimal aerodynamic lift is developed at low rotational speeds of the cylinder for a given tension level in the film transparency, this would be a determination of a minimum radius for the cylinder.

Another factor that should be considered in determining the size of the transparent plastic or glass cylinder is the centrifigually induced tangential or "hoop" stress. A stress of

2 approximately 500 lb/in (3.446 Mpa)* was found to constitute an upper limit especially in light of the elevated temperatures that are likely to occur when using an incandescent lamp inside the cylinder. Glass was found to be a preferred material when incandescent lamps were used, resulting in higher temperatures; thermoset plastic was preferred with lower temperature light sources such as fluorescent. Plastic was also found to be satisfactory from a cost standpoint when fabrication is considered.

The maximum tangential (hoop) stress and radial stress as a function of rotational velocity are given by the following respective equations:

(Oh nx

where: σn - maximum tangential stress in the cylinder; σ - maximum radial stress in the cylinder ω = rotational velocity r_ r. - outer and inner radii respectively;

V - Poisson's ratio of cylinder material; γ - mass density of cylinder material; g - standard acceleration due to gravity.

*Mρa=Micro Pascal

Fig. 5 illustrates a graph for glass cylinders of various sizes plotted for max hoop stress in psi vs rotational speed in RPM.

Film Strip Transparency Flexural Rigidity Photographic film transparencies generally display thicknesses between 0.0005 inches (.013mm) and 0.0070 (.178mm) inches and are commonly produced from acetate or polyester materials with flexural

₂ modulae ranging from 200,000 lb/in (1,380 MPa) to 2,000,000 lb/in ² (13,780 MPa) respectively.

Combining the effects of these thicknesses and material stiffnesses, these properties combined yield flexural rigidities of from 2.1 x 10 ^~ lb-in ² (6.03 x 10 ^~9 Nm ² )* to 6.0 x lθ ^"2 lb-in ² (1.72 x 10 ^~4 Nm ² )* per inch of width.

Common widths for film transparency strip materials currently include 16mm (0.630"), 35rnm (1.378") and 70mm (2.756") sizes. Consequently, overall flexural rigidity that is likely to be encountered by this type of device can range from 1.3 x 10 —6 lb-in2

(3.73xlθ ^"9 Nm ² ) to 0.2 lb-in ²

(5.74x10 -4Nm2) , a range of a factor of 105.

Accordingly, selection of a cylinder diameter and other bearing parameters must consider the flexural rigidity of the film in conjunction with the amount of tension to ensure that the force, or lift developed by the air bearing will be sufficient to preclude touch-down of the film to the surface of the cylinder. Wrap Angle

The limits on the angle of wrap around the circumference of the cylinder that the film subtends are determined by the end effect undulations mentioned previously and the rigidity of the film *Nm 2«Newton-meter2

which would limit the wrap angle to some angle less than that which would cause permanent damage to the film. In addition, if the longitudinal dimension of the effective bearing area is not great enough, the nonuniformities in the air bearing gap will be noticeable in the scanning process.

Transparency Strip Tension

Tension levels for handling strip materials depend upon thickness and are generally limited to the following values to preclude undue stress and attendant deformation:

0.0005 inches (.013mm) thick - 0.25 lb per inch of film width; (43.8 N per m of film width)

0.0001 inches (.025mm) thick - 0.5 lb per inch of film width; (87.6 N per m of film width)

0.005 inches (.127mm) thick - 2.5 lb per inch of film width; (437.8 N per m of film width)

0.007 inches (.178mm) thick - 3.5 lb per inch of film width. (612.9 N per m of film width) Substituting the following values in the air gap equation for h _fi :

K = .643 (a di ensionless shape/stiffness factor) ;

_ ₃ r = 0.75 inches (19.05 x 10 m) μ = 2.62 E-α lb/sec/in (18.1μN - sec/m )

U = 392.5 inches/sec (9.975 m/sec) (based upon rotational speed of 5,000 rpm) results in a theoretical air gap of 0.000160 inches (4.06 μM) which is more than sufficient to preclude visible interface patterns (using incandescent or fluorescent light or other light source having relatively low coherence) by approximately a factor of three. Air Viscosity

Although the performances of the apparatus described herein are relatively insensitive to variations in air viscosity such as may occur at higher elevations, it is obviously not suitable for use in such rarefied atmospheres that may be encountered in aerospace applications without the supply of a controlled atmosphere such as nitrogen gas.

The air bearing configuration described above is the simplest configuration possible for operation in that it uses a featureless cylinder. It should be understood that the performance of the bearing can be enhanced, by the addition of certain features to surface 59 of rotating cylinder 58. Such a feature could be the "tilted shoe" or "taper pocket" bearing. This type of design provides a number of tilted surfaces or ramps followed by a strip which leads up to the next tilted surface 68 of cylinder 58 as illustrated in Figure 6. Essentially, each of the ramps acts as a wedge or pump to increase the efficiency of the air bearing. While the invention has been described in conjunction with the specific embodiment thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description.

For example, the incorporation of a remote light source and attached fibre optic light pipe could be used in place of the lamp in the center of the cylinder. Such a modification could significantly reduce the potential heat build-up that would be associated with a lamp in proximity to the cylinder and film.

Accordingly, it is intended to embrace all such alternatives, modifications, and variations as

fall within the spirit and broad scope of the appended claims. Advantages and Industrial Applicability

The present invention is useful in a microfilm reader for the purpose of projecting images onto a solid state sensor for the purpose of converting the images into digital data for subsequent processing and/or storage. The invention is useful in systems which project the microfilm image onto a linear sensor as opposed to area sensors; and is capable of accommodating a range of media widths without alteration to the general configuration of the apparatus. The improvement offered by the invention increases the flatness of the film across the width of the scanned area, which is important in systems with limited depths-of-focus. In addition, optical interference patterns between the film and support surface have been eliminated, as well as a reduction in abrasion and wear between the film and support surfaces. These advantages are achieved without the requirement for any external source of pressurized air for the air bearing film gate.