GB2166880A - Flash exposure - Google Patents

Abstract

In full frame imagewise exposure on a moving photoconductive belt using flash illumination of an opaque original, a shadow cast by the lens 32 is removed by means of auxiliary non-imagewise exposure (before, during or after the imagewise exposure). A predetermined area of the belt may be illuminated from the lamp housing as shown (during the imagewise exposure) or independently by means of an LED array (50, Fig. 1) at a location spaced from the imagewise exposure station. <IMAGE>

Description

SPECIFICATION Full-frame flash illumination system The present invention relates generally to an illumination system in a document reproduction machine and more particularly, to a fullframe flash illumination system contained within a light housing.

As demands for faster copying and duplicating have increased, conventional machines which scan documents in incremental fashion to provide a flowing image on a xerographic drum have proved inadequate. New high speed techniques have evolved which utilize flash exposure of an entire document (fullframe) and the arrangement of a moving photoconductor in a flat condition at the instant of exposure. Full efficiency of such systems has been realized by enclosing the illumination system within a light housing. The interior walls of such a housing may be diffusely or specularly reflective to produce multiple reflection of light emanating from the illumination source contained within the housing. A uniform level of illumination is produced at the object plane; generally a transparent platen upon which the document to be reproduced is positioned. U. S.Patent 4,250,538 is representative of such systems.

In these prior art systems, the imaging lens is positioned within the floor of the light housing and, in a multi-magnification mode, is moved laterally and vertically to positions required by magnification changes. One problem with these prior art imaging systems is that an image of the lens face is reflected from the bottom surface of the transparent platen and is projected back through the lens onto the photoreceptor surface. This negative disc image, or "shadow", is subsequently developed as part of the document image and, upon transfer to a recording sheet, appears as a reiatively dark spot on the output copy. Various expedients have been tried to reduce this problem. For example, as disclosed in U. S.

Patent 4,250,538, the lens barrel is painted white to reduce the size of the reflected lens image.

It is therefore an object of the present invention to reduce or eliminate the "shadow" or "black hole" problem in full-frame flashtype systems.

According to the present invention, there is provided a flash illumination and optical imaging apparatus for producing, on a photoreceptor belt member, images of documents on a transparent object plane, the apparatus comprising, a document illumination housing positioned adjacent said object plane, a flash lamp positioned within said housing and adapted to be pulsed to provide a generally uniform level of illumination at said object plane, a projection lens mounted in the surface of said housing opposite said object plane, said lens being adapted to project an image of a document onto said photosensitive belt member, and supplementary illumination means adapted to illuminate an area of said belt upon which, during operation of the flash lamp, a shadow of said lens is formed by reflection from the object plane.

According to a first aspect of the invention, the light component is generated by a separate illumination means positioned in proximity to the belt surface. According to a second aspect, the illumination component is derived from light generated within the light housing.

A flash illumination and optical imaging apparatus according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a partial perspective view of a full-frame illumination and imaging system showing a first embodiment of a daerk hole neutralization device positioned in proximity to the photoreceptor surface.

Figure 2 shows a second embodiment of a neutralization device.

Figure 3 shows a third embodiment of a neutralization device.

Figure 4 shows a side view of an illumination and imaging system which incorporates a first embodiment of an optical charge neutralization device.

Figure 5 shows an illumination housing incorporating a second embodiment of an optical charge neutralization device.

Referring to the drawings, Fig. 1 shows a document imaging system 10 which includes an integrating optical cavity 12 and a photoreceptor belt 14 (only a portion of which is shown). Cavity 12 is a completely enclosed housing, generally rectangular in shape, having a first pair of opposing side walls 15, 16 and a second pair of opposing side walls 18, 20.

An upper, or top wall is formed by seating a glass platen 22 into aperture 24. The lower, or bottom wall 28 has an aperture 30 therein which accommodates a circular lens 32.

Mounted in the lower half of side wall 20 is flash illumination lamp 34 which may be, for example, a Xenon gas lamp. The lamp is connected to pulsing circuitry (not shown) which, when activated, pulses the lamp, resulting in an illumination flash of appropriate duration.

The interior walls of the cavity have substantially diffusely reflecting surfaces which cause the flashed light to undergo multiple reflections from the walls, providing a uniform level of illumination at the underside of platen 22.

Blockers 36 and 38 prevent direct light from reaching platen 22 and lens 32, respectively.

In operation, an original document to be copied (not shown) is placed on platen 22.

Upon triggering of an illumination flash, the document is uniformly illuminated by the light, diffusely reflected from the cavity walls. The light rays are reflected from the document platen and are projected as a light image of the original document through lens 32 onto photoreceptor belt 14. The surface of belt 14 has been charged at a point prior to the exposure station to a uniform charge level by a corona generating device 42. As the light image of the document strikes the surface, informational areas are discharged to form an electrostatic latent image 44 conforming to the original document image. The belt is moved in the indicated direction, passing a low level illumination member which, in this embodiment is an LED array 50 comprising a series of light emitting diodes (LEDs) used for purposes discussed more fully below.As the belt continues its movement, the belt passes through a developing station (not shown) where the latent image is developed by coating it with a finely divided electrostatically attractable powder referred to as a "toner". Thus, a toner image is produced in conformity with a light image of the document being reproduced.

Generally, the developed image is then transferred to a suitable transfer member such as paper and the image is fused. The specific mechanics for accomplishing the development transfer and fusing are not shown but are well known in the art, e. g. U. S. Patent 4,318,610.

Referring still further to Fig. 1, after lamp 34 has flashed, cavity 10 effectively acts as the light source. In addition to producing the original level of illumination at the document surface, the cavity also generates a stray light component which is reflected from the underside (and top) of the platen and is projected through the lens to discharge areas of the photoreceptor. In some systems, this stray light can be compensated for by increasing the amount of charge deposited on belt 14 by corona device 42 or by applying an anti-reflection coating to the underside and top of platen 22. However, a remaining problem is that the face of lens 32 is seen as a black disc in the image reflected from the bottom of the platen.That is, if viewed through the lens from a point on the photoreceptor surface, an inversely located cavity would be seen with a black disc at the upper "surface" corresponding to the lens location. This black disc is projected onto the surface of belt 14 forming a generally circular shadowed image area 66 approximately 1/2 the size of the actual lens (at 1:1 magnification) and having a higher charge level than desired. The edges of the image would be slightly out of focus. Image area 66 would thus be developed as an information area superimposed on either an actual image area or an area otherwise white (noninformation) area. The developed "black hole" image would then be transferred to the recording paper resulting in an objectionable output copy. This is especially conspicuous if it happens to fall in a large light grey image area.

According to a first aspect of the invention, the area on the belt upon which the black disc image of the lens is projected is predetermined for a particular magnification mode. A programmable "erase" component such as LED array 50 is then positioned at an appropriate position above the belt and energized so as to reduce the charge level of that area of the sheet. Fig. 1 shows a portion of belt 14 with LED array 50 positioned above the belt at a point subsequent to (downstream) from the exposure zone. The array 50, comprised of a plurality of separately addressable LEDs 50I, 502, SON is actuated by application of suitable control potentials from control circuit 64.The general operation and construction of these arrays is well known and described for example, in ''Electronic Design" September 27, 1966 at page 67 et seq. and in U. S. Patent 3,967,893. Assuming a magnification of 1:1 has been selected, the precise location of image 66 has been predetermined and control circuit 64 has been programmed to separately actuate those LEDs required to direct radiation onto the disc area 66 as it passes beneath the array in the indicated direction. Thus, referring to Fig. 1, as the right hand edge of the disc begins to move beneath array 49 the central LEDs are initially activated. Further LEDs are actuated until the maximum dimension of the disc (roughly the diameter) is beneath the array. The diodes are subsequently and sequentially inactivated as the left side of the disc passes beneath the array.Thus, the surface area of disc 66 undergoes a charge reduction sufficient to compensate for the black disc imaging. As shown, LED array 50 extends over a significant portion of the belt width and can be addressed to discharge a disc area projected onto a different portion of the belt (for a magnification change) or a disc with different dimensions (again dependent on magnification). Array 50 can also be located upstream from the exposure zone so as to discharge the disc area prior to exposure.

The negative image of the disc can be erased by other mechanisms than the LED array. For example, as shown in Fig. 2, a mask 70 or series of masks having an aperture 72 generally conforming to disc image 66 can be introduced between a light source 74 and belt 14 in the same location occupied by the LED array in Fig. 1. In this case, the light shown is pulsed at the precise point that the negative disc image underlies the mask aperture.

Alternatively, mask 70 can be adapted to have an aperture whose dimensions can be changed. For this technique the mask aperture would initially be completely closed until the negative disc image begins to pass beneath.

The mask aperture would then progressively open from some central point outward and then progressively close again, simulating the action of the programmable LED array of Fig.

1. For this embodiment lamp 74 can be maintained in the energized state.

According to another embodiment of the present invention, a projected image of the black disc can be projected onto the photoreceptor during the actual image exposure. This embodiment is enabled when a transparent photoreceptor belt 80 is used as shown in Fig. 3. As shown, Fig. 3 shows a perspective view of the belt at the exposure station. A mask 82 with a disc aperture 84 therein is positioned beneath transparent photoreceptor belt 80. Lamp 86 is adapted to be energized at the same time and for the same duration as the document illumination lamp 34. This embodiment, as in the Fig. 2 embodiment, permits the illumination source to be positioned upstream or downstream from the exposure zone. Changes in disc image size and location can be similarly compensated for. For this embodiment, additional light may be required by lamp 34 to compensate for the substrate optical density.

The charge reduction device embodiments shown in Figs. 1-3 constituted separate illumination devices which required independent energization. According to a second aspect of this invention, the charge neutralization light can be derived from light generated from within the optical cavity. Figs. 4 and 5 show a modification of optical cavity 12 which accomplishes this end.

Referring to Fig. 4, there is shown a first embodiment of an optical cavity modified to produce and direct a light component onto the disc image area 66.

Referring to Fig. 4, the area on the belt upon which the black disc image of the lens is projected is predetermined for a particular magnification mode. An optical component is mounted in the floor 28 of cavity 12 and arranged so as to direct a portion of the stray light from within the housing onto image area 66 so as to reduce the charge level at that area. A light pipe 90 is mounted at a preferential angle so as to direct illumination onto area 66 at a time coinciding with the image exposure. The length and diameter of the pipe are dictated by the size of image area 66 and the amount of light needed for exposure. A typical construction for pipe 90 would consist of a hollow cylinder 92, containing a centrally located pinhole at each end. The length of the cylinder and diameter of the first (object side) pinhole dictate the irradiance distribution at the photoreceptor.The diameter of the second (image side) pinhole controls the exposure level. The two pinholes, in combination, control both the shape and absolute level of the exposure.

Cylinder 92 is mounted on lens carriage 94 and is adapted to move with the lens during magnification changes so as to maintain the proper position for desired charge level reduction. Reflector cap 96 may be added to avoid introduction of document reflected light into the cylinder.

Fig. 5 shows a second embodiment of a modified cavity wherein the optical component is a positive projection lens 100 movably mounted adjacent lens 32. Lens 100 is used in conjunction with transparency 102 positioned above the lens. Transparency 102 is formed with transmission characteristics containing an inverse image of the stray light pattern. This pattern is then projected onto area 66 producing a "soft" image discharge. Lens 100 can be a wide angle lens designed to provide stray light control beyond the dimensions of area 66. As in Fig. 4, a reflector cap 104 is added to prevent light reflected from the document from being projected by the lens.

While either light pipe 90 or lens 100 can be located in any appropriate location on the housing floor for a unity magnification system, a multi-magnification system, for example, a reduction system, requires that lens 32 move towards the image plane in a three-dimensional path so as to maintain proper image registration. The reflected image of the lens thus changes position on the belt requiring that light pipe 90 or lens 100 be movable so as to direct light onto the new "black hole" location. This movement can be accomplished by mounting the component on the lens carriage as described above. A camming arrangement (not shown) may also be desirable to translate lens movement into a simultaneous repositioning of the optical component so that the light being directed (by pipe 90) or projected (by lens 100) falls on the new location.

While specific embodiments on the invention have been described, other changes or modifications are possible consistent with the principles of the present invention. For example, for the embodiments shown in Figs. 1-3, a laser scan system or alternative illumination source, may be used to provide the required negative image. Also, while the various embodiments are directed toward solving the lens "black hole" problem, the photoreceptor may suffer from stray light non-uniformities in areas other than the lens image. Therefore, more than one separate illumination source may be required for the Figs. 1-3 embodiments or more than one optical component may be placed in the cavity floor to compensate for a more complex stray light pattern on the photoreceptor.

Claims (11)

1. A flash illumination and optical imaging apparatus for producing, on a photoreceptor belt member, images of documents on a transparent object plane, the apparatus comprising, a document illumination housing positioned adjacent said object plane, a flash lamp positioned within said housing and adapted to be pulsed to provide a generally uniform level of illumination at said object plane, a projection lens mounted in the surface of said housing opposite said object plane, said lens being adapted to project an image of a document onto said photosensitive belt member, and supplementary illumination means adapted to illuminate an area of said belt upon which, during operation of the flash lamp, a shadow of said lens is formed by reflection from the object plane.

2. The apparatus of claim 1 wherein said supplementary illumination means is positioned adjacent said belt at a position downstream from the exposure station.

3. The apparatus of claim 1 or claim 2 wherein said supplementary illumination means comprises an LED array containing a plurality of separately addressable LEDs and control means to activate and deactivate said LEDs so as to irradiate said shadowed area of the belt.

4. The apparatus of claim 1 or claim 2 wherein said supplementary illumination means comprises a light source and a mask having an aperture conforming to the shape of said shadowed area, said mask being located between said light source and said photoreceptor belt.

5. The apparatus of claim 1 wherein said belt is transparent and wherein said supplementary illumination means is positioned on the non-image forming surface of the belt, and wherein an optical mask having an aperture' conforming to said shadowed area is positioned between said supplementary illumination means and said non-image forming surface of the belt.

6. The apparatus of claim 1 wherein said supplementary illumination means comprises: at least one optical means for directing a portion of the illumination produced by said flash lamp within said housing onto said shadowed area of said belt member.

7. The apparatus of claim 6 wherein said optical means is a light pipe positioned in said surface of the housing opposite the object plane.

8. The apparatus of claim 6 wherein said optical means is a second projection lens positioned in said surface of the housing opposite the object plane.

9. The apparatus of any one of claims 6 to 8, wherein said optical means is repositionable on changes in magnification mode of operation so that light emitted by said optical means is redirected towards a new location for said shadowed area of the belt.

10. The apparatus of any one of claims 6 to 9 wherein at least two optical means direct portions of the illumination produced within the housing onto separate areas of the belt member.

11. An apparatus substantially as hereinbefore described with reference to the accompanying drawings.