PHOTOGRAPHIC FILM PROCESSING

Abstract: In known film processing techniques, the amount by which a film is processed at each stage is determined chiefly by time. If a film is either over- or underexposed, it may be incorrectly processed producing unsatisfactory results. The present invention utilises an arrangement which measures changes in the infrared density of the film during processing to ensure that satisfactory results are produced. The arrangement comprises an infrared light emitting diode (22) and an infrared sensitive photodiode detector (26) which are both mounted in a support (20). Film (34) passes between the diode (22) and the detector (26) so that the amount of infrared radiation being transmitted through the film can be determined to provide a measure of the infrared density of the film at each stage during its processing.

Description: PHOTOGRAPHIC FILM PROCESSING This invention relates to photographic film processing, and is more particularly concerned with the control of such processing. In standard film processing techniques, the exposed film is developed, bleached and then fixed prior to printing. At each stage of the processing technique, the amount of developing, bleaching, or fixing tends to be determined by time. This can lead to incorrect processing of films which are either over- or under—exposed thereby producing unsatisfactory results. It is Known that during film processing, the levels of silver and/or silver halide present in the film change in each of the development, bleaching and fixing stages. It is an object of the present invention to utilise means for measuring such changes in the silver and/or silver halide levels to control the processing process. According to one aspect of the present invention, there is provided film processing apparatus comprising a plurality of film processing stations, each station executing a predetermined step in the processing operation, characterized in that each station includes infra red monitoring means for determining the infra red density of the film. By this arrangement, optimum processing of an exposed film can be achieved. Advantageously, the monitoring means comprises an infra red source and an infra red detector. In a preferred embodiment, the infra red light source is an infra red light emitting diode, and the infra red detector is an infra red sensitive photodiode. Preferably, the source and detector are spectrally matched, and operate on a wavelength of around 950nm. In a preferred embodiment of the invention, a computer is used for sensing the output of the detector, and for controlling switching of the film being processed from one station to another in response to that output. Control strips of known exposure can be processed in a similar way to film having unknown exposures in order to monitor and control the process , activity. For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawing in which:— Figure 1 illustrates a schematic block diagram of film processing apparatus for use with a device according to the invention; and Figure 2 illustrates an embodiment of an infra red monitoring device used in the apparatus of Figure 1. It is known that the quantity of silver or silver halide in a film can be measured using infra red techniques, the infra red density being a function of the quantity of silver or silver halide present. An unprocessed film consists mainly of silver halide and has a predetermined infra red density value. This value of infra red density will not vary substantially from film to film and is independent of the latent image present on the film itself. Referring to Figure 1, film processing apparatus is shown in block diagram form. The apparatus comprises a developing station 10, a bleaching station 12, and a fixing station 14. The exposed film is processed by passing it through each station 10, 12, and 14, in turn before passing it to a photographic printer (not shown). The time for which the film is at each one of the stations in the processing apparatus is determined using a device as shown in Figure 2. Figure 2 shows a device for sensing the infra red density of a film and can be used at any one of the three processing stations mentioned above. The device comprises a support 20 which carries an infra red light emitting diode (LED) 22, and an infra red photodiode detector 26. The LED 22 and detector 26 are sealed in respective transparent plastics tubes 24, 28 and they are spaced apart by the support 20 as shown. Film 34 is arranged to pass close to the detector 26 so that the infra red density sensed by the amount of radiation passing from the LED 22, through the film 34, and on to the detector 26, approximates to the diffuse density of the film. The absolute value of the density is unimportant. The LED 22 is driven at a constant current from a power supply 16 (see Figure 1) by means of connections 30. The detector 26 is spectrally matched to the LED 22. The wavelength of the infra red radiation emitted by the LED 22 is around 950 nm. The detector 26, when operating in its linear short circuit current mode, produces a signal which represents transmission of infra red radiation through the film 34. The signal from the detector 26 is converted to a density value by a monolithic logarithmic amplifier (not shown) to provide an output signal which corresponds to the density value. This signal is monitored by a computer 18 (see Figure 1) through connections 32, and when it reaches a predetermined value, depending on the processing station, completes that particular processing step and allows the film to be passed on to the next station (or to the printer). Whilst the film is in the developing station 10, the silver halide present in the film is converted to metallic silver. This causes a rise in the infra red density. The rate at which this rise occurs depends on the exposure of the film. If left in the developer, the rate of rise of the infra red density would continue to rise until all the silver halide had been converted to metallic silver. However, under normal conditions, the development process is terminated after a predetermined default time, for example after 3.25 minutes as in the C41 process, and the film is passed to the bleaching station 12. if the rate of rise of the infra red density in the film is above a predetermined limit, then the film can be considered to be over—exposed. The development time is reduced to compensate for this over-exposure. If the rate of rise of the infra red density is below another predetermined limit, then the film can be considered to be under—exposed and the development time is increased to compensate. Within these two predetermined limits exists a range of infra red density values for which the default development time can be used. In the bleaching station 12, the film is bleached which causes the metallic silver to be oxidised back to silver halide. As a consequence, the infra red density returns to its original value prior to development. Once this density value has been attained, that is the infra red density has fallen to a steady low level, bleaching is completed and the film is then passed to the fixing station 14. In the fixing station 14, the film is fixed, and the silver halide is made soluble so that it can be washed out of the film. This has the result that the infra red density falls to zero. Once a zero reading is sensed, the film is then washed, stabilised and dried before being passed to the printer. Therefore, by carefully monitoring the infra red density of the film during each of its processing stages, processing can be accurately controlled to produce optimum results prior to printing. This invention can be used for any silver based film process but it is particularly applicable to cyclic film processes in which the film circulates around a fixed path until that stage of the process is complete, and the film is switched to the next stage of the process. Control strips of known exposure can be used as standards. When these strips are processed, the rate of rise of the infra red density should always be the same, and therefore the processing time should be the same for each strip. Any variation in the processing time in one or each of the stations can be used to check the process activity. Using apparatus according to the invention, the following can be monitored or detected:— (a) completion of the bleaching stage; (b) completion of the fixing stage; (c) aging of the fixer solution; (d) under— or over—exposed film to allow compensation for this in the development time to permit optimum development for the film; and (e) development rate of control strips as a check on the process activity.

Claims: CLAIMS: 1. Film processing apparatus comprising a plurality of film processing stations (10, 12, 14), each station (10, 12, 14) executing a predetermined step in the processing operation, characterized in that each station (10, 12, 14) includes infra red monitoring means (20, 22, 24, 26, 28) for determining the infra red density of the film (34). 2. Apparatus according to claim 1, wherein the monitoring means (20, 22, 24, 26, 28) comprises an infra red source (22) and an infra red detector (26). 3. Apparatus according to claim 2, wherein the infra red light source (22) is an infra red light emitting diode. 4. Apparatus according to claim 2 or 3, wherein the infra red detector (26) is an infra red sensitive photodiode. 5. Apparatus according to any one of claims 2 to 4, wherein the source (22) and detector (26) are spectrally matched, and operate on a wavelength of around 950nm. 6. Apparatus according to any one of claims 2 to 5, wherein the source (22) and detector (26) are sealed in respective transparent tubes (24, 28) and mounted in a support (20), the support (20) providing a sufficient spacing between the source (22) and the detector (26) to allow passage of the film (34) being processed. 7. Apparatus according to any one of the preceding claims further comprising a computer (18) for sensing the output of the monitoring means (20, 22, 24, 26, 28) and for controlling switching of the film (34) being processed from one station to another in response to that output. 8. Apparatus according to any one of the preceding claims, wherein control strips of known exposure are processed to monitor and control the process activity.

Inventors: SYKE DAVID ALBERT, SYKE, DAVID, ALBERT
Applicants: EASTMAN KODAK CO, KODAK LTD, EASTMAN KODAK COMPANY, KODAK LIMITED
Application year: 1990
Main classification: G03D13
IPC codes: G03D3, G03D13