METHOD AND APPARATUS FOR ILLUMINATING A DOCUMENT TO BE PHOTOGRAPHED

Abstract: Method and apparatus for facilitating the sitting of the ratio between front and back illumination of a document (6) to be photographed to optimise the definition on the eventual image (microfilm) obtained. The overall brightness of the illuminated image can be set to an optimum level determined by an operator and adjustment of the ratio between front and back illumination can be optimised without changing the set level of the overall brightness. Following setting of the ratio, the exposure can be made (possibly at a different brightness level) without affecting the preset ratio.

Description: METHODANDAPPARATUSFORILLUMINATINGA DOCUMENTTOBEPHOTOGRAPHED Technical Field This invention relates to a method of, and apparatus for controlling the ratio between top and bottom lighting in the recording of document images e.g. on microfilm. Background Art Documents are commonly recorded on microfilm and in the case of an engineering drawing, this is commonly recorded on microfilm using a large planetary camera. Engineering draw¬ ings are produced on a wide variety of materials including comparatively opaque white paper and translucent materials such as tracing paper and diazo coated films. The original drawings which a draughtsman prepares are usually made on pre¬ printed translucent sheets and sometimes the preprinted infor¬ mation is printed on the top surface and sometimes on the bottom surface of the sheet. The draughtsman normally draws the image on the top side of the sheet. Full size contact prints may be made on iazo duplicate paper and the dye of this duplicate image may be on the top or bottom face of the diazo sheet. These variations in the original documents presented for microfilming lead to the need in a planetary camera for bottom lighting, top lighting, or a mixture of top and bottom lighting to produce the test contrast in the microfilm recordings. A planetary camera used for microfilming large documents such as engineering drawings commonly has a table with a diffusing opalescent white plastics top surface which the document to be microfilmed rests cn. A lower set of lights is commonl provided below the table surface with a control to vary the brightness of the bottom lighting from zero to the maximum the lower set of lights car. provide. A separate BAD ORIGINAL upper set of lights is provided above the document table which is independently controllable to vary the brightness of the upper set from zero to its maximum. These two independently controllable sets of lights can provide a wide range of illumination conditions for any given document. The problem arises, however, of providing a logi¬ cal set of instructions to the camera operator to ensure that the document illumination is set to the optimum for a particu lar document. The purpose of the illumination system is to provide sufficient light intensity to produce the required image and at the same time to maximise the contrast in the image, par¬ ticularly in cases where the original document contrast is low. Prior art illumination systems have either left it to th camera operator to judge when maximum contrast has been achieved by varying the proportion of top to bottom lighting while observing the document from above, or else have relied on set ratios for particular classes of documents. Neither of these prior art systems is wholly satisfactory Relying on the experience of the operator to set the ratio may be reasonable in the case of a fully experienced operator, but it is exceedingly difficult (if not impossible) to make an assessment of maximum contrast in conditions in which the brightness of the " viewed image is changing. Relying on the use of some arbitrarily preset ratio must be an approximation to a correct setting in most cases. The present invention provides an improved method and apparatus which avoids the aforementioned disadvantages. Disclosure of the Invention According to one aspect of the invention a method of controlling the ratio between top and bottom illumination of a document for imaging comprises maintaining the apparent - .5 - brightness of the document at least substantially constant while varying the proportion of top to bottom lighting. By operating in accordance with the method of the inven¬ tion, an operator can observe the apparent density of any chosen line on a document and can adj ' ust the ratio between reflected and transmitted light (i.e. top to bottom lighting) to obtain the maximum visual line density. 3y ensuring that the overall brightness of the document does not vary during the ratio adjusting operation it has been found that optimum ratio setting can be obtained by even inexperienced operators. The set level of overall brightness used during the ratio adjustment need not be that required for the exposure to make the image, provided overall brightness can be adjusted without affecting the pre-set ratio. In terms of apparatus, the invention comprises a light- transmitting document-support, first lighting means positioned to illuminate the support from one side, second lighting means positioned to illuminate the support from the opposite side, image-forming means on said one side of the support for producing a permanent image of a document placed against said support, lumen sensing means on said one side of the support to monitor the apparent brightness of a document placed against said support, a brightness ratio control means to vary the ratio between the brightness of the first and the second lighting means and a control circuit which energises the two lighting means to provide the brightness ratio set by said control means at the pre-set overall brightness determined by said lumen sensing means. The image-forming means can be a camera using photo¬ graphic film or diazo film or it could be a copying machine based cn the xerographic principle. Incandescent lamps anό/or fluorescent lamps can be used for both lighting means but incandescent lamps may be preferred for the first and flucr- BAD ORIGINA - ij - escent tubes for the second. The support can be of white translucent material and is preferably disposed horizontally so that said one side is above the support. Brief Description of Drawings The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a schematic illustration of a camera layout for operating in accordance with the method of the invention, Figure 2 is a block diagram to illustrate the mode of operation of the layout of Figure 1 during ratio setting, and Figures 3a_ and 3b_ are two alternative arrangements for the high gain feedback amplifier used in the brightness con- trol circuit of a practical embodiment as shown in Figure 1. Preferred Embodiment The camera shown in Figure 1 comprises a horizontal table 1 of white diffusing translucent material below which is located a ban ' of parallel bottom lights 2. A pair of top lights 3 illuminate the table 1 from above. A camera - and an adjacent photocell 5 are positioned to receive light reflected from and light transmitted through, a document 6 resting cn the table 1. The photocell 5 monitors the bright¬ ness of the document β as seen by the camera U . The brightness of the top lights 3 is controlled by a power control circuit K,_ fed from a potentiometer R m and the brightness of the bottom lights 2 is controlled by a power control circuit K-, fed from a potentiometer ?.-_ . these two potentiometers being ganged together and simul aneously operated by a ratio knob 7. The output from the photocell 5, which senses the lumens issuing from the document β (i.e. those reflected from the lights 3 and those trans¬ mitted from the lights 2), is fed to a brightness control circuit 8 which supplies the electrical power to the potent¬ iometers Rm and R„. The mode of operation of the apparatus shown in Figure 1 can be deduced from Figure 2. In this Figure, the voltage fed to the control circuit K is V τ and that fed to K β is V β . The designations B τ and Bg refer to the brightness of the top and bottom lights, respectively, and the designations 3V m and BV R represent the viewed brightness of the document 6 due, respectively, to the top light only and to the bottom light only. Rm and KT 3 represent, respectively, the reflec- tance coefficient of the document β in respect of the light from the lights 3 and the transmission coefficient in respect of the light from the lights 2. BV Q represents the overall document brightness as viewed by the photocell 5 and B D is a preset signal for the overall illumination required during the visual setting of the ratio knob 7- V 0 is the output from the control circuit 8 and is the voltage fed to the potentiometers R„ and R g . In practice the voltage B D can be derived from a potentiometer under operator control since the optimum overall brightness at which an operator finds it easiest to set the ratio knob 7 for maximum line density can vary slightly from operator to operator. Once the ratio ^ /V- has been set for a particular document, V- can be altered to that required for optimum imaging in the camera -4 (e.g. in the case of very fast silver halide film in the camera a reduction in Y Q may be desirable during exposure to put the latter within the shutter speed range of the camera) or when using a very slow film process an increase in Y~. may be desirable to minimise exposure time and maximise the production of microfilm images) and this alteration of Q can be effected automatically (using the output of the photocell 5) or manually. BAD ORIGINAL (ft Mode of Operation The mode of operation of the equipment showr. in Figure 1 will be clear from the above description and it will be apprec iated that the operator, having set the overall brightness level most suited to him/her, will adjust the knob 7 to optimise the apparent density of lines on the document to be photographed, and will then (using any conventional technique to set the brightness for exposure) record the image in the camera at the appropriate brightness for exposure. Whatever brightness setting is used for the exposure it will retain the ratio of top to bottom lighting he/she had originally set, and will thus secure optimum clarity of the eventual image. The control circuit 8 in Figures 1 and 2, should be able to generate the signal V- from very small differences between the brightness demanded 3 Q and the brightness viewed overall BV Q . A suitable circuit is shown in Figure 3a_ where a direct negative feedback high gain amplifier is shown. The inputs BV 0 and B D are fed to the input of a high gain amplifier 10 via resistors R* j _ and P^a a variable feed back resistor R- being used to control the gain. This circuit gives a very rapid response of V Q to any signal 5 " Q -B Q . Figure 3b_ shows an integrating amplifier,in which the high gain amplifier 10 is placed in parallel with a capacitor C. This has the advantage of providing a slow response of V Q to changes in BV Q -3 D and makes for a stable control arrange ment in the face of lamps which change their brightness relatively slowly compared to the change in power fed to them. BAD ORIGINAL

Claims: CLAIMS 1. A method of controlling the ratio between top and bottom illumination of a document for imaging which comprises maintaining the apparent brightness of the document at least substantially constant while varying the proportion of top to bottom lighting. 2. A method as claimed in claim 1, in which following an adjustment of the said ratio the set level of overall brightness used during the ratio adjustment is adjusted to that required for the exposure to make the image without affecting the pre-set ratio. 3. A method of controlling the ratio between top and bottom illumination of a document for microfilming substan¬ tially as hereinbefore described with reference to and as illustrated in, the accompanying drawings. - . Apparatus for producing a permanent image of a docu¬ ment comprising a light-transmitting document-support, first lighting means positioned to illuminate the support from one side, second lighting means positioned to illuminate the support from the opposite side, image-forming means on said one side of the support for producing a permanent image of a document placed against said support, lumen sensing means on said one side of the support to monitor the apparent brightness of a document placed against said support, a brightness ratio control means to vary the ratio between the brightness cf the first and the second lighting means and a control circuit which energises the two lighting means tc provide the brightness ratio set by said control means at the pre-set overall brightness determined by said lumen sensing means. 5. Apparatus as claimed in claim b 3 in which means is provided tc permit the overall brightness employed for sett¬ ing of the brightness ratio, ro be at " BAD ORIGINAL $ - 3 - of said ratio, when the image forming means is to be used to produce a permanent image. 6. Apparatus as claimed in claim 4 or claim 5 , in which the document-support is a sheet of white translucent material. 7. Apparatus as claimed in claim 6, in which the sheet is disposed horizontally. 8. Apparatus as claimed in any of claims 4 to 7, in whic the image-forming means is a camera. 9. Apparatus as claimed in any of claims to 8, in which the first lighting means is one or more incandescent lamps and the second lighting means is a blank of fluorescent tubes. 10. Apparatus as claimed in any of claims 4 to 9, in which the lumen sensing means is a photocell disposed adjac¬ ent to the image-forming means. 11. Apparatus as claimed in any of claims 4 to 10, in which the brightness ratio control means comprises a manually adjustable ganged pair of potentiometers. 12. Apparatus as claimed in claim 11, in which the control circuit receives an input from the lumen sensing means and supplies an output to each of the ganged pair of potentiometers. 13. Apparatus as claimed in claim 12, in which the control circuit includes an integrating amplifier. 1-4. A planetary camera for microfilming documents substantially as hereinbefore described with reference to, and as illustrated in, Figures 1 and 2 of the accompanying drawings or Figures 1, 2 and 3a or Figures 1, 2 and 3b_ of the accompanying drawings /"If OMPI

Inventors: SEIB M
Applicants: SEIB LTD MARTIN, SEIB M, MARTIN SEIB LTD
Application year: 1981
Main classification: G03B27
IPC codes: G03B27