Processor chemistry control strip reader and replenishment system

Abstract: A voltage set point system is employed to provide a constant illumination of a photographic test strip (10). The set voltage is used as the supply for a voltage divider comparator network (63, 64, 66) for accurately determining exposed film density levels.

Description: This invention relates to a method and to a system for controlling the disposal and/or replenishment of developer in a photographic film processor. During continuous use of a developer solution for developing latent images in silver halide photographic film, the composition of the solution undergoes change as a result of several factors, including: (1) consumption of developing agents and oxidation-inhibiting compounds, (2) entry of silver halide ions into the solution, and (3) reaction of the solution with oxygen in the atmosphere. The relative amounts of developing agents in the solution become reduced by chemical reactions with exposed silver halide and by inevitable removal of developer liquid together with the photographic film segments as they leave the solution. The rate at which the developer solution becomes exhausted in the absence of replenishment depends on various factors, including: (1) temperature, (2) the extent of agitation of the solution, (3) the percent of developed (exposed or fogged) silver halide in the individual photographic film segments, and (4) the type of processed photographic film. In the machine processing of silver halide film materials, eg graphic arts materials, automatic replenishment systems are commonly used for effecting automatically-controlled addition of incremental amounts of one or more replenisher solutions to the developer solutions at appropriate times in order to maintain the development conditions within prescribed tolerances. The activity of a developer solution can be periodically assessed by using the solution to develop latent images of sensitometric control strips and subsequently comparing optical density characteristics thereof with analogous data pertaining to one or more reference control strips produced from an identical latent control strip image using a developer of a chosen standard activity level as benchmark (e.g. see U.S. Patent No. 4,081,280 and U.S. Patent No. 4,642,276 both of which are incorporated herein by reference). Relative changes in the chemical composition of a developer solution during film processing can be measured by comparing the distance (hereinafter referred to as the density range) separating particular image density values on a test control strip, with the distance separating the same image density values on a reference control strip. Typical comparative image values on a reference control strip are, for example, 0.30 and 3.5. The extent of exhaustion of developer chemistry can be assessed by comparing the location of specified optical density values measured along a reference control strip with the location of corresponding optical density values along a developer test control strip. The difference between said locations can be quantified and then be used to control the extent of automatic replenishment of developer chemistry. To facilitate these location measurements, a calibrated linear distance scale comprising regularly-spaced lines (commonly called "tick marks") runs parallel with and alongside the developed images of the reference and test control strips. These tick marks can be counted by a photodetector in a system for making automated distance measurements. The control strips can also be provided with a distinct form of mark to permit automatic detection of the positioning of the control strip in the control strip reading device. Such a position locating mark can be, for example, in the form of a notch which can be detected by mechanical means, or a spot which can be detected by means of a photodetector. It is a common problem in the prior art that the above-described density measurements are often adversely affected by unwanted variations in the intensity of the light being employed to project through the film onto calibrated photodetectors. Also temperature changes of the photodetectors can cause changes in the "dark current" over time and can be a serious problem, since it is often impractical to require the photodetectors to be used in a constant temperature environment. These effects can lead to improper chemistry replenishment and inconsistent film development quality. Patent specification US-A-4,128,325 describes a method of controlling the disposal and/or the replenishment of developer in a photographic film processor, comprising the steps of: projecting light along a path from a source to a detector and producing a detector signal; developing a test control piece in the developer; inserting the developed piece in the light path; and determining whether to dispose of the developer and/or determining whether to replenish the developer in dependence upon a current value of the detector signal with the developed piece inserted; The present invention is concerned with providing a method and apparatus which can obtain an accurate indication of the developing activity of a photographic developer, said indication being substantially independent of the temperature of the photodetector and substantially uninfluenced by large changes in the intensity of the test light source. The method according to one aspect of the present invention is characterised with respect to the method of US-A-4,128,325 by the steps of: using the detector signal in a feedback loop to regulate the intensity of light emitted by the source prior to insertion of the developed piece; holding the value of the detector signal in the feedback loop upon insertion of the developed piece; and making said determination in dependence upon the held value of the detector signal in addition to the current value of the detector signal. According to another aspect of the present invention, there is provided a system for controlling the disposal and/or replenishment of developer in a photographic film processor, comprising: a light source for projecting light along a path; a detector for detecting the light in the path and producing a detector signal; means for holding a developed test control piece in the light path between the source and the detector; and means for determining whether to dispose of the developer and/or for determining whether to replenish the developer in dependence upon a current value of the detector signal with the developed piece inserted in the light path; characterised by: a feedback loop for regulating the intensity of light emitted by the source; means for holding the value of the detector signal in the feedback loop upon insertion of the developed piece; and said determining means being operable to make said determination in dependence upon the held value of the detector signal in addition to the current value of the detector signal. A specific embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a system designed in accordance with the principles of the instant invention; Figure 2 is an enlarged depiction of the portion of the system relating to the processing of the control strip; Figure 3 depicts a test control strip as it might appear during a typical reading; Figure 4 shows a typical arrangement of a photodetector array used in the system. Referring now to the drawing, when the apparatus is on standby, a light source 18 in a calibration circuit projects onto an array of photodetectors 40, 42, 44 and 46 contained in detector system 16. An electrical output signal of detector system 16 proportioned to the light levels sensed by the photodetectors is transmitted along wire 30 to amplifier 32. The amplified output signal is transmitted along wire 34 to a "sample-and-hold" network 36 which, corresponding to the intensity of light source 18, generates and maintains a substantially constant reference voltage V ref which in turn energizes a voltage divider network consisting of resistors 63, 64 and 66. A typical value for V ref for applicant's preferred embodiment is 10 volts. Should there be a variation in light source intensity detected because of, for example, dust on the source lamp, an inverse feedback amplifier 74 monitoring output signal in wire 52 via wire 68 adjusts the light intensity via an output signal transmitted along wire 76 to maintain a voltage V′ near said reference voltage V ref . The rate at which the developer solution depletes in the absence of replenishment depends on a variety of factors, including: (1) how much film has been processed, (2) the reaction of the solution with oxygen in the air, (3) temperature, (4) agitation of the solution, and (5) the type of film developed. The practitioner then, bearing in mind the above factors, periodically inserts a freshly-developed test control strip 10 into the calibration unit to determine (within prescribed tolerances) the consistency of the developer chemistry and effect automatic replenishment as necessary. A set of drive rollers 12, 14 is used to feed the test control strip 10 between the light source 18 and the photodetectors 40, 42 and 44 contained in detector system 16 for making density measurements and concurrently between light source 18 and photodetector 46 for "tick mark" counting. As the control strip is inserted into the calibration unit, but before passing between the light source and photodetectors, it trips a sensor 48 which sends an output signal via wire 50 to the "sample and hold" network 36 to freeze the voltage of signal 52 at its present value V′ near reference voltage V ref . The "sample and hold" network 36 will maintain a substantially constant output signal equal to V′ on wire 52 and ignore further output signal changes on wire 34 as long as said output signal in wire 50 is present indicating a control strip is being read. A 20 volt power supply represented by 38 should adequately maintain the voltage V′. In this manner, one may be sure that the calibration system is reading in a like manner as it was just prior to the time that the test control strip was read when no film was present between light source 18 and detectors 40, 42, 44 and 46. Film rollers 12 and 14 advance said control strip into detector 16 and a first tick mark is read as it crosses between light source 18 and photodetector 46. A signal 78 is sent to CPU 80 to begin counting tick marks. The frozen voltage V′ is applied via wire 52 to the top of said voltage ratio network comprising resistors 63, 64 and 66. Resistors 63, 64 and 66 of said voltage ratio network are chosen such that a voltage V₁ at the junction of 63 and 64 equals the voltage on wire 34 when the optical density of the reference control strip equals 0.3. Said voltage input V₁ to comparator 58 is taken from wire 56 at the junction between 63 and 64 in said voltage ratio network. A second input to comparator 58 reads the amplified output signal transmitted along wire 34 representing the optical density of the test control strip as film rollers 12 and 14 continue to advance the freshly developed test control strip between the light source 18 and photodetectors 40, 42, 44 and 46. When the amplified electrical signal along wire 34 produces a voltage level equal to the 0.3 reference density level equivalent V₁, comparator 58 sends an output signal along wire 70 to CPU 80 to record the current number of tick mark counts as representing the 0.3 optical density on the test control strip 10. Said tick mark counts are received by CPU 80 along wire 78. Similarly, resistors 63, 64 and 66 are also chosen such that a voltage V₂ at the junction of resistor 64 and resistor 66 equals the voltage on wire 34 when the optical density of the reference control strip equals 3.5. Said voltage input V₂ to comparator 60 is taken from wire 62 at the junction between 64 and 66 in said voltage ratio network. A second input to comparator 60 reads the amplified output signal transmitted along wire 34 representing the optical density of the test control strip. In due course the control strip advances to the point where the amplified output signal along wire 34 produces a voltage V₂ at the input to comparator 60 that is equivalent to the 3.5 reference density voltage level V₂ along wire 62. An output signal from comparator 60 is sent via wire 72 to CPU 80 indicative of this second milestone being reached. The tick mark count is noted via the signal along wire 78 and the test control strip is reversed to again measure its 3.5 and 0.3 density equivalent level voltages. The number of tick marks between said voltages on the return trip is averaged with the number counted on the forward trip and this average number or gradient for the test control strip is then compared with the number of tick marks stored in CPU 80 representing the original gradient between the 0.3 and 3.5 density values on the reference control strip; this comparison can then be used to determine the quality of the developer chemistry. The gradient values thus determined may be plotted along the ordinate of a graph vs. time along the abscissa. The gradients normally form lines of roughly zero slope. A sharp downward slope in a gradient line represents a degradation in the developer chemistry and the entire batch should be discarded and replaced. The deviation in the number of tick marks measured for the 3.5 optical density level of the test control strip relative to the number of tick marks stored in CPU 80 for the reference control strip indicates the strength or weakness of developer activity and triggers automatic replenishment of the developer chemistry via an output signal along wire 82 as needed.

Claims: A method of controlling the disposal and/or the replenishment of developer in a photographic film processor, comprising the steps of: projecting light along a path from a source (18) to a detector (16) and producing a detector signal; developing a test control piece (10) in the developer; inserting the developed piece in the light path; and determining whether to dispose of the developer and/or determining whether to replenish the developer in dependence upon a current value (34) of the detector signal with the developed piece inserted; characterised by the steps of: using the detector signal in a feedback loop (68, 74, 76) to regulate the intensity of light emitted by the source prior to insertion of the developed piece; holding the value of the detector signal in the feedback loop upon insertion of the developed piece; and making said determination in dependence upon the held value (V ref ) of the detector signal in addition to the current value of the detector signal. A method as claimed in claim 1, wherein said determination is made in dependence upon at least one comparison between the current value of the detector signal and a respective predetermined proportion of the held value of the detector signal. A method as claimed in claim 1, wherein the test control piece is in the form of strip and, when developed, has an optical density which varies along the length of the strip, the inserting step comprising feeding the developed piece across the light path, the method further comprising detecting the position of the developed piece across the light path, and said determination being made in dependence upon at least one detected position of the developed piece when the current value of the detector signal is equal to a respective predetermined proportion of the held value of the detector signal. A system for controlling the disposal and/or replenishment of developer in a photographic film processor, comprising: a light source (18) for projecting light along a path; a detector (16) for detecting the light in the path and producing a detector signal; means (12, 14) for holding a developed test control piece (10) in the light path between the source and the detector; and means (58, 60, 63, 64, 66, 80) for determining whether to dispose of the developer and/or for determining whether to replenish the developer in dependence upon a current value of the detector signal with the developed piece inserted in the light path; characterised by: a feedback loop (68, 74, 76) for regulating the intensity of light emitted by the source; means (36) for holding the value of the detector signal in the feedback loop upon insertion of the developed piece; and said determining means being operable to make said determination in dependence upon the held value (V ref ) of the detector signal in addition to the current value (34) of the detector signal. A system as claimed in claim 4, wherein said determining means includes at least one comparator (58, 60) for comparing the current value (34) of the detector signal and a respective predetermined proportion (V1, V2) of the held value of the detector signal. A system as claimed in claim 4 for use with a test control piece in the form of a strip which, when developed, has an optical density which varies along the length of the strip, wherein the holding means is operable to feed the developed piece across the light path, the system further comprising means (46) for detecting the position of the developed test piece across the light path, and the determining means being operable to make said determination in dependence upon at least one position detected by the position detecting means when the current value of the light detector signal is equal to a respective predetermined proportion of the held value of the light detector signal.

Inventors: GRIFFIN CHARLES DEWITT, GRIFFIN, CHARLES DEWITT
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1990
Main classification: G03C5
IPC codes: G03C5, G03D3