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The present invention relates to systems for transferring electronic image
data to film images and, more particularly, to a system for producing a
film image which when displayed will correspond in brightness and contrast
to a video image.
In multi-format camera (MFC) systems presently used in transferring
electronic image data to film images, the image to be transferred is
generally previewed by an operator viewing a television monitor. The
operator may select from the monitor certain views which are to be
coverted to permanent images on film. In these systems the image data is
usually manipulated to control the intensity of the image as presented on
both the video monitor and the film. For example, MFC systems are widely
used in medical diagnostic systems for transferring data obtained by x-ray
imaging, magnetic resonance imaging and other diagnostic techniques to
film.
It is desirable and expected by the operator that the image captured on
film be substantially identical to that observed on the monitor, i.e., the
image intensity and contrast on the monitor should match that of the film
backlit by a lightbox. The intensity and contrast of the film displayed on
the lightbox depends on the intensity of the backlighting and the optical
density of the film at any given point. Any deviations or differences
between the two images may effect the ability of a physician to accurately
diagnose medical conditions.
The regulation of film density is described in U.S. patent application Ser.
No. 761,439 now U.S. Pat. No. 4,700,058, filed Aug. 1, 1985 and assigned
to the assignee of the present invention. The text of that application is
hereby incorporated by reference. In essence, that application provides a
method and apparatus for producing uniform film images from electronic
data. The factors which influence the conversion of electronic data to
film image are primarily variations in the intensity of the film writing
light source and variations in film processing due to contamination and
changes of chemical used in the processing. The inventive system of the
aforementioned U.S. patent application utilizes electronic controls for
periodically calibrating the film writing device and for adjusting the
image data to maintain desired film density constant over time. The film
density is compared to a set of intensity values which represent a desired
intensity or film density. Deviations in measured film density when
compared to the desired values are corrected by adjusting the "intensity"
represented by the electronic image data. Accordingly, it is desireable to
provide a set of intensity values representative of the values to be
reproduced on film. The intensity values may represent grey scale values
or the familiar red, green and blue color scale values.
SUMMARY OF THE INVENTION
Among the objects of the present invention is the provision of a method and
apparatus for establishing a set of electronic intensity values for
controlling film density in a system for converting or transferring
electronic image data to film images. This and other objects are attained
in a system in which electronic image data is converted to a video image
on a television monitor. Apparatus is provided for measuring the intensity
values appearing on the monitor under desired conditions and for storing
the measured intensity values as a set of calibration standards. A film is
prepared in a film writing system using the same electronic image data and
the resultant intensity values on the film compared to the calibration
standards. Any deviations determined by the comparison are corrected by
adjusting the intensity values of the electronic image data coupled to the
film writing system. In an illustrative embodiment, the image intensity on
the monitor is determined by a portable apparatus including a photocell
for producing an electrical signal representative of light intensity or
brightness. The photocell signal is appropriately scaled and stored. The
stored values are transferred to a calibration data storage device in the
electronic image transfer and conversion system for use in calibrating
film images. In another arrangement, the apparatus may also be used to
directly measure film density for comparison against the data obtained
from the monitor. The measurement of film density may require measurement
while the film is backlit by a lightbox.
DESCRIPTION OF THE DRAWINGS
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 block diagram of a system for transferring
electronic data to film images using the present invention;
FIG. 2 is a diagram of one method for obtaining film density values; and
FIG. 3 is one form of apparatus according to the present invention for
obtaining video intensity and film density values for use in the system of
FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
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 with which the present invention is
particularly useful. The overall system function is to convert image data
supplied from electronic source 10 into viewable images on either positive
or negative film in film processor 12. The image data is generally
supplied from a storage medium. It is desireable that the data converted
to an image on film accurately represent the image data and also
correspond to a monitored video image on video monitor 14. It is also
important that the image represented on the film be of a high quality, 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 a constant relationship between image data and film
density over time, and yet provide for a minimum amount of setup and
adjustment time.
The image data from source 10 may be either in analog or digital form. A
display controller 16 converts the data to a standard format for
application to television type monitor 14. Controller 16 also includes
circuitry for selectively switching to either presentation of image data
or presentation of calibration or test data from block 18. The test data
may comprise, for example, a set of grey scale values formatted to create
a sensitometric strip 20 on monitor 14 and on the processed film. Such a
strip 20 is described in the aforementioned U.S. patent application Ser.
No. 761,439 now U.S. Pat. No. 4,700,058. Alternately, the test data may
provide a plurality of different color sensitometric strips such as, for
example, red, green and blue scale strips represented by 20A, 20B and 20C.
For color applications, the strips 20 may be generated simultaneously or
sequentially. The formatted data from controller 16 is coupled to a
controllable gain block 22 which may be appropriately characterized as a
transfer device having a predetermined transfer function for converting an
input signal V.sub.in to an appropriately corresponding output signal
V.sub.o.
The output signal developed by the block 22 is connected through an
electronic switch 24 to a further block 26. The block 26 represents a
second amplifier which also has an adjustable gain and, in addition, is
capable of having zero offset which is variable.
The signal produced by the block 26 is coupled to appropriate input
terminals of a film writing device 28 which may be a cathode ray tube
(CRT) or laser, both of which are well known in the art. As is well known,
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. A laser
writing source could be advantageous in a color image system since laser
light is monochromatic and would enable color writing without filtering
although requiring at least three writing sources. While a color CRT would
permit simultaneous writing of all colors, the resolution is generally not
as good as multiple CRT's or lasers.
Continuing with the block diagram, there is shown a final block 12 which
represents the film developing and processing in order to develop the
final film image. As will be appreciated, the ouput signal indicated as
produced by block 28 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
are also non-linear and such characteristics are well known in the film
processing art.
The control system includes two feedback loops, a first of which monitors
the light intensity from the film writing device 28 and provides a
feedback signal to the variable gain amplifier 26 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
22 which adjusts the magnitude of the image data signal supplied to the
block 26 in order to drive the film density in a direction to correct for
any deviations from a desired density. In the operation of the system,
there is provided a calibration mode in which the initial values of light
intensity output from the film writing device 28 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 28, the switch 24 is placed in a
position such that a voltage reference signal V.sub.r is applied as an
input signal to the amplifier 26. 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 30 with a feedback signal
representative of write intensity or light output of the film writing
device 28, 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 set of reference data values, i.e., test
data, as an input signal to the block 22 while monitoring the density of a
film produced at block 12. The feedback loop provides signals indicative
of film density which are then compared or summed in summing block 32 with
a set of desired film density signals provided by calibration data block
34.
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 exposure are read to determine whether the write
intensities must be adjusted such that the desired film densities are
achieved. In order to accomplish these functions, the write intensity
output of the CRT or laser device 28 is 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 28 and the
calibrated light output of the writing device 28 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 28 is first
passed through a calibrated sensitometric block 36 which is mounted
adjacent the film writing device 28 in a position abutting the point at
which film is inserted to be exposed to the light from the film writing
device 28. In the calibration mode the film is placed in position so that
the light beam passes through both the sensitometric block 36 and a film
38. The film 38 also includes an exposed sensitometric strip 40 normally
positioned along an unused edge of the film. The sensitometric strip is
created by the test data signals from block 18 applied to the film writing
device 28 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, the regulated
light beam from the writing device 28 is scanned in a vertical direction
along the edge of the film strip 38 first passing through each step of the
sensitometric block 36 and then through each of the intensity blocks on
the exposed sensitometric strip 40. A photodetector 42 located behind the
film strip 38 senses the light from the film writing device 28 which
passes through the sensitometric block 36 and sensitometric strip 40 and
provides output signals proportional to the magnitude of the light
impinging on it. The photodetector 42 preferably comprises a single
photodetector which scans in synchronism with the regulated light beam
from the film writing device 28 so as to receive the light beam as it
scans down the edge of the film 38. The output signals developed by the
photodetector 42 are proportional to the amount of light passing through
each section of the sensitometric block 36 and sensitometric strip 40.
Since the sensitometric block 36 is a highly calibrated block, the initial
portion of the scan is used to calibrate the photodetector 42. As the
light beam thereafter passes through the sensitometric strip 40, the
photodetector 42 will provide output signals representative of the density
of the image on the film 38. It should be noted that the image forming the
exposed sensitometric strip 40 is created by a step voltage. The use of a
step voltage to produce the sensitometric strip 4 is well known in the
art. Thus, it will be seen that by using the calibrated light output from
the film writing device 28, film density can be controlled by periodically
creating a sensitometric strip 40 on an edge of a film 38 and inserting
the developed film in front of the writing device to allow the
photodetector 42 to compensate the amplifier 22 for any deviations in film
density from the desired value. Although the disclosed method of
calibration of film density is through use of the regulated light beam
from the film writing device 28, it will be appreciated that an external
calibrated light beam could be used for reading the film densities. For
example, the film density could be determined by analysis in a lightbox
such as is indicated at 41 in which the film 38 is displayed. The film
density could be read using the photodetector 42 of FIG. 2. However, an
improved method will be described with regard to FIG. 3.
By performing the calibration sequence described above, it can be seen that
the transfer device 22 can be adjusted so as to modify image data in a
manner to create a desired film density.
In the case of a look-up table, i.e., a digital memory system for
processing digital image data, the transfer function of device 22 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 cability 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 28 can be
accomodated. However, the use of image data in an analog form provides a
much simpler approach since the transfer device 22 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.
A detailed implementation of the system of FIG. 2, with the exception of
the development of the calibration data signals, is shown in U.S. patent
application Ser. No. 761,439, now U.S. Pat. No. 4,700,058 and will not be
repeated herein. Turning now to FIG. 3, there is shown a simplified block
diagram of one form of apparatus for obtaining calibration data signals.
As previously described, it is desirable to generate an image on film
which, when displayed on a lightbox, has the same intensities as an image
on an electronic video monitor. The apparatus 44 of FIG. 3 is designed to
be a portable light meter which can "read" intensity and convert the read
intensity to a form suitable for use as calibration data. A light detector
such as a photo-diode 46 is positioned adjacent an open end of a hollow
cylinder 48, which cylinder 48 has a second end connected to a light
impervious box 50. The open end of cylinder 48 is also provided with a
hinged cover 52 for blocking light to the photo-diode 46 to permit dark
current auto zero set. A push-button 54 protrudes from the open end of
cylinder 48 and is connected to a switch (not shown) which actuates
storage of the signal produced by photo-diode 46.
Within the box 50 there is provided a power source indicated as a battery
56 for supplying power to a plurality of electronic circuits. The circuits
include a pre-amplifier 58 for amplifying the signal from the photo-diode
46, an auto-zero circuit 60 for setting a dark-current level, an analog to
digital converter 62 for converting the analog output signals from
photo-diode 46 into digital data signals, a digital memory 64 for storing
the digital data signals and an RS-232 serial link interface 66 for
transferring data from the memory 64 to an external storage device such as
that shown at 34 in FIG. 1. The connection to the interface 66 may be by
means of a three terminal phone plug 68. Each of the circuits within the
box 50 are commercially available circuits and their function and
operation well known. The unique combination of these circuits provides a
portable apparatus capable of reading light intensity levels, converting
those levels to a digital signal and storing the digital signals for later
transfer to a utilizing system.
In the use of the calibration apparatus 44, the end of the cylinder 48 is
pressed against a face of the monitor 14 adjacent the sensitometric strips
20. The intensity of the scale value "seen" by the photodiode 46 is
transferred to the digital memory 64. The apparatus 44 is then
sequentially positioned to read each of the other scale values in a like
fashion so that a complete set of calibration data is generated. The
system is then caused to write the same scale values on film 38 and the
film is developed and placed in lightbox 41. The apparatus 44 is used to
read the intensities of the light through the scale 20 on film 38 in the
same manner as was used to read the intensities on monitor 14. The data
obtained by the film reading can then be compared with the data from the
monitor reading and any difference used to adjust the amplifier 22. In
essence, the data provided by apparatus 44 in reading film 38 becomes the
feedback data applied to summing circuit 32.
It will be appreciated that what has been disclosed is a system for
developing a set of calibration data which can be used to assure a match
between an electronic image on a video monitor and a hard copy
reproduction of that image on film. The disclosed system achieves this
result by transferring appropriate intensity information from the viewing
monitor to the film in a calibration process. While the invention has been
disclosed in what is presently considered to be a preferred embodiment,
many variations and modifications will become apparent to those skilled in
the art. It is intended therefore that the invention be given the full
spirit and scope of the appended claims.
* * * * *
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