GB2261737A - Scanning ultra violet exposure unit - Google Patents

Abstract

The present invention uses ultra violet tubes instead of a single point light source. The tubes are scanned across the exposure area on a flat bed and at a very close distance. This reduction of distance between the light source and the exposure area is enabled by use of a linearising screen which is placed between the tubes and the exposure area. This linearising screen thus simulates the angle at which the light hits the exposure area on a conventional exposure machine with the advantage of removing the great distance between the light source and the exposure area. <IMAGE>

Description

PHOTOGRAPHIC EXPOSURE UNIT This invention relates to a photographic exposure unit, of use in the photographic and reprographic trades for various kinds of "contact printing" process, for example for photographic printing from negatives, for preparation of half-tone plates for offset litho processes and so on. Conventionally, such "contact printing" processes are performed using ultraviolet light and photographic materials which are relatively insensitive to ambient lighting in the visible spectrum.

One conventional apparatus for use in such a ??contact printing process utilises a metal halide lamp which in use is spaced some distance from the photographic sheet material to be exposed and illuminates the entire area to be exposed. Such a metal halide lamp is conventionally of very high power, for example from 1 kw to 6 kw and has a relatively short service life (around 500 hours). The high electrical power required by such a lamp necessitates a heavy duty power source, so that, for example, a 3-phase supply may be required if the apparatus is to be mains operated. As the major part of the electrical power supplied to the lamp is converted into heat, in order to prevent overheating, a cooling fan is required to remove the heat given off.A further disadvantage of this conventional apparatus is that the lamp must be kept continually lit during use to ensure that the correct colour temperature is maintained.

This, in turn, means that a shutter arrangement is required to exclude light from the image area to prevent fogging of film or other photographic material when loading the same into the image area. In order to provide adequate support for the metal halide lamp, the cooling fan and shutter mechanism, a heavy and bulky construction is necessary for structural parts of the apparatus and the weight and bulk of the unit is further increased by the necessity to provide an electrical transformer to handle the electrical power involved.

An exposure unit for use in "contact printing" is also known which utilises an array of ultra-violet discharge tubes and lamps disposed relatively close to the image area. With such an arrangement, some of the problems noted above are alleviated, in that the individual power requirements for the discharge tubes and lamps in the array is lower than that for the single metal halide lamp in the first-mentioned conventional apparatus and the time required for the discharge tubes and interspersed lamps to reach their colour temperature equilibra is minimal so that there is no need to run the tubes and lamps continuously.However, it is not possible, with such an arrangement, to ensure completely uniform illumination of the entire image area, so that in certain applications, objectionable bands or stripes may be produced in the developed image, corresponding with the individual UV discharge tubes. A further disadvantage of the last-noted apparatus is that the illumination is diffuse, in the sense that not all of the light reaching the photographic material through the superimposed negative, mask or the like passes approximately perpendicularly to the plane of the material, so that the sharpness of the image obtained is reduced.

It is an object of the present invention, in one aspect, to provide an improved exposure unit which is lighter in construction and more economical to operate than the known kinds of apparatus mentioned.

According to this aspect of the invention there is provided a photographic exposure unit comprising means for supporting a sheet of photo-sensitive material in contact with a negative, screen, optical mask or the like, in an image area, a light emitting device affording a light emitting region which is elongate along a first axis to extend across the image area, and means mounting said light emitting device, in close proximity to said image area, for movement in a scan along the image area transversely to the direction in which said axis extends, whereby during such scan a band of illumination is caused to scan transversely through said image area to expose such photosensitive material in the image area, through any said negative, screen or mask superimposed therewith.

It is an object of the invention, in another aspect, to provide an improved photographic exposure unit of the kind including a rigid transparent plate superimposed on a flexible impermeable blanket to define therewith a space to receive a sheet of photographic film or paper for exposure, either alone or in a sandwich including a mask or photographic negative, with means being provided for evacuating said space to cause said photographic sheet or sandwich to be pressed flat against the transparent plate by air pressure acting on said blanket.

Known units of the last-noted type suffer from the drawback that it is necessary to evacuate said space relatively slowly7 for too rapid an evacuation, or a sudden surge in evacuation, can cause air pockets to be trapped by the blanket, whereby the photographic material is not held flat against the plate in the area of these air pockets and the photographic print is impaired. Accordingly, the space between the plate and the blanket has to be evacuated slowly and evenly for all of the air between the plate and blanket to be removed.

According to said other aspect of the invention, there is provided a photographic exposure unit including a rigid transparent plate superimposed on a flexible impermeable blanket to define therewith a space to receive a sheet of photographic film or paper for exposure, or a sandwich including a sheet of photographic film or paper and a mask or photographic negative and means for evacuating said space to cause said sheet of film or paper, or said sandwich to be pressed firmly flat against the transparent plate by air pressure acting on said blanket, wherein said blanket is secured at the end at a position adjoining or engaging said plate and at its opposite end at a position spaced from the plane of said plate, whereby during evacuation of said space, a boundary line between the portions of the blanket already engaging said plate or such photographic film, sheet, or sandwich and the portions of the blanket which have yet to do so moves across the plate from said one end of the blanket towards the other, as evacuation proceeds.

Because the blanket is effectively rolled onto the plate from one end to the other air is reliably expelled from between the plate and the blanket in a progressive manner along a front moving from one end of the unit towards the other allowing much more rapid evacuation than with conventional exposure units.

An embodiment of the invention is disclosed below by way of example with reference to the accompanying drawings in which: FIGURE 1 is a schematic perspective view illustrating an apparatus embodying the invention; FIGURE 2 is a partial view, in section along the line A-A of Figure 1; FIGURE 3 is a fragmentary plan view, under substantial magnification, of a linearising screen; FIGURE 4 is a diagram illustrating spread in a photographic contact printing process, and FIGURE 5 is a schematic longitudinal section view illustrating the arrangement of a vacuum exposure unit embodying the invention.

Referring to the drawings, the apparatus comprises a holder 8 for holding the photographic film or plate to be exposed, the negative to be printed, half-tone screen if required etc., the holder 8 comprising, in a conventional manner, a glass cover plate 10, a flexible impermeable blanket 12 arranged below the plate 10 generally parallel therewith and a flexible seal lit around the edges of the blanket 12, which seals the blanket 12 with respect to the plate 10.

In use, a sheet of photographic material, for example photographic film, is placed on the blanket 12, whilst the plate 10 is removed, the appropriate negative, halftone screen, if required, etc. are placed on top of the film and the glass plate 10 replaced, the volume between the blanket 12 and the plate 10 being then evacuated by means of a vacuum pump (not shown) to cause the plate 10 and blanket 12 to be forced together by atmospheric pressure to clamp the photographic material and the superimposed negative, screen, etc. firmly against the underside of the plate 10 and thus in a flat condition.

Such an arrangement is known per se.

Arranged above the holder 8 for traversing movement across the latter, in the direction of the arrow 16 in Figure 1, is a movable scanning head 18 which carries two elongate ultra-violet discharge tubes 20 extending, parallel with each other and perpendicular to the arrow 16, entirely across the image area, the tubes 20 also extending parallel with the plane of the plates 10 and 12. The scanning head 18 comprises a frame supporting the tubes 20 and carrying a reflector arrangement (not shown) for the tubes and opaque top and side walls. The frame of the scanning head 18 is open at the bottom, for transmission of light from the tubes 20 into the image area defined by the holder 8.If desired, a linearising screen, indicated at 22 in Figure 2, and described in greater detail below, may be fitted detachably across the underside of the frame of the scanning head 18 so that the light from the tubes 20 will only reach the image area after passing through the screen 22. As best shown in Figure 2, the tubes 20 are disposed very close to the upper surface of plate 10, with just sufficient spacing therefrom to accommodate the linearising screen 22 with a running clearance relative to the upper surface of plate 10. The apparatus has a support structure (only part of which is indicated at 19, which carries the holder 8 and supports the head 18.

In use, with the holder 8 loaded as described above, the tubes 20 are energised and the whole frame 18 is traversed across the image area of the film holder, in the direction of the arrow 16, from one edge of the film area to the other, so that the transverse band of illumination provided by the tubes 20 sweeps across the photographic material and superimposed negative, screen, etc. in a single steady movement from one edge of the material to the other, so that in the course of the complete scan, each part of the image area receives the same amount of illumination, although some parts of the image area receive their quota of illumination later than others.

It will be appreciated that, in order to ensure uniform illumination, it is preferable for the head 18 to start its scanning movement from a position substantially in advance of the upstream edge of the image area and to terminate its scanning movement substantially beyond the downstream edge of the image area, so that the head 18 "overshoots" the image area at either end of the scan.

Any appropriate arrangement may be utilised for supporting the head 18 for the scanning movement described and for effecting such scanning movement and such arrangement is not illustrated in detail. By way of example, however, the opposite ends of the head 18 may be slidingly supported on rails or channels fixed in the structure of the machine and extending parallel with the arrow 16, whilst the actual movement of the scanning head may be effected in any convenient manner, for example, as illustrated in Figure 1, by means of a pinion 30 fixed on the shaft of an electric motor 32 mounted on the head 18, and which pinion engages a rack 34 extending longitudinally along a side member of the structure 19. Other means for moving the head may be used, for example, a system of coordinated cables, or a chain-drive mechanism, or some similar arrangement.The power supply to the tubes 20 (and the motor 32 in the illustrated embodiment), may be, for example, by a flexible lead from a stationary part of the apparatus.

The linearising screen 22 referred to above may simply comprise an opaque plate, of light weight but substantial thickness, formed with an array of narrow bores extending perpendicularly to the plane of the plate. Figure 3 illustrates, in plan, a fragment of such a plate. In the array illustrated, successive rows of such narrow bores are staggered relative to the scanning direction of the head so as to ensure that, during a scan, the variation in illumination of the image area, due to one transverse row of bores is entirely cancelled by the corresponding variation in the succeeding staggered row of bores. The effect of such a linearising screen is simply to block the passage, from the tubes 20 to the holder 8, of any light rays which are not directed substantially parallel with the axes of said narrow bores and thus perpendicular to the plane of screen 22.As a consequence the light transmitted by the linearising screen 22 approximates to a parallel beam thus ensuring that the images formed in the contact printing process (being effectively simple shadows) are sharp.

It will be understood that such a linearising screen in fact cuts out a relatively large part of the light from the tubes 20 which would otherwise reach the image area of the holder 8, so that the increased sharpness in imaging resulting from the use of the linearising screen is obtained at the cost of some increase in exposure time. To minimise the light loss, it is preferred that the ratio of the area of the bores through the screen 22, to the area of the interstices therebetween, should be as large as possible, consistent with the screen 22 having adequate physical strength to support itself. To decrease exposure time as far as possible, higher-output ultra-violet tubes 20, with internal reflectors, may be used.

In the embodiment of the invention described, since there are only a few ultra-violet tubes required to produce the illumination for exposure, (and, in principle, a single tube could be used), power requirements, weight and running costs in terms of replacement of tubes, etc. as well as consumption of power are correspondingly reduced. The tubes used have a long working life (around 1000 hours to the point at which light output has dropped to 80% of its original value). The tubes used are preferably instant-striking, to ensure optimum lamp life. The tubes used reach their full output and desired colour temperature within 3 seconds of being switched on, so that there is no need to run the tubes except during, and shortly before, individual exposures.

The power consumption of the tubes 20 is relatively low, for example 100 watts, so that heat output is low and bulky transformers etc. are not required.

The apparatus can therefore be of lightweight construction.

Due to the scanning action of the light source, uniform exposure of the image area can be ensured by ensuring a uniform scanning speed, which presents no difficulty in practice.

Another distinct advantage of the scanning exposure system of the present invention over a conventional metal halide system is the ability to give an "even spread". Spread is printers terminology for enlarging an image. This is achieved by placing distance between the film and mask emulsion layers (conventionally done by making a sandwich of an acetate layer (foil) between the film and mask, and using diffused light. The problem of spread in a metal halide system is illustrated in Figure it.

It will be noted from Figure it that light which reaches the regions adjacent the edges of the image area is at an angle relative to the plane of the photographic material. This tends to cause the image to be displaced in position, according to size of this angle. With the apparatus of the present invention, this net displacement of the image cannot occur, because, averaged over the whole exposure, the mean angle of incidence, relative to the normal to the plane of the plate 10, is substantially zero throughout-the image area.

As described above, the linearising screen 22, disposed between the light source and image area, is carried by the head 18 and may, for example, be fixed in place with screws and either obscures the unwanted diffused component of light from the image area permanently, or does so until the screen 22 is removed manually.

In a further development, the linearising screen 22 is itself mounted so that it can be moved rapidly and automatically between an operative position in which it is interposed between the tubes 20 and the image area and an inoperative position in which is clear of the open or light-transmitting underside of the head 18 and thus does not obstruct the passage of light from the tubes 20 to the image area.In a preferred arrangement, the screen 22 is mounted for scanning movement in the direction of arrows 16 independently of head 18, under the action of its own independent drive means, for example under microprocessor control, so that the screen 22 can be made, at will, to scan the image area with the head 18, whilst remaining directly below the head 18, or can scan the image area with the head 18 whilst remaining slightly out of register with the head 18 so that some of the light from the tubes 20 reaches the image area without passing through the screen 22 and is thus relatively diffuse, whilst the remainder passes through the screen 22 and is thus linearised. Alternatively, the head 18 may be arranged, for example, to make one scan without the screen 22 and, immediately after, a second scan with the screen 22 fully in place.

In this way, a combination of diffused and linearised light can be obtained.

The diffused light output can be used to good effect in certain applications, for example, to "spread" the image (enlarge and reduce). Using such an independently movable linearising screen, much reduced exposure times are obtained without the screen obscuring the tubes. In most circumstances the diffused light will not cause substantial deterioration in image quality and in these circumstances the screen 22 will not be required. Thus the operator would be able to choose between: A) high quality "linearised" light with longer exposure times, B) diffused light affording brief exposure times, or C) any combination between the extremes indicated at A and B.

Referring to Figure 5, an exposure system embodying the invention is preferably fitted with a vacuum holder, for holding the sheet photographic material and superimposed mask, negatives etc, which differs from the conventional vacuum holder, for a photographic exposure unit, in the respects seat out below.

The main drawback with the conventional vacuum holder system is the time taken to evacuate the space between the plate 10 and the blanket 12. Too rapid an evacuation, or a sudden surge in applied negative pressure, can cause air pockets to be trapped between the blanket and the plate 10 so that the photographic materials do not, in such pockets, assume the desired flatness and a faulty print results. It is therefore necessary to evacuate the space between plate 10 and blanket 12 slowly and evenly to allow all the air to be expelled.

Referring to Figure 5, in the embodiment of the invention illustrated therein, the above noted disadvantages are avoided by so configuring and supporting the blanket 12 that it assumes a curved shape in section, with the convex side facing towards the plate 10, the blanket 12 being adjacent the plate 10 at one end and being spaced relatively far from the plate 10 at a location 40 adjacent the outlet (not shown) from which air is drawn from the space between the plate 10 and blanket 12 during evacuation. Means, not shown in Figure 5, are provided at the location 40 for holding the respective end of blanket 12 away from the plate 10.

It will be appreciated that as evacuation of the space between blanket 12 and plate 10 progresses and thus the pressure differential between the atmosphere and the space being evacuated increases, the tension in the blanket 12 increases and the curvature, as viewed in Figure 5 is of the region still out of contact with the plate 10 also increases so that the blanket 12 first engages the plate 10 adjacent the end (at the left in Figure 5) initially nearest the plate 10, and subsequently the point where the blanket just makes contact with the plate 10 moves progressively to the left in Figure 5 so that the blanket 12 is effectively "rolled" onto the underside of the plate 10 with the air between the plate 10 and blanket 12 being expelled just ahead of the advancing point of contact.In practice, considering the device in place, there will be a continuous line along which the blanket 12 just makes contact with the plate (i.e. a line dividing the region of contact from the region of non-contact).

In this way, in use, during evacuation, air is expelled from one edge of the film (between plate 10 and blanket 12) to the other and the formation of air pockets is avoided.

The speed with which the space between the plate 10 and blanket 12 can be evacuated can be enhanced by the use of vacuum reservoirs which are evacuated when the machine is idling. When an exposure is to be made, solenoid valves connecting the reservoirs with said space are opened allowing rapid removal of air from said space into said reservoirs, so decreasing evacuation times.

It will be appreciated that, for purposes of illustration, the separation of the blanket 12 from the plate 10 and the curvature of the blanket 12 are much exaggerated in Figure 5.

It will be appreciated that a vacuum holder such as described with reference to Figure 5 may also be utilised in a conventional exposure unit, for example having a single metal halide light source.

Claims (8)

CLAIMS:

1. A photographic exposure unit comprising means for supporting a sheet of photo-sensitive material in contact with a negative, screen, optical mask or the like, in an image area, a light emitting device affording a light emitting region which is elongate along a first axis to extend across the image area, and means mounting said light emitting device, in close proximity to said image area, for movement in a scan along the image area transversely to the direction in which said axis extends, whereby during such scan a band of illumination is caused to scan transversely through said image area to expose such photo-sensitive material in the image area, through any said negative, screen or mask superimposed therewith.

2. A photographic exposure unit according to Claim 1 wherein said light emitting device comprises at least one electrical discharge tube which is elongate along said axis.

3. A photographic exposure unit according to Claim 1 or Claim 2 including screen means associated with said light emitting device to prevent the passage of light from said light emitting device to said photosensitive sheet material otherwise than substantially perpendicular to such material.

it. A photographic exposure unit according to Claim 3 wherein said screen means comprises an opaque plate having a plurality of light transmitting narrow bores or the like passing perpendicularly therethrough.

5. A photographic exposure unit substantially as hereinbefore described with reference to and as shown in Figures 1 to 3 of the accompanying drawings.

6. A photographic exposure unit including a rigid transparent plate superimposed on a flexible impermeable blanket to define therewith a space to receive a sheet of photographic film or paper for exposure, or a sandwich including a sheet of photographic film or paper and a mask or photographic negative and means for evacuating said space to cause said sheet of film or paper, or said sandwich to be pressed firmly flat against the transparent plate by air pressure acting on said blanket, wherein said blanket is secured at the end at a position adjoining or engaging said plate and at its opposite end at a position spaced from the plane of said plate, whereby during evacuation of said space, a boundary line between the portions of the blanket already engaging said plate or such photographic film, sheet or sandwich and the portions of the blanket which have yet to do so moves across the plate from said one end of the blanket towards the other, as evacuation proceeds and the blanket is progressively stretched.

7. A photographic exposure unit substantially as hereinbefore described with reference to, and as shown in Figure 5 of the accompanying drawings.

8. Any novel feature or combination of features disclosed herein.