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Discussion Starter · #1 ·
What is the effect on the CMS settings/CIE diagram for a projector with an aging bulb if the grayscale is routinely recalibrated as the bulb dims and changes its spectral output?


Does the continuous Correction of the Grayscale over a bulbs life automatically mean the CMS settings used previously will continue to be correct as the bulb ages --or is the color correction of Gain and Bias for the grayscale essentially independent and ineffective in providing correction for errors in the color gamut due to bulb aging?


I have recalibrated my JVC RS20 projector's Grayscale to correct for the expected shift due to the aging of the bulb (now at 880 hours) and for the grayscale shift that accompanied a change of bulb power to the high setting to maintain the brightness. (I am using a new Eye 1 D2 meter and Calman software for all my measurements.)


After correcting for Grayscale drift I have found. in my first THX measurements, that the THX color Gamut is off -- rotated slightly clockwise. Yellows a have a bit too much red, Reds have a bit too much Blue etc., so I believe the meter is correct. This suggests that the factory settings for THX (which can not be adjusted) no longer can be relied on ..and that simply correcting the grayscale will not restore the THX performance to the "as new - close to REC 709" performance reported by most owners of the RS20.


So a subset of my question is this: Are the factory THX settings really only useful when the bulb life is new, and will never have a chance to be correct again (even if the grayscale is corrected) untill the bulb is replaced?


Thoughts on this and the reasons why, or why not, the Color Gamutt must always be recalibrated along with the Grayscale as a bulb changes over it's lifetime would be appreciated.


KT
 

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Discussion Starter · #3 ·

Quote:
Originally Posted by ChrisWiggles /forum/post/17017985


With the CMS, not sure the THX setting is useful at all if you're gonna calibrate the CMS. If you're just going out of the box it's quite good, but if you've got the tools...

Thanks Chris, but the THX mode is not the real issue here. I just used it as an example of the changes I have seen as the bulb aged. Let me try to restate the issue.


Suppose, after calibrating the Grayscale, the CMS of my RS20 is used to bring the projector's user modes into REC 709 calibration within a 1976 Delta E value of 3 or less. Now suppose that over many hours the lamp dims and the power to the lamp has to be raised and so the Grayscale is clearly off and therefore I have to recalibrate it.


My question is this .. will the CMS settings now also have to be re-done or will the color CIE performance remain in the Delta E 3 range because all the color drift in BOTH the Grayscale AND the CIE Color performance were corrected when the Grayscale was recalibrated?


KT
 

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The main answer is really quite easy... the underlying issues are a little more complex.


Making the grayscale accurate controls how colors mix to create a neutral white or gray. You can make the grayscale quite accurate on a TV/projector with primaries that are not accurate. You do this by setting the TV to control the amount of each primary needed to create a neutral gray for each step (or as close as you can get with the 2 adjustments you usually have for each primary). As you move away from gray towards any primary, the errors will slowly increase as you get farther and farther away from your neutral reference point and closer and closer to the (inaccurate) primary.


That said, inaccurate primary colors seem to be less visible/noticeable than an inaccurate grayscale when the inaccuracies are of similar magnitude. So if you have a choice of adjusting only grayscale or only primaries, grayscale should give you more noticeable improvement (provided both have errors of similar magnitude).


How much primary and/or complimentary colors drift depends a lot on how the lamp is split to RGB... color wheel, prism, stationary filters, etc. and in how effective those "splitting" devices are at allowing only the correct wavelength(s) through for each primary. Since the combination of wavelengths that are allowed through to create the red primary change as the lamp ages, it is difficult in real-world products priced for use at home to eliminate the drift completely. Using red, green, and blue lasers as a light source should be an ideal solution since each laser should emit just a single frequency - those may change in intensity/amplitude as the laser ages, but the color wouldn't drift. So you could go through the life of the lasers re-tweaking grayscale while retaining "ideal" grayscale and primary (and presumably complimentary) colors. But projection lamps are wide-bandwidth devices and each frequency is subject to change as the lamp ages so "comtrol" is quite difficult.
 

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Quote:
Originally Posted by KTTV Images /forum/post/17018158


Thanks Chris, but the THX mode is not the real issue here. I just used it as an example of the changes I have seen as the bulb aged. Let me try to restate the issue.


Suppose, after calibrating the Grayscale, the CMS of my RS20 is used to bring the projector's user modes into REC 709 calibration within a 1976 Delta E value of 3 or less. Now suppose that over many hours the lamp dims and the power to the lamp has to be raised and so the Grayscale is clearly off and therefore I have to recalibrate it.


My question is this .. will the CMS settings now also have to be re-done or will the color CIE performance remain in the Delta E 3 range because all the color drift in BOTH the Grayscale AND the CIE Color performance were corrected when the Grayscale was recalibrated?


KT

No you'd have to do both. CMS is a different operation, well I won't say entirely different or completely unrelated, but it's quite different.


How much things may move on this projector I don't know yet, I don't have that many hours on mine.



But yes you should check the primaries again, though realistically I don't suspect they will move a whole bunch.
 

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Discussion Starter · #6 ·

Quote:
Originally Posted by Doug Blackburn /forum/post/17020034


The main answer is really quite easy... the underlying issues are a little more complex.


Making the grayscale accurate controls how colors mix to create a neutral white or gray. You can make the grayscale quite accurate on a TV/projector with primaries that are not accurate. You do this by setting the TV to control the amount of each primary needed to create a neutral gray for each step (or as close as you can get with the 2 adjustments you usually have for each primary). As you move away from gray towards any primary, the errors will slowly increase as you get farther and farther away from your neutral reference point and closer and closer to the (inaccurate) primary.


That said, inaccurate primary colors seem to be less visible/noticeable than an inaccurate grayscale when the inaccuracies are of similar magnitude. So if you have a choice of adjusting only grayscale or only primaries, grayscale should give you more noticeable improvement (provided both have errors of similar magnitude).


How much primary and/or complimentary colors drift depends a lot on how the lamp is split to RGB... color wheel, prism, stationary filters, etc. and in how effective those "splitting" devices are at allowing only the correct wavelength(s) through for each primary. Since the combination of wavelengths that are allowed through to create the red primary change as the lamp ages, it is difficult in real-world products priced for use at home to eliminate the drift completely. Using red, green, and blue lasers as a light source should be an ideal solution since each laser should emit just a single frequency - those may change in intensity/amplitude as the laser ages, but the color wouldn't drift. So you could go through the life of the lasers re-tweaking grayscale while retaining "ideal" grayscale and primary (and presumably complimentary) colors. But projection lamps are wide-bandwidth devices and each frequency is subject to change as the lamp ages so "comtrol" is quite difficult.

Very interesting answer. Since the RS20 primaries are pseudo-primaries the changes as the lamp ages in spectra for -say red - involve even more shift than that in the red panel alone since some of the other 2 primary colors may be mixed in with the red to hit the REC 709 or SMPTE-C red target.


Thanks

KT
 

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Discussion Starter · #7 ·

Quote:
Originally Posted by ChrisWiggles /forum/post/17020159


No you'd have to do both. CMS is a different operation, well I won't say entirely different or completely unrelated, but it's quite different.


How much things may move on this projector I don't know yet, I don't have that many hours on mine.



But yes you should check the primaries again, though realistically I don't suspect they will move a whole bunch.

Thanks Chris. The THX mode on my RS20 is now off as I described at the top of this thread - and according to my measurements at 880 hours - the 1976 delta E ranges between about 6.5 to 11 for all 6 colors. It would be interesting to see if others have seen similar shifts in THX with bulb age.


KT
 
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