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I've come up with a way to adjust greyscale on a projector for only about $60 worth of materials, rather than paying $900 for a TVSPro Optical Comparator, or more for a color analyzer.
An optical comparator is a source of light at a very specific color temperature (color). The basic idea is that you hold the comparator up near the screen while showing test patterns that have various values of grey, and adjust the gain and bias controls on the projector until all the greys are the same color temperature. Joe Kane has written at
http://www.avstandards.com/page3.html that optical comparators can actually work better than color analyzers with some of the newer display technologies.
The parts I used were:
- $16.95 An 18" full spectrum 6500K light bulb from http://www.cinemaquestinc.com . This bulb is actually made by Lumiram http://www.lumiram.com and is the Lumichrome 1XX. It was chosen because it has the highest color rendering index available, CRI 98, which makes the color less viewer-dependent. Lights with lower CRIs would look like different colors to different viewers who might adjust the projectors differently to compensate.
- $7.95 A plastic GE 18" flourescent light fixture with diffuser, switch and cord from Home Depot. To get the right one look for one that handles F15T8 bulbs. This was chosen because of its compactness and smooth diffuser (frosted plastic bulb cover) that doesn't seem to change color temperature much. I actually preferred this unit over the fixture that cinemaquest sells (which I'm returning), because the diffuser smooths out the light, making it easier to see just one intensity of light at a time
- $22 for four Neutral Density filter gels from www.filmtools.com to make different parts of the comparator more dim than others. These were branded as "Lee filters" and were numbers 209, 210, 211, and 299, in strengths of .3, .6, .9, and 1.2 respectively. After I bought these I noticed that cinemaquest literature says they sell a set of Rosco ND gels for $12.95 complete, so you might want to investigate that first; they are probably fine.
Building it was a matter of putting the bulb in the fixture, and then cutting the filters to size and putting them between the diffuser and the bulb. My goal was to divide the 18" into quarters, with the left quarter about 100 IRE, and the right quarter about 20 IRE, and the middle two values in between. Determining which combinations of filters will achieve the correct levels is done by putting the test patterns up on the screen with brightness and contrast ballpark correct, and then experimenting with covering the light with different filters or combinations of filters. It will be different for different projectors depending on the tube brightness, but doesn't have to be exactly spot on. The filter stack I ended up with was like this:
Full width: 1.2 filter
3/4 width: .9 filter
1/2 width: .6 filter
1/4 width: .3 filter
I then put the molded diffuser on over the filters, and put black electrical tape around the edges of the diffuser to cover the places where light was leaking around the edges of the filters. I also covered most of the fixture in black electrical tape, wherever light was leaking through the plastic. It didn't seem too hot, but do this at your own risk; it is theoretically possible that these modifications could cause the fixture to overheat and be dangerous.
So then I put the completed comparator on a speaker in front of the screen, and got to work calibrating my warmed up projector. I brought up the Avia DVD on my HTPC, and switched my Sony 1272 projector into the service mode to adjust the electronic bias and gain controls. By holding down the pattern button I got it to show the Avia test patterns rather than the projector's test patterns. After trying a number of different patterns, I learned that my high-index Draper M2500 screen actually did shift the colors so that the right hand of the screen was more red, and the left was more blue. So it worked best to adjust with patterns that had the greys in the middle of the screen. I alternated between a vertical grey step, and the central rectangles of solid greys of different values. I sometimes used towels to cover the parts of the comparator that were of different intensity from what I was working on... make sure the towels are not colored so they don't add a color cast to the comparator (I confused myself for a little while using orange towels).
I would first adjust the bias (Red and Blue) to some of the darker values on the comparator, and then adjust the gain (Red and Blue) to match the lighter values. There is definitely a lot of interaction between the two controls, so it is necessary to go back and forth a few times to get everyting to look right.
Then I saved the settings to memory, and slightly tweaked the brightness and contrast settings to get those back to where they should be. The proof that I had gotten the greyscale linear was that I could turn the contrast from all the way down to all the way up, and the overall color cast of the image would remain constant (except for when it approached 100%; then it would be more orange because the blue gun goes into saturation).
Because the ND filters on the comparator allowed me to adjust both highlights and lower values, I didn't need to get the greyscale tracked with the more laborious Guy Kuo Lenscap/solarcell/Voltmeter method.
I had adjusted the greyscale manually in the past without a comparator, so it was somewhat close, but not quite right. The difference for me was not night and day, but I definitely noticed that colors (especially fleshtones) looked more accurate, especially when watching NTSC sourced material. One surprising thing is that I saw more shadow detail, probably because the shadows are now correctly grey rather than losing one or more gun's output.
With the diffuser and ND filters on top of the tube, I am not positive I have calibrated to exactly 6500K; It might be off by a few hundred degrees, but that absolute value makes almost no difference as far as I am concerned. The important thing is to get close, and to get the greyscale linear so it is all one temperature from 1 IRE to 100 IRE.
In the couple of years I have owned this projector I had always said I would get an ISF calibration "when the system is finally settled down". But I've realized that I will probably keep changing DVD players, HTPC video cards, HDTV receivers, or line doublers throughout the life of this thing, so it is never going to be totally "done". By having the ability to adjust greyscale (and everything else) myself, I don't have to worry "I'd like to change something, but that would mean another $400 ISF visit"... I can just redo it myself on an as needed basis.
-Tom
An optical comparator is a source of light at a very specific color temperature (color). The basic idea is that you hold the comparator up near the screen while showing test patterns that have various values of grey, and adjust the gain and bias controls on the projector until all the greys are the same color temperature. Joe Kane has written at
http://www.avstandards.com/page3.html that optical comparators can actually work better than color analyzers with some of the newer display technologies.
The parts I used were:
- $16.95 An 18" full spectrum 6500K light bulb from http://www.cinemaquestinc.com . This bulb is actually made by Lumiram http://www.lumiram.com and is the Lumichrome 1XX. It was chosen because it has the highest color rendering index available, CRI 98, which makes the color less viewer-dependent. Lights with lower CRIs would look like different colors to different viewers who might adjust the projectors differently to compensate.
- $7.95 A plastic GE 18" flourescent light fixture with diffuser, switch and cord from Home Depot. To get the right one look for one that handles F15T8 bulbs. This was chosen because of its compactness and smooth diffuser (frosted plastic bulb cover) that doesn't seem to change color temperature much. I actually preferred this unit over the fixture that cinemaquest sells (which I'm returning), because the diffuser smooths out the light, making it easier to see just one intensity of light at a time
- $22 for four Neutral Density filter gels from www.filmtools.com to make different parts of the comparator more dim than others. These were branded as "Lee filters" and were numbers 209, 210, 211, and 299, in strengths of .3, .6, .9, and 1.2 respectively. After I bought these I noticed that cinemaquest literature says they sell a set of Rosco ND gels for $12.95 complete, so you might want to investigate that first; they are probably fine.
Building it was a matter of putting the bulb in the fixture, and then cutting the filters to size and putting them between the diffuser and the bulb. My goal was to divide the 18" into quarters, with the left quarter about 100 IRE, and the right quarter about 20 IRE, and the middle two values in between. Determining which combinations of filters will achieve the correct levels is done by putting the test patterns up on the screen with brightness and contrast ballpark correct, and then experimenting with covering the light with different filters or combinations of filters. It will be different for different projectors depending on the tube brightness, but doesn't have to be exactly spot on. The filter stack I ended up with was like this:
Full width: 1.2 filter
3/4 width: .9 filter
1/2 width: .6 filter
1/4 width: .3 filter
I then put the molded diffuser on over the filters, and put black electrical tape around the edges of the diffuser to cover the places where light was leaking around the edges of the filters. I also covered most of the fixture in black electrical tape, wherever light was leaking through the plastic. It didn't seem too hot, but do this at your own risk; it is theoretically possible that these modifications could cause the fixture to overheat and be dangerous.
So then I put the completed comparator on a speaker in front of the screen, and got to work calibrating my warmed up projector. I brought up the Avia DVD on my HTPC, and switched my Sony 1272 projector into the service mode to adjust the electronic bias and gain controls. By holding down the pattern button I got it to show the Avia test patterns rather than the projector's test patterns. After trying a number of different patterns, I learned that my high-index Draper M2500 screen actually did shift the colors so that the right hand of the screen was more red, and the left was more blue. So it worked best to adjust with patterns that had the greys in the middle of the screen. I alternated between a vertical grey step, and the central rectangles of solid greys of different values. I sometimes used towels to cover the parts of the comparator that were of different intensity from what I was working on... make sure the towels are not colored so they don't add a color cast to the comparator (I confused myself for a little while using orange towels).
I would first adjust the bias (Red and Blue) to some of the darker values on the comparator, and then adjust the gain (Red and Blue) to match the lighter values. There is definitely a lot of interaction between the two controls, so it is necessary to go back and forth a few times to get everyting to look right.
Then I saved the settings to memory, and slightly tweaked the brightness and contrast settings to get those back to where they should be. The proof that I had gotten the greyscale linear was that I could turn the contrast from all the way down to all the way up, and the overall color cast of the image would remain constant (except for when it approached 100%; then it would be more orange because the blue gun goes into saturation).
Because the ND filters on the comparator allowed me to adjust both highlights and lower values, I didn't need to get the greyscale tracked with the more laborious Guy Kuo Lenscap/solarcell/Voltmeter method.
I had adjusted the greyscale manually in the past without a comparator, so it was somewhat close, but not quite right. The difference for me was not night and day, but I definitely noticed that colors (especially fleshtones) looked more accurate, especially when watching NTSC sourced material. One surprising thing is that I saw more shadow detail, probably because the shadows are now correctly grey rather than losing one or more gun's output.
With the diffuser and ND filters on top of the tube, I am not positive I have calibrated to exactly 6500K; It might be off by a few hundred degrees, but that absolute value makes almost no difference as far as I am concerned. The important thing is to get close, and to get the greyscale linear so it is all one temperature from 1 IRE to 100 IRE.
In the couple of years I have owned this projector I had always said I would get an ISF calibration "when the system is finally settled down". But I've realized that I will probably keep changing DVD players, HTPC video cards, HDTV receivers, or line doublers throughout the life of this thing, so it is never going to be totally "done". By having the ability to adjust greyscale (and everything else) myself, I don't have to worry "I'd like to change something, but that would mean another $400 ISF visit"... I can just redo it myself on an as needed basis.
-Tom
