How do the various monitor controls affect ON/OFF contrast?

[tony22]

I just noticed something odd while calibrating my Sony GDM-FW900 CRT. I used i1Match and my i1Pro to calibrate, and then ColorHCFR to run a full colors and grayscale immediately afterward to get all the graphs and numbers. For those who are unfamiliar with the software, look here

http://www.homecinema-fr.com/ColorHCFR/index_en.php

It's used primarily in the video display side of the world, but produces all the charts and graphs similar to the LaCie package seen used on some of the computer display review sites.

Anyway, for whatever reason I ran a second calibration about one hour after the first (even before the first case the display had been on in non-screen saver mode for at least 6 hours), and when I looked at the ON/OFF contrast after running the patterns in ColorHCFR I was shocked to see such a difference. The first time through the ratio was 585:1. Not bad, not great, but my old CRT is reaching the end of its useful life. The second time through the ratio was 418:1!

In the first case the RGB-601 Black point was .271 for Red (as an example) and the White 98.638. In the second, .298 and 99.102. Is it simply a matter of having had a higher Black point (which seems not that great a difference) which caused such a huge change (Green was also slightly higher, but Blue was actually slightly better in the 418:1 case)? I'm assuming it was, but the real question is why this happened. I say that because in the second case, the measurement tick in i1Match for Bightness was actually one notch to the left of center (suggesting it was just a bit lower than optimum), while in the higher contrast ratio case the brightness measurement was dead on center.

I'm thinking it was probably more the difference in the RGB values that did it. I had to raise Green and Blue Gain in the second calibration. Why, I don't know, but I think it was enough to blow the Black point numbers. Would it have been better if I had approached the RGB targets just on the low side with the Gain controls, and then tweaked them in with the Bias? Or could I have recovered the original contrast value by just having dropped the Brightness until it was hitting the target limit, but on the low side?

Note that in this exercise the Brightness and Contrast controls from one calibration to the next did not change by more that 1 or 2 ticks. In the case of the RGB, as indicated above, Green and Blue Gain were upped by about 2 ticks.

[Czornyj]

Blackpoint of a CRT is well below the i1pro measurement range, so for such low luminance values it basically reads its noise rather than the real luminance of the display. The reported contrast values are completly random as a result.

[shewhorn]

I'd say your problem is that you're using an Eye One Pro to measure contrast ratio. It can't be done with any degree of accuracy, the Eye One Pro is just not consistent enough when measuring black point and as such, you'll get wildly different results reported for contrast ratio. I've seen two readings for an identical black point luminance that were 100% off from one another. Ethan Hansen recently confirmed my own observations (see his thread re: measuring the performance of various colorimeters and spectrophotometers). His tests have indicated that below 0.35 cd/m^2 you get into the noise floor of the Eye One Pro and this results in... well... noise.

Cheers, Joe

[tony22]

Ah, thanks for pointing out Ethan's info; I had scanned through it before but not thoroughly. Interesting. I had heard about this problem with the i1 (and spectros in general) but had not experienced it before since I used to use a Spyder. As soon as I got my i1Pro I compared my Spyder to it and saw the Spyder had drifted over time so that the color filters were pretty far off. However, could I still use that to get a reasonable contrast ratio number? Hmm, maybe not. My recollection is that the color values were so far off from low end to high (compared to the i1) that I probably couldn't count on it for contrast.

So what does this mean for trying to hit the Brightness target in i1Match?

[shewhorn]

So what does this mean for trying to hit the Brightness target in i1Match?

Luminance and white point should be fairly consistent and closer to the mark than most colorimeters.

Cheers, Joe

[tony22]

Thanks Joe. I guess what I meant to ask was, if the i1Pro is not so good at low luminances, the how could one reliably set the Brightness using i1Match for the lowest Black Point? Seems using an i1Pro you could never really get to an optimized setting.