Dynamic gain correction for CRT printing

A system is provided that causes the CRT of a CRT printer to produce images on the screen of the CRT, which when projected onto a photographic media will have uniform illumination at all points on the photographic media. The system accomplishes the foregoing by dynamically changing the amplitude (gain) of the video image signal applied to the CRT.

FIELD OF THE INVENTION 
The present invention relates to a photographic printer in which a CRT is 
used as a imaging source for producing prints on photosensitive media. 
BACKGROUND OF THE INVENTION 
A cathode ray tube (CRT) converts information contained in an input signal 
to electron beam energy and converts that energy into light energy to 
provide a visual information output on a phosphorous screen. The amount of 
beam modulation of the CRT and thus, the amount of light output of the CRT 
is a function of the voltage difference between the cathode and the first 
grid of the CRT (the grid closest to the cathode). If the voltage of the 
first grid of the CRT is held constant, then the cathode may be modulated 
with the voltage that represents the video information i.e. the input 
signal. In the event the voltage on the cathode is held constant then the 
voltage on the first grid may be modulated with the voltage that 
represents the video information i.e. the CRT input signal. 
A video amplifier circuit amplifies and processes the video input signal 
and applies it to either the first grid or cathode of the CRT. A means of 
scanning the electron beam horizontally and vertically over the screen of 
the CRT is provided. The combination of scanning and electron beam 
modulation by the input signal produces an image on the screen of the CRT 
that will be transferred to the photographic media. The video amplifier 
provides a means of blanking the video signal during the horizontal and 
vertical retrace periods i.e. the time it takes for the electron beam to 
return to its starting point for scanning a horizontal line and the time 
it takes for the electron beam to return to its starting point for 
scanning the image vertically. 
Photographic printers have been utilized that use a CRT as an imaging 
source for producing a print on a photosensitive media. A monochromatic or 
color CRT may be used as a imaging source and a monochromatic or color 
photosensitive media such as photosensitive paper or film may be used to 
record the CRT produced image. Typically a monochromatic image is 
displayed on the phosphorous screen of a CRT by modulating the electron 
beam of the CRT with dark to light gradations of the image and 
simultaneously deflecting the CRT electron beam to achieve the position of 
the pixels on the CRT screen. The image on the screen of the CRT may be 
transferred to the photographic media by any known means i.e. contact 
printing (the photographic media is placed directly on the CRT screen), 
projected by one or more lenses and/or one or more mirrors. 
A monochromatic CRT may be used to expose a color picture onto photographic 
media. When a monochromatic display is used, three sequential exposures 
through red, green and blue filters will individually expose the 
photographic media to the red, green and blue components of the image 
displayed sequentially on the screen of the CRT. 
A color CRT may also be used to display a color picture on a photographic 
media. If a color CRT was used, the color CRT would expose in one sequence 
the red, green and blue portions of the image on the screen of the CRT 
simultaneously onto the photographic media. 
Assume the CRT is displaying an image from a video input signal. The 
illumination of the image will decrease from the center to the edges of 
the screen of the CRT with a gradient. The reason for the foregoing is 
that the electron beam has to travel an increased distance when it travels 
to the edge of the CRT screen. Also, the beam is less perpendicular to the 
phosphor screen when it is near the edges of the screen. 
If the image on the screen of the CRT is transferred to the photographic 
media by one or more lenses, the lens or lenses will project greater 
illumination at the center of the photographic media than at the edges of 
the photographic media. The reason for the above is vignetting and cosine 
fourth falloff. Vignetting reduces illumination of the image for off axis 
image points and is related to the size and quality of the lens. Cosine 
fourth falloff pertains to the physics of projecting a round point of 
light through a lens at a off-axis angle. The projected point of light is 
an ellipse instead of round and its intensity is reduced compared to a 
round point of light resulting from an on-axis projection through the 
lens. 
Thus, if a negative photographic media receives an image from the screen of 
a CRT, the density at the center of the print will be greater than the 
density near the edges of the print. That is, the center of the print will 
be darker than the edges of the print for the same video input signal 
amplitude. Furthermore, if a positive photographic media receives the CRT 
image, the center of the print will be lighter than the edges of the print 
for the same video input signal amplitude. 
PROBLEMS TO BE SOLVED BY THE INVENTION 
One of the problems encountered by the prior art was that for a given CRT 
input signal the CRT would not have uniform illumination at all points on 
the screen of the CRT. Thus, the photographic media would not be able to 
produce prints of uniform density at different locations on the print. The 
reason for the above, is that the CRT electron beam has to travel 
different distances. Also, the beam is less perpendicular to the phosphor 
screen when it is near the edges of the screen. 
Another problem encountered by the prior art is that when the image on the 
screen of the CRT is transferred to a photographic media by one or more 
lenses, the lens or lenses will project greater illumination at the center 
of the photographic media than at the edges of the photographic media. 
SUMMARY OF THE INVENTION 
This invention overcomes the disadvantages of the prior art by compensating 
for the falloff in illumination towards the edges of the CRT screen when 
an image on the screen of the CRT is transferred to photographic media by 
contact printing. 
The invention also compensates for the falloff in illumination towards the 
edges of the image projected onto photographic media by a lens system from 
the screen of the CRT. In this instance, the invention compensates for 
illumination falloff in both the CRT and the lens system, so that the 
image projected onto the photographic media will have uniform illumination 
at all points. 
The system accomplishes the foregoing by dynamically changing the amplitude 
(gain) of the video input signal. 
ADVANTAGEOUS EFFECTS OF THE INVENTION 
The above CRT system causes the CRT to produce images on photographic media 
that have uniform illumination at all points on the photographic media. 
This invention compensates for the CRT falloff so that prints having 
uniform density at all points may be obtained. 
The foregoing is accomplished by providing an analog dynamic gain 
correction system for controlling the illumination of an image that 
appears on the screen of a CRT of a CRT printing device, from a video 
source, producing an analog video signal, that represents the image and 
producing summed horizontal and vertical parabolic signals that are 
synchronized with the analog video signal, the system comprises: a 
multiplier coupled to the video source for receiving the video signal, the 
summed horizontal and vertical parabolic signals, and to the CRT for 
controlling gain of the video signal and producing an image on the screen 
of the CRT; whereby, when a photographic media is placed in front of the 
image appearing on the screen of the CRT, the photographic media will have 
the same illumination at all points on the photographic media for the same 
video signal amplitude.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings in detail, and more particularly to FIG. 1, 
the reference character 10 represents a video source. Video source 10 may 
be a frame store, a television camera or a scanner or other video source. 
The video input signal produced by source 10 i.e.. the signal that 
represents the image that is to be displayed on the screen of CRT 20 is 
coupled to one of the two inputs of multiplier 11. 
When a portion of the video input signal has a certain voltage, for 
instance 2 volts, a white image will be produced on the screen of CRT 20 
and when the video input signal has a voltage of approximately 0 volts a 
black portion of the image will be produced on the screen of CRT 20. The 
other input to multiplier 11 is a voltage that will control the system 
gain. The aforementioned voltage is transmitted on line 12 and is 
generated as follows. The horizontal sync output of video source 10 is 
connected to horizontal parabola generator 13. Generator 13 generates a 
parabolic signal at a horizontal rate, that is transmitted to adder 14. 
The vertical sync output of video source 10 is connected to vertical 
parabola generator 15. Generator 15 generates a parabolic signal at a 
vertical rate that is transmitted to adder 14. Adder 14 adds the above 
signals and transmits them to multiplier 11 via line 12. 
Multiplier 11 multiplies its two input signals and transmits the multiplied 
signal to video amplifier 16. The aforementioned signal is supplied such 
that black is at 0 volts or close to 0 volts and white is at a positive 
voltage for instance 2 volts. The result of having black at 0 volts is 
that multiplier 11 does not change the output voltage representing black 
as a function of the parabolic waveform on line 12. The result of having 
white and shades of grey at non-zero voltages is that the white and grey 
portions of the signal on line 12 are multiplied by multiplier 11 
according to their magnitude. 
The output of amplifier 16 drives the cathode or grid of CRT 20 to provide 
an image on the screen of CRT 20. The parabolic signals on line 12 have 
their greatest amplitude when the electron beam is near the edges of the 
screen of CRT 20. Consequently, when the signal on line 12 is multiplied 
by the video input signal, the signal at the input to amplifier 16 will 
have increasing amplitude or gain as the electron beam approaches the 
edges of the screen of CRT 20. Thus, the output of amplifier 16 causes the 
screen of CRT 20 to have greater illumination at its edges than it would 
heretofore have had. 
A focusing device 21 comprised of one or more lenses and/or one or more 
mirrors may be placed in front of the screen of CRT 20. Focusing device 21 
causes the image appearing on the screen of CRT 20 to be exposed to 
photographic media 22. The focusing device 21 contributes to the falloff 
of illumination at the edges of photographic media 22. However, this 
falloff has been compensated for by the multiplication of the video input 
signal with the parabolic waveform appearing on line 12. Thus, the screen 
of CRT 20 will have greater illumination at its edges than it otherwise 
would have had. 
If focusing device 21 is not placed between CRT 20 and photographic media 
22, the image appearing on the screen of CRT 20 may be directly exposed 
onto photographic media 22. 
FIG. 2 is a drawing of a digital dynamic gain correction system for a CRT 
printer. Oscillator 9 outputs a pixel clock signal that is coupled to the 
input of digital video source 10 and pixel counter 31. 
Digital video source 10 may be a frame store, a television camera or a 
scanner etc.. The digital video signal produced by source 10 i.e.. the 
signal that represents the image that is going to be displayed on the 
screen of CRT 20 is coupled to one of the two inputs of multiplier 11. The 
horizontal and vertical sync signals are used to synchronize the digital 
video signal. A certain value of the digital video signal will produce 
white on the screen of CRT 20 and a zero value of the digital video signal 
will produce black on the screen of CRT 20. The values of the digital 
video signal between white and black will produce shades of grey. 
The horizontal sync output of source 10 is coupled to the reset input of 
pixel counter 31. The output of oscillator 9 is coupled to the clock input 
of pixel counter 31. Pixel counter 31 is used to generate sequential 
addresses at a rate governed by oscillator 9. The horizontal sync input to 
the pixel counter 31 resets the address count to a starting point at the 
beginning of each horizontal line. The digital addresses that are output 
by counter 31 are transmitted to Look-up-Table 33. 
Look-up-Table 33 produces a parabolic signal at a horizontal rate that is 
transmitted to adder 35. 
The vertical sync output of source 10 is coupled to the reset input of line 
counter 32. The horizontal sync output of digital video source 10 is 
coupled to the clock input of line counter 32. Line counter 32 is used to 
generate sequential addresses at a rate governed by the horizontal sync. 
The vertical sync input to line counter 32 resets the address count to a 
starting point at the beginning of each vertical sweep. The digital 
addresses that are output by counter 32 are transmitted to Look-up-Table 
34. 
Look-up-Table 34 produces a parabolic signal at a vertical rate that is 
transmitted to adder 35. 
Adder 35 adds the above signals and transmits them to one of the inputs of 
multiplier 11. Multiplier 11 multiplies the digital video signal by the 
horizontal rate correction parabola added to the vertical rate correction 
parabola. 
The digital output of multiplier 11 is transmitted to digital to analog 
converter 36. Converter 36 converts its digital input signal to an analog 
output signal. The above signal is transmitted to amplifier 16. 
The output of amplifier 16 drives the cathode or grid of CRT 20 to provide 
an image on the screen of CRT 20. The parabolic signal at the output of 
adder 35 has its greatest value when the electron beam is near the edges 
of the screen of CRT 20. Consequently, when multiplier 11 multiplies the 
digital video signal by the output of adder 35, the resulting magnitude of 
the digital video input signal to converter 36 will be increased as the 
pixel positions approach the horizontal and vertical edges of the raster 
on the screen of CRT 20. Thus, the output of amplifier 16 causes the 
screen of CRT 20 to have greater illumination at its edges than it would 
heretofore have had. 
A focusing device 21 comprised of one or more lenses and/or one or more 
mirrors is placed in front of the screen of CRT 20. Focusing device 21 
causes the image appearing on the screen of CRT 20 to be exposed to 
photographic media 22. The focusing device 21 contributes to the falloff 
of illumination at the edges of photographic media 22. However, this 
falloff has been compensated for by the multiplication of the video input 
signal with the parabolic waveforms appearing on line 12. Thus, the screen 
of CRT 20 will have greater illumination at its edges than it otherwise 
would have had. 
If focusing device 21 is not placed between CRT 20 and photographic media 
22, the image appearing on the screen of CRT 20 may be directly exposed on 
photographic media 22. 
The above specification describes a new and improved dynamic gain 
correction system for a CRT printer. It is realized that the above 
description may indicate to those skilled in the art additional ways in 
which the principles of this invention may be used without departing from 
the spirit. It is, therefore, intended that this invention be limited only 
by the scope of the appended claims.