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REFERENCE TO CO-PENDING APPLICATIONS
Reference is made to co-pending applications by J. Pone entitled
"Photographic Printer with Automatic Density and Color Corrections for
Paper Gamma"; by J. Pone and P. Seidel entitled "Photographic Printer with
Automatic Slope Compensation"; and by J. Pone entitled "Photographic
Printer with Interactive Color Balancing" which were filed on even date
and are assigned to the same assignee as the present application. These
co-pending applications describe photographic printing systems which may
use the exposure time control of the present invention.
BACKGROUND OF THE INVENTION
The present invention relates to photographic printing systems. In
particular, the present invention is an improved method and apparatus for
controlling exposure times.
Photographic printers produce color or black and white prints or
transparencies from photographic film originals (generally negatives).
High intensity light is passed through the film and imaged on the
photosensitive print medium (film or paper). The photographic emulsion
layer on the print paper or film is exposed and subsequently processed to
produce a print or transparency of the scene contained in the original.
In order to increase efficiency and minimize time required to fill customer
orders, high speed printers have been developed in which many exposures
are made on a single roll of print paper. After the exposures are made,
the roll is removed from the printer, is photoprocessed to produce prints,
and is cut into individual prints. The prints are then sorted by customer
order and ultimately packaged and sent to the customer.
A critical portion of a photographic printer is the exposure time control,
which controls the duration of the exposure of the photosensitive medium
in order to assure that the image on the photosensitive medium is properly
exposed. The exposure time control may utilize inputs from several
different sources in order to determine the proper duration of the
exposure. For example, most automatic printers use large area transmission
density (LATD) sensors to sample the light transmitted by the negative
either prior to or during the exposure. Control of the exposure time is
determined using a method known as "integration to grey". In addition,
many automatic printers include an automatic density correction (ADC) or
color scanning station which scans the negative prior to printing and
corrects the exposure time in the event of an abnormality in illumination
of the negative known as "subject failure". Finally, the operator may
enter density or color correction signals from the operator control panel.
Based upon some or all of these input signals, the exposure time control
determines the proper exposure time for each of the color channels or for
one black and white channel.
The significant advances in digital electronics and digital computers in
recent years has led to the development of computer control of
photographic printers. One of the functions controlled by the computer
(which has typically been a minicomputer) is the exposure time control
function. In the past, a significant amount of computing time has been
dedicated to the control of the exposure time.
SUMMARY OF THE INVENTION
The present invention is an improved exposure time control which uses a
digital processor, but which reduces the amount of processing time used by
the digital processor in controlling the exposure time. In the present
invention, the digital processor derives, from input signals, a digital
count for each channel and a clock control signal for each channel.
Storage means stores the digital counts, and variable clock means provide
clock (or interrupt) signals at rates determined by the clock control
signals. The digital processor changes the digital counts for each of the
color channels from their initial values in response to the clock signals.
The exposure time of each color channel is controlled as a function of the
time required to change each corresponding digital count from its initial
value to a predetermined final value.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a system block diagram of a photographic printer.
FIG. 2 is a block diagram of a preferred embodiment of the exposure time
control of the present invention.
FIG. 3 is a block diagram of a preferred embodiment of a portion of the
exposure time control of FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a block diagram illustrating a photographic printer. In the
printer, an image contained in film 10 is printed onto photosensitive
paper 12. Light from print lamp 14 is passed through a frame of film 10
and is focused by optics 16 onto an appropriate portion of paper 12.
The exposure time during which paper 12 is exposed to the image from film
10 is determined by the position of filter paddles and shutter 18. The
filter paddles typically include a subtractive filter for each color
channel (red, blue, and green). Filter paddles and a shutter 18 are
controlled by exposure time control 20.
In the embodiment shown in FIG. 1, exposure time control 20 receives input
signals from LATD sensors 22, from density or color sensors 24, and from
operator control panel 26. Not all of these sources of input signals are
required in every system, and similarly, other sources of input signals
which affect the exposure time may be used in the printer.
FIG. 2 shows a preferred embodiment of the exposure time control of the
present invention. The control system includes a digital processor 28,
which is preferably a microprocessor such as the Intel 8080A. Analog
signals from the LATD sensors 22 and the sensors 24 are multiplexed by
multiplexer 30. The analog signals are converted to digital signals by A/D
converter 32, and supplied as input signals to microprocessor 28. Input
signals from the operator control panel 26 are also received by
microprocessor 28. In one preferred embodiment, the red, green, and blue
signals from the LATD sensors are each converted by A/D converter 32 into
12-bit digital signals, as are the signals from density or color sensors
24 for individually measured segments of the film.
Based upon the input signals from A/D converter 32 and operator control
panel 26, microprocessor 28 generates a digital count and a clock control
signal for each color channel. The red, green, and blue digital counts are
each stored in random access memory (RAM) 34. The clock control signals
are preferably digital numbers n.sub.R, n.sub.G, and n.sub.B, which are
supplied to variable clocks 36a, 36b, and 36c.
The clock signals from variable clock 36a-36c are generated at rates
determined by the value of n.sub.R, n.sub.G, and n.sub.B. The output
signals of clocks 36a-36c are supplied to multilevel interrupt circuit 38,
which supplies interrupt signals to microprocessor 28 as a function of the
clock signals.
Each time microprocessor 28 receives an interrupt signal, it changes the
appropriate count in random access memory 34. When the count of a
particular color channel has been decremented to zero, microprocessor 28
energizes the appropriate one of the filter paddle solenoids 39, thereby
terminating the exposure in that color channel.
FIG. 3 shows a more detailed block diagram of one preferred embodiment of a
portion of the exposure time control. In this embodiment, read only memory
(ROM) 40 is provided in addition to random access memory 34. Connected to
the data/address/control bus 42 are two multiport I/O circuits 44a and
44b. Divide-by-n counters 46a and 46b receive numbers n.sub.R and n.sub.G
from multiport I/O circuit 44a, while divide-by-n counter 46c receives
number n.sub.B from multiport I/O circuit 44b. Counters 46a, 46b, and 46c,
together with common time base generator 48 perform the functions of
variable clocks 36a, 36b and 36c, respectively, shown in FIG. 2.
The carry outputs of counters 46a, 46b and 46c are supplied to interrupt
enable circuits 50a, 50b and 50c. The outputs of interrupt enable circuits
50a-50c are supplied to multilevel interrupt circuit 38, which supplies
interrupt signals to microprocessor 28.
Multiport I/O circuit 44b also provides three outputs which are supplied
through buffers 52a, 52b and 52c to solenoid drivers 54. Filter paddle
solenoids 39 are driven by solenoid drivers 54.
In a preferred embodiment of the present invention, the three digital
counts supplied to RAM 34 are in the range of 50-99. The numbers n.sub.R,
n.sub.G, and n.sub.B, which control the rate at which interrupt signals
are supplied, are in the range of 0 to 256, and common time base generator
48 provides a time base of 1 millisecond. The interval between interrupts
for a particular color channel, therefore, can range from 1 to 256
milliseconds, depending on the value of n.sub.R, n.sub.G, or n.sub.B.
Each time multilevel interrupt 38 supplies an interrupt signal to
microprocessor 28, the appropriate digital count in RAM 34 is decremented
by microprocessor 28. The exposure time for each color channel is
determined by the time required to decrement the digital count in RAM 34
for that color channel from its initial value to zero (or some other
predetermined final value). When zero is reached, microprocessor 28
provides a signal through multiport I/O circuit 44b to one of the solenoid
drivers 54, thereby terminating the exposure in that color channel.
The present invention allows great flexibility in the making of corrections
to the exposure times for each color channel. The initial count of between
50 and 99 is a base count which can be modified at any time during the
exposure by simply increasing or decreasing the count. For example, in one
preferred embodiment of the present invention, filter paddle compensation
is provided each time one of the filter paddles is driven into the light
path to terminate the exposure in a particular color channel. The filter
paddle compensation is achieved by microprocessor 28 by increasing the
digital counts contained in RAM 34 for the remaining color channel or
channels. This increase in the digital counts, and therefore the increase
in exposure time, compensates for the unwanted absorption in the remaining
color channels by the filter paddle which has just been driven into the
light path.
The digital counts for each color channel and the clock control numbers for
each channel can be derived in many different ways depending upon the
particular input signals supplied to microprocessor 28. In one preferred
embodiment, the determination of the digital counts and the clock control
numbers is based upon the premise that the intensity times the time that
the photosensitive medium is exposed must be a constant regardless of the
density of the film negative. In other words, the less intensity of the
light, the longer the exposure time must be. Since the density of the
negative to be printed is measured by LATD sensors 22, since the total
time-intensity product desired for the particular photosensitive medium is
known, and since the time base and the permissible range of values of the
digital counts and the clock control numbers are specified, it is possible
to derive the digital count and clock control number for each color based
upon the LATD signals for that color. Any other corrections to exposure
time, such as required by signals from density or color sensors 24 or
operator control panel 26 may be used to modify either the digital counts
or the clock control numbers, or both.
In one preferred embodiment of the present invention, the digital signals
from LATD sensors 22, density or color sensors 24 and operator control
panel 26, are converted to modified log.sub.2 values by multiplying the
log.sub.2 of the reciprocal of the LATD signal by a constant (which is a
scaling factor). The use of log.sub.2 values permits easy handling of the
signals by simple addition and subtraction rather than multiplication.
After the red, green, and blue LATD signals have been corrected based on
values from density or color sensors or operator control panel signals, or
by other corrections required by the printer, a modified antilog is taken
of the corrected value. The antilog consists of two digital numbers for
each color channel. One number is the digital count and the other is the
clock control number n. The digital count is supplied to RAM 34 and
stored, and the clock control count is supplied to one of the multiport
I/O circuits 44a or 44b and is used to control the rate of clock signals
generated by counters 46a-46c.
An important advantage of the present invention is that the clock or
interrupt rate is independent for each channel. In addition, independent
optimization of the rates for each of the color channels is possible. The
microprocessor is not burdened, therefore, with extremely long counts and
the updating which is required for those long counts.
If all three color channels normally terminated in "dead heat" (i.e., at
the same time), only one clock operating at a given rate for all three
channels would be necessary. In practice, however, the channels do not
generally finish in "dead heat". If a single time base were provided for
all three channels, therefore, it would have to be of extremely fine
resolution. As a result, the microprocessor would be burdened with
extremely long counts which would take up a significant portion of the
microprocessor's time simply to update and change counts.
With the present invention, on the other hand, the microprocessor is free
to perform calculations for the next exposure or to perform other machine
control functions. This allows a system of the present invention in many
cases to utilize a microprocessor rather than a minicomputer to control
exposure time.
In conclusion, the exposure time control of the present invention
represents a significant improvement over the prior art exposure control
systems. It permits the use of a microprocessor to perform all of the
exposure time control functions and calculations without overburdening the
microprocessor with an extremely fine time base. Although the present
invention has been described with reference to preferred embodiments,
workers skilled in the art will recognize that changes may be made in form
and detail without departing from the spirit and scope of the present
invention.
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