Color Film Writing System With Closed Loop Control for Electronic Images

The present invention relates to systems for transferring electronic data to film images and, more particularly, to a system for producing uniform grey scale and color film images from electronic images.

Currently available film writing systems for transferring electronic data to film images are subject to variations in their light output which give rise to variations in film density. Such film density variations may effect the ability to discern certain detail in a developed film image. For example, in medical film imaging, such variations may impede a physician's ability to accurately diagnose medical problems using film analysis. The problems of film density variation are also aggravated by variations in the film itself and in the film processing. Since film processing involves a chemical treatment and reaction, the chemicals become contaminated with repeated use resulting in film density variations as a function of time and use.

Compensation for film density variations in the currently available processing systems have relied on trial and error techniques. Typically, a known reference image signal is provided to the system and a film exposed and developed. An operator then evaluates the image density on the film and adjusts the gain of the system in a direction to compensate for any deviations in film density from a desired norm. This process is repeated until a satisfactory film density is attained. Unfortunately, instabilities in the control system for the light source and changes in the film processing result in early deterioration of film quality thus requiring a repeated alignment procedure by trial and error adjustment. Furthermore, the prior art systems have not addressed the need for color image film processing.

It is an object of the present invention to provide an improved system for converting electronic images to color film images.

It is another object of the present invention to provide an improved system for converting electronic images to color film images which automatically compensates for variations in light output.

It is a further object of the present invention to provide an improved system for converting electronic images to color film images which includes compensation for variations in film density.

In accordance with one embodiment of the present invention an electronic film writing system and a film processing system are combined into a control system including a first control loop to regulate light output of the film writing device and a second control loop to compensate for variations in developed film density. In the film writing system a light source such as a cathode ray tube (CRT) or laser provides a light output which can be used to impress an image on a film. If the light source is not color controllable, e.g., a black-and-white CRT, light filtering may be used for each of a plurality of colors using either one or more light sources. Each light source is driven by a variable gain, adjustable offset amplifier at a level to produce sufficient light to properly expose a film for each color. In order to maintain the light output at a constant regulated level, a light sensor is connected in a light path adjacent each light source and provides a control signal representative of light intensity. The control signal is applied to the amplifer associated with each light source as a feedback signal to adjust the drive signals supplied to the light source in a direction to compensate for deviations from a desired or set level.

The film density is controlled by a second feedback loop which incorporates a film density measuring system to provide a density signal representative of film density. The density signal is applied as a gain varying and offset signal to control the magnitude of video image data signals coupled to the driving amplifier for each film writing device. For each color component used to create the color image, there will be a corresponding set of image data signals and a corresponding feedback loop to adjust the magnitude of the signals. Each of the control system feedback loops are synchronized to selected cycles of operation of the system in order to avoid mutural interference. In an alternate embodiment, the density control loops are incorporated in the intensity control loop.

For a better understanding of the present invention, reference may be had to the following detailed description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a simplified functional block diagram of a control system for controlling constant film density over time;

FIG. 2 is a simplified diagram of a film density metering system;

FIG. 3 is a functional block diagram of a feedback control system for controlling write intensity output;

FIG. 4 is a functional block diagram of an alternate embodiment of the system of FIG. 3 in which the density control function is incorporated in the write intensity feedback control loop;

FIG. 5 is a block diagram of an automatic density control apparatus including an alternate form of calibration film holder cassette for use with the embodiment of FIG. 4;

FIG. 6 is an illustration of minimum and maximum video intensity calibration signals;

FIG. 7 is a simplified block diagram of the system of FIG. 1 modified for processing color film; and

FIG. 8 is a modification of FIG. 2 for color film.

Referring now to FIG. 1, there is shown a simplified block diagram of an electronic image processing system including a film processing system incorporating a control system in accordance with the present invention. The overall system function is to convert electronic image data supplied to the system at point 10 into viewable images on either positive or negative film at point 12. In effecting this result, it is necessary that the data converted to an image on film accurately represent the image data supplied at point 10. It is also important that the image represented on the film be of a high quality such as, for example, to enable accurate diagnosis of any medical problem appearing on the film. In achieving this last function, the conversion system must be capable of maintaining uniform film density and yet provide for a minimum amount of setup and adjustment time. The electronic data may be obtained from any of several sources such as, for example, medical diagnostic equipment such as x-ray systems or magnetic resonance systems.

The image data supplied at point 10 may be either in analog or digital form. If the image data is supplied as an analog signal, the block 14 may comprise a variable gain linear or piecewise linear amplifier. In the case of image data supplied in digital form, the block 14 may comprise a digital memory device and associated processing circuitry to enable the image data to act as an address or a pointer to identify particular values stored in memory in what is well known in the art as a look-up table. The block 14 may appropriately be characterized as a transfer device having a predetermined transfer function for converting the input signal V.sub.IN to an appropriately corresponding output signal V.sub.O.

The output signal developed by the block 14 is connected through an electronic switch 16 to a further block 18. The block 18 represents a second amplifier which also has an adjustable gain and in addition, is capable of having a zero offset which is variable in response to a control signal. As can be seen from the graph of the voltage input to block 18 labelled V.sub.O and the voltage output labelled V.sub.D, the amplifier 18 provides an additional gain and offset for the V.sub.O signal.

The V.sub.D signal produced by the block 18 is coupled to appropriate input terminals of a film writing device which may be a cathode ray tube (CRT) or laser, both of which are well known in the art. The CRT may be part of a monochrome monitor, i.e., a black and white television viewer, or may comprise a color monitor. As will be appreciated by the shape of the graph shown in the block 20, the CRT or laser characteristic tends to be non-linear with respect to the input drive signal, i.e., the brightness of the generated light beam is a non-linear function of the amplitude of the input drive signal.

Continuing with the block diagram, there is shown a final block 22 which represents the film developing and processing in order to develop the final film image at point 12. As will be appreciated, the output signal indicated as produced by block 20 is in reality the light output of the CRT or laser which is used to expose the film prior to developing. The effect of any given writing intensity, I, is to produce a film density, D, on the exposed film. The film density versus exposure intensity characteristics shown in the block 22 is the well known characteristic curve of the film and the processing system.

In accordance with the present invention, the control system includes two feedback loops, a first of which monitors the light intensity from the film writing device 20 and provides a feedback signal to the variable gain amplifier 18 in order to maintain the light output of the film writing device at a desired level. The second feedback control loop monitors the density of the exposed and developed film and provides a feedback signal to the variable gain amplifier block 14 which adjusts the magnitude of the image data signal supplied to the block 18 in order to drive the film density in a direction to correct for any deviations from a desired density. As will be appreciated, there is provided a calibration mode in which the initial values of light intensity output from the film writing device may be established followed by a subsequent step of providing a reference data signal which allows the film density to be adjusted to a desired value. In the calibration of the film writing device 20, the switch 16 is placed in a position such that a voltage reference signal V.sub.R ' is applied as an input signal to the amplifier 18. Obviously, the use of the V.sub.R ' signal is to assure that a known reference is applied for calibration purposes. A signal I.sub.R, which is summed in summing block 24 with a feedback signal representative of write intensity or light output of the film writing device 20, is simply an adjustment signal which can be varied in order to adjust the light output to a desired level. Similarly, film density is established by providing a known reference signal V.sub.R as an input signal to the block 14 while monitoring the density of a film produced at point 12. The feedback loop provides signals indicative of film density which are then summed in summing block 26 with a variable signal D.sub.R which can be adjusted to obtain the desired film density at point 12.

It will be apparent that the closed loop control of film density requires that the film first be exposed with a known scale of write intensities and thereafter developed. Subsequently, the densities produced on the film in response to the scaled exposure must be read to determine whether the write intensities must be adjusted such that the desired film densities are achieved. In order to accomplish these functions, it is necessary that the write intensity output of the CRT or laser device 20 be adjusted to provide an initial known light intensity. Once the film has been exposed and then developed, it can be returned to the writing point in front of the device 20 and the calibrated light output of the writing device 20 can be used to read the actual film density. One form of a reading process is illustrated in FIG. 2. As is shown the regulated light beam from the writing device 20 is first passed through a calibrated sensitometric block 28 of a type well known in the art. The sensitometric block 28 is mounted adjacent the film writing device 20 in a position abutting the point at which film is inserted to be exposed to the light from the film writing device 20. In the calibration mode the film is placed in position so that the light beam passes through both the sensitometric block 28 and the film 30. The film 30 also includes an exposed sensitometric strip 32 normally positioned along an unused edge of the film. The sensitometric strip is created by the calibration drive signals applied to the film writing device 20 during the calibration cycle. Creation of a sensitometric strip on an unused edge of film is well known in the art and will not be described in further detail. During the calibration process of the present invention, the regulated light beam from the writing device 20 is scanned in a vertical direction along the edge of the film strip 30 first passing through each step of the sensitometric block 28 and then through each of the scal blocks on the exposed sensitometric strip 32. A photodetector 34 located behind the film strip 30 senses the light from the film writing device 20 which passes through the sensitometric block 28 and sensitometric strip 32 and provides output signals proportional to the magnitude of the light impinging on it. The photodetector 34 preferably comprises a single photodetector which scans in synchronism with the regulated light beam from the film writing device 20 so as to receive the light beam as it scans down the edge of the film 30. The output signals developed by the photodetector 34 are proportional to the amount of light passing through each section of the sensitometric block 28 and sensitometric strip 32. Since the sensitometric block 28 is a highly calibrated block, the initial portion of the scan is to be used to calibrate the photodetector 34. As the light beam thereafter passes through the sensitometric strip 32, the photodetector 34 will provide output signals representative of the density of the image on the film 3. It should be noted that the image forming the exposed sensitometric strip 32 is created by a step voltage V.sub.R applied to the first amplifier 14 of FIG. 1. The use of a step voltage V.sub.R to produce the sensitometric strip 32 is well known in art. Thus, it will be seen that by using the calibrated light output from the film writing device 20, film density can be controlled by periodically creating a sensitometric strip 32 on an edge of a film 30 and inserting the developed film in front of the writing device to allow the photodetectors 34 to compensate the amplifier 14 for any deviations in film density from the desired value. Although the preferred method of calibration of film density is through use of the regulated light beam from the film writing device 20, it will be appreciated that an external calibrated light beam could be used for reading the film densities.

By performing the calibration sequence described above, it will be appreciated that the data supplied to the transfer device 14 can be used to establish a table of film density as a function of the reference voltage V.sub.R. In the case of a look-up table, i.e., a digital memory system for processing digital image data, the transfer function will be modified to provide an output voltage for any given input voltage command necessary to produce the desired film densities. In the look-up table format, the system has a greater capability of compensating for non-linear variations in the overall image conversion process. The look-up table can be constructed to have as many reference density settings as there are reference input voltage levels. By so doing, film gamma correction and/or corrections for the non-linearities of the film writing device 20 can be accommodated. However, the use of image data in an analog form provides a much simpler approach since the transfer device 14 may comprise a linear operational amplifier circuit with variable offset and variable gain. The gain and offset can then be adjusted so that the minimum and maximum film density signals produce the desired result and the other signals falling between the minimum and maximum levels would then produce an output signal varying linearly or piecewise linearly between the minimum and maximum values.

The above description has dealt generally with a feedback control system for regulating film density. Where the received image data represents information for color film images, the data will be supplied in the form of intensity information for each color component of the film image. Typically, such data represents the red, green and blue (RGB) color components since these colors can be used to form all others. Accordingly, data for each color must be processed and the transfer device 14 therefore represents at least three controllable devices.

In addition, the film will incorporate a sensitometric strip 32 for each primary color for which data is received and the feedback signals from reading the film density on each strip 32 will be provided to a corresponding one of the devices 14. While time sharing of the readout of strip 32 by photodetector 34 is possible, by providing a photodetector 34 for each strip, the design of the feedback system can be simplified.

Since there are three sets of color image data in an RGB system, the amplifier 18 is necessarily adapted to handle three sets of data, either on a time share basis or by being comprised of three separate amplifiers. If the display device 20 is a color monitor, three amplifiers are normally provided. If device 20 is a monochrome monitor, only one amplifier 18 need be provided since the data is supplied in a time sequential manner. However, with a monochrome monitor, color is obtained by placing a filter between the light source 20 and the film during exposure, the color of the filter corresponding to the light beam color represented by the data being processed during exposure.

Another alternative is to provide a monochrome monitor for each color desired and to sequentially position the film before each monitor. It will be appreciated that this arrangement will also require the use of color filters; however, the filters can be fixed in placed rather than sequentially used as in the arrangement using only one monitor. In order to expose the film to all three monitors, either the film can be moved or a lensing arrangement utilized to focus light from each monitor onto the film at a common location. The lensing arrangement has the advantage of allowing either simultaneous or sequential exposure. Since no film or filter movement is involved, time delays between sequential exposures are minimized.

Turning briefly to FIG. 7, the above mentioned system using three monchrome monitors 20A, 20B and 20C is shown. Image data are provided to a red, green, blue separator 11 of a type well known in the art which provides separated red, green and blue (RGB) data. The RGB data is supplied to respective RGB transfer devices 14A, 14B and 14C. The relative color component intensity is controlled by a feedback loop for each color. Each of the feedback loops is identical to that described above except being limited to a single color. The adjusted intensity data is supplied to corresponding amplifiers 18A, 18B and 18C, each of which drives one of the monitors 20A, 20B and 20C. The monitors expose the film which is then developed as indicated by block 22. During a calibration cycle, the sensitometric blocks 32A, 32B and 32C (see FIG. 8) are used to provide the feedback data to control blocks 14A, 14B and 14C. The intensity control loops for the monitors 20 are the same as is shown in FIG. 1 and are omitted for simplicity in FIG. 7.

If a color monitor is used for device 20, only one device 20 will be required and the signals from the amplifiers 18A, 18B and 18C can be used to drive the different color electron guns of the device 20. Of course, with a color monitor, the RGB filters can be eliminated. If a single monochrome monitor is used for device 20, only one amplifier 18 may be required for device 20, only one amplifier 18 may be required for light source intensity control as is shown in FIG. 1. However, three devices 14 will still be required to provide time sequential signals for each color to the device 20. As is well known, lasers produce monochromatic light. Thus, if a laser is used as a light source, the color filters may be eliminated.

As will be realized from the above description, converting a monochrome system to a color system involves the provision of one of the devices 14 for each color component of the images along with a feedback loop for each color component. Depending on the selection of either a monochrome or color monitor or multiple monochrome monitors, the amplifier 18 and display device 20 may be multiple or single units. While use of a laser for light source 20 will require multiple units just as for the monochrome monitor, the color filters can be eliminated. Accordingly, the following description will be directed to a specific implementation using only a single color, e.g., black and white. The extension of the description to multiple colors will be apparent from the above disclosure. The simplest extension, from a system understanding approach, is to provide one of the systems disclosed in FIGS. 3-5 for each color (e.g., red, green and blue) for which data is to be processed. Modifications to simplify that arrangement will be apparent from the above description.

In FIG. 3, there is shown in more detail the calibration and control loops for regulating the writing intensity of writing device 20. Before beginning the detailed description, it might be noted that the image data supplied at terminal 10 is in the form of a video signal having a typical television format. In particular, the image data is of a type which can be displayed by application to a typical television video monitor. Referring briefly to FIG. 6, there is shown a typical video signal having synchronizing (sync) pulses, back porch reference, video black level reference, and video signal. The upper waveform differs from the lower waveform only in the amplitude of the video signal. Since the illustrative signals are used for calibration, the black cal signal is of smaller amplitude than the white cal signal. As can be seen in FIG. 3, the write intensity feedback loop actually comprises two separate loops, one for detecting the brightest output of the writing device 20 and one for detecting the minimum brightness of the writing device 20. For purposes of this description, it will be assumed that the writing device 20 comprises a CRT. As will be understood, the calibration of the output of the writing device 20 can be done on the periodic basis between film exposures. In order to achieve this function, the image data is coupled into a sync pulse separator 36 which detects the synchronizing pulses contained in the image data signal and provides appropriate timing signals to various sections of the calibration and feedback control loops to insure that the calibration cycles occur at appropriate times. Such synchronizing circuits are well known in the video processing art and will not be described herein.

Referring first to the black level control loop which establishes the intensity of the minimum light output of the writing device 20, there is provided a photosensor 38 positioned adjacent the CRT 20 which provides an output signal to a black sample gate 40. The output signal from the photodetector 38 is proportional to the light output of the CRT 20. However, during the time that the output of the black sensor 38 is sampled, the system is set to produce a minimum grey scale or minimum light output signal at the point adjacent the photodetector 38. For example, the detector 38 typically samples only a single scan line on the face of the CRT at any one instant. Consequently, the gate 40 need only synchronized sampling to a time at which a scan line appears in front of detector 38. The black sample gate 40 is an electronic sampling gate of a type well known in the art and may comprise for example, a gated digital register or an FET switch connected to supply the sampled signal to a storage capacitor. The signal sampled by the black sample gate 40 is applied to a comparator 42 in which it is compared to a black reference signal established by a black reference potentiometer 44. By adjusting the potentiometer 44, the black level or minimum grey scale value of the CRT 20 can be adjusted. The comparator 42 produces an error signal which is applied to a black sample storage curcuit 46, which circuit 46 may include a capacitor storage circuit having sufficient capability to hold a particular voltage level signal at a substantially constant value between calibration times. The circuit 46 acts as a proportional plus integral circuit whose output value is incremented or decremented by the signal from comparator 42 in order to generate an appropriate black level control signal. It will be noted that both the black sample gate 40 and the black sample store circuit 46 are operated by a gating signal supplied by an auto-density control circuit 48. The control circuit 48 is in turn provided with a synchronizing signal from the sync separator 36. The control circuit 48 provides the appropriate reference signal V.sub.R ' for driving the CRT 20 during the calibrating cycle and also provides the timing signals to allow the black sample gate 40 and black sample store circuit 46 to sample the signal from the photodetector 38 at the appropriate time. The control circuit 48 also supplies the signals for the white sample control loop which controls the maximum write intensity of the writing device 20.

Continuing with the black level control loop, the black sample signal store in circuit 46 is coupled through an amplifier 50 to a clamp circuit 52. The clamp circuit 52 is driven also by the sync separator 36 which provides a control signal to the black control amplifier 54 at a predetermined time. More particularly, the clamp 52 assures that the black level for the video image data is set only when the minimum black video signal is present. The adjustment of amplifiers for driving cathode ray tubes and in particular the setting of black levels is well known in the television art and will not be described herein.

The setting of the maximum intensity level of light output of the writing device 20 is very similar to the setting of the black level. A white sensor 56 is positioned adjacent the face of the CRT 20 and provides an output signal to a white sample gate 58 which is clocked by the control circuit 48 to sample the output (maximum white level) of the phototransistor white sensor 56 at the appropriate time. The sampled output of the white sensor 56 is compared with a white reference signal in a comparator 60. The resulting signal from comparator 60 is coupled to a white sample store circuit 62 which is also gated by the control circuit 48 at appropriate times. An output signal developed by the white sample store circuit 62 is coupled through an amplifier 64 to an automatic contrast control circuit 66. The auto contrast control circuit 66 may be of the type typically used in television receivers. In essence, the auto contrast control 66 is a variable gain amplifer which adjusts the amplitude of the image data applied to it in a manner to compensate for any deviations from the desired maximum intensity at the CRT 20. The video amplifier 68 couples the video and calibration signals to the CRT and is similarly a standard video amplifier of a type well known in the art.

Although the black level, actually the minimum desired grey scale, is set first, it will be appreciated that the setting or adjusting procedure must be repeated after the white level is set since the white adjustment controls the gain of the amplifier 66 which will vary the amplitude of the entire video signal including that portion corresponding to video black which is used to clamp the black level. Accordingly, the control circuit 48 is adapted to reiterate the black level set procdure after the white level is set.

It will be noted that a sensitometer generator 70 produces the aforementioned calibrated sensitometer output signal utilized to generate the sensitometric strip 32 on the film 30 as shown in FIG. 2. The sensitometer generator 70 is also provided with a synchronizing signal from the sync separator 36. A density calibration circuit 72 which is manually activated by a density calibration input signal from an operator, is utilized to activate a switch 74 during the density calibration mode. The density calibration circuit 72 provides a signal to cause a switch 74 to switch from the video image terminal 10 to a sensitometer generator terminal 76. During the density calibration cycle, the auto density circuit 48 opens the switch 16 to disconnect the image data to allow the density calibration circuit 72 to supply a known reference signal to the write control system. The signal from density calibration circuit 72 is supplied through a summing junction 78 to an input terminal of the auto contrast control circuit 66. The auto density circuit 48 also has control of the electronic switch 80 to move it to bypass the auto contrast control circuit 66 during the black level restoration process. As will be appreciated, the auto contrast control would attempt to compensate for low light levels if the switch 80 were left connected to the contrast control circuit 66 during initial calibration. During the reiterated black level calibration steps, the switch 80 is left in the position such that the video signal passes through the amplifier 66 so that the black level clamp voltage includes the gain of amplifier 66 to compensate for the gain shift of the actual grey level above the back porch level.

Although the photodetectors or sensors 38 and 56 have been illustrated as two separate sensors, it will be appreciated that a single sensor could be utilized by sequential reading of black and white levels. In the system in which both sensors are used, the black and white sensors can be read in parallel by appropriately controlling the drive signals to the CRT so that white or maximum brightness is attained adjacent the white level sensor and black or minimum brightness is attained adjacent the black or grey level sensor. In either case, the auto density circuit 48 provides a synchronized signal to the sample gates 40 and 58 to latch the data obtained from the black and white sensors. The sampled black level signal from gate 40 is compared in the comparator 42 with the reference signal on a repetitive cycle until the desired black level is achieved. The white sample circuit 62 is then operated to adjust the white level signal until the white level reaches a desired magnitude. The black level can then be re-checked and the process iterated until both white and black levels are at the desired level or until the error signals produces by the comparators 42 and 60 are sufficiently small.

Once the write intensity of the film writing device 20 has been set to a calibrated level, the film density loop can be activated to permit the gain of the first transfer device 14 to be adjusted to a level to compensate for any deviations from desired film density. It should be noted that calibration of the write intensity output of the film writing device 20 does not require that film be exposed. A shutter can be provided and kept closed during the write intensity calibration process.

FIG. 4 is another embodiment of the present invention in which the film density control loop is incorporated in the write intensity control loop. Specifically, the black sample store circuit 46 and the white sample store circuit 62 are each replaced by a density control circuit 82 and 84, respectively. The circuits 82 and 84 function in the same manner as the circuits 46 and 62 but additionally include a direct current (DC) level shift capability which is responsive to a density control signal for shifting the DC level of their respective output signals. The black level control loop is responsive to a minimum density control signal while the white level control loop is responsive to a maximum density control signal.

In order to clarify the operation of the system of FIG. 4, reference is now made to FIG. 5 in which there is shown a block diagram of apparatus for generating the minimum and maximum density control signals. The block 86 labeled auto-density calibration controller is a part of the block 48 of the system block diagram. The block 86 is described separately here inasmuch as the prior description has already disclosed the various other functions which signals from the auto-density control circuit 48 is labeled as an auto-brightness controller in FIG. 4 even though it is essentially the same circuit 48 as described with respect to FIG. 3, the auto-brightness label is applied here since the circuit functions in conjunction with the brightness or intensity control loop for the CRT 20. As shown in FIG. 5, the auto-density calibration command which is generally a manual operation command initiates the auto-density calibration cycle. The manual initiation is required since the calibration cycle requires that an exposed calibration cassette such as that illustrated at 88 or an exposed portion of a film be placed in a position to be analyzed by the photo detector associated with the system. The calibration cassette 88 is an alternate embodiment of the previously described calibration film strip using the sensitometer strips and calibrated sensitometer blocks. In the calibration cassette, two reference film densities 90 and 92, are provided to enable the photo detector 94 to become calibrated to particular maximum density and minimum density levels represented by the two reference levels. The photo detector positioner 96 which may be a motor driven positioner is mechanically coupled to the photo detector 94 and arranged to position it at locations wherein light passing through the maximum and minimum film density reference blocks 90 and 92 can be sampled by the photo detector 94. Actuation of the detector position 96 is by a signal coupled from the auto density calibration controller 86. Operation of the detector positioner may be either in a timed sequence or there may be provided feedback signals from the detector 96 to identify the particular location of the detector as it moves the photo detector 94 by the reference film strips. The photo detector also is driven past the sensitometer strip 98 which is the same type of sensitometer strip described previously with regard to FIG. 2. More particularly, the sensitometer strip is exposed along an edge of an x-ray film or calibration cassette by light emitted from the CRT or laser device 20. Thus, as the photo detector passes along the sensitometer strip 98, it can pick out those particular blocks within that strip which correspond to the desired maximum and minimum density values represented by the blocks 90 and 92. It will be appreciated of course, that the photo detector does not actually do the comparison but does provide the signals which are coupled through an amplifier 100 to the circuitry which actually compares the maximum and minimum density values to the values developed along the edge of the film in the sensitometer strip 98.

The signals developed by the amplifier 100 are coupled through an analog to digital converter 102 to a reference register 104 and a sample register 106. The values of the maximum and minimum density strips 90 and 92 are stored in the reference register 104 while the sample register 106 is used to store the values in digital form of each of the scaled density areas in the sensitometer strip 98. Clock signals provided from the auto density calibration controller 86 are used to clock the reference register 104 and sample register 106 to both allow the sample values to be stored and also to further clock the values from the registers into a digital comparator 108.

The results of the digital comparison in comparator 108 are coupled through AND gates 110 and 112 to respective minimum density register 114 and maximum density register 116. The AND gates 110 and 112 are gated or selected by signals supplied from the auto density calibration controller 86. Obviously, if the comparator 108 is searching for a minimum density match, then the results of that comparison should be stored in the minimum register 114. Likewise, a maximum density match would be stored in the maximum density register 116. Thus, the controller 86 provides select signals to each of the AND circuits 110 and 112 in order to gate the appropriate AND circuits to enable the proper register 114 or 116 to receive the step code information from controller 86. The minimum density register 114 has its output connected to a look-up table 118 which is used to convert the density step code to an appropriate gain control or DC offset value, in this instance a minimum density control value, which is supplied to the black level density control block 82 in FIG. 4. Similarly, the density step code from register 116 is also applied to a look-up table 120 in which it is converted to a maximum density control signal supplied to the density control block 84 in FIG. 4.

It will be apparent that the various control signals from the auto density calibration controller are necessary in order to synchronize the operation of the photo detector 94 and to appropriately compare each of the grey scale values sequentially in the sensitometer strip 98 to the appropriate maximum or minimum reference value 90 or 92 in order to determine the appropriate density control value to be applied to the circuits 82 and 84.

The automatic brightness control of the present invention may be initiated automatically or by command from an x-ray machine operator. The brightness calibration sequence corrects for any change in brightness and/or offsets which are due to component changes with time and temperature. When the automatic brightness command is received, the controller first opens the switch 16 to isolate the video input signal from the control system. The shutter between the CRT and the film is normally closed at this point to prevent exposing the film during the brightness calibration sequence. The auto brightness circuit then changes the position of switch 80 and begins the black level set procedure by generating a black sample signal at the same time that the calibration pulses are supplied to the black control circuit 54. Changing the position of the switch 80 by-passes the automatic contrast control or variable gain amplifier 66 which would defeat the initial setting of the black level control. After the black level control loop has provided a satisfactory minimum black loop error out of the comparator 42, the switch 80 is released back to its normal position and the white calibration signal and white sample pulses are generated to initiate the white level calibration sequence. When the white level loop error is determined to be at a desireable minimum level, the controller goes back to repeat the black loop setting with the auto contrast control 66 in the system. As was mentioned previously, this is necessary since the gain of the auto contrast control 66 will affect the level of the minimum video or black level signal. The system will reiterate the black and white level control loop calibration cycles until the loop error in each loop is within a minimum range. In the embodiment shown in FIG. 4, the maximum density and minimum density control signals are set at zero during the brightness control calibration cycles since it is necessary to isolate the density control offset values during the brightness calibration cycles.

In order to automatically calibrate the system for film density variations, it is necessary to process a film using appropriate calibration signals as described above to generate the sensitometry strips necessary to allow the system to match desired reference values with actual measure values. In the preferred emodiment illustrated in FIG. 5, the film is processed and developed and then placed in the special calibration cassette holder 88. The calibration cassette 88 with the test film is loaded in the system and an auto density calibration command is activated to begin the auto density calibration process. The auto density controller 86 illuminates the test film with flat white light from the CRT. The detector 94 is positioned to read the minimum density reference shown in the film strip 92 and the value of that reference is then temporarily stored in the reference register 104. The detector is then moved down to view the exposed film sample strip 98 to search for an equivalent density level on the test film. When the comparator 108 detects a match, the controller outputs a step code which is stored in the register 114. The minimum density step code is a code value which is available in the auto density calibration controller 86 as it positions the photo sensor 97 in search of the step on film strip 98 which matches the reference density value. More particularly, the film density strip 98 is generated by a sequence of step voltage values which are incremented across the CRT during the calibration cycle. These values expose the film creating a sequence of steps varying from below the minimum reference density value to above the maximum reference density value. The controlled sequences the step values in the order they appear on the film strip. Accordingly, when the comparator detects a match between a particular step value or film density value and the reference value, the controller then picks from its memory the particular voltage or density value which created that particular density value on the film. That value is then stored in the minimum density register. Similarly, once the minimum density value has been found, the photo detector then moves into position to store the value associated with the maximum density reference at strip 90 and then moves down to monitor each of the values in the film strip 98 until a corresponding maximum density value is isolated. When the comparator 108 provides a matched signal to the controller 86, the controller then provides a step code to the maximum density step register corresponding to that particular value of film density. The register then outputs that value to a look-up table which provides the appropriate gain control signal to the white level density control loop.

The sensitometer signal generator 70 generates a conventional stair step signal which is identical to the normal input video in amplitude and set up. This signal is used to expose a piece of film as a periodic verification of the performance of the system. The process of generating the stair step voltage and providing a sensitometer strip are well known in the x-ray film art.

It should now be appreciated that this invention provides a method and apparatus for automatically controlling the processing of electronic images to film images with compensation for variations in the light output of a film writing device and variations in the film and film processing device.