Revision [1653]

Last edited on 2011-01-14 19:37:34 by AlanRobinson
Deletions:
We have tested this device using a PR650 and a UTD photometer. We found that the color channels are independent, with little to no additivity errors. Less clear is that the internal gamma correction is consistent over time. The LEDs apparently wear out fairly quickly (see HP manual), so there is some kind of internal feedback system that is supposed to compensate for this. Leaving the monitor on for 400 hours, however, did result in a noticeable change in the brightness of the color channels, necessitating a new calibration.
In addition, it appears that there may be additional internal compensation operating over a shorter time scale, as measuring brightness in an orderly fashion from DAC 0-1024 produces a different luminance curve than when the same values are measured in random order. We have yet to determine exactly what causes this. If you interleave each orderly measurement with a period of black and color channel=max, the curve matches the one made from random order pretty well, suggesting that the issue may involve some kind of feedback control with the back-light (kind of like dynamic contrast) that behaves poorly when the max luminance stays relatively constant for a long period of time. More tests are necessary.
One strange 'issue' to know about is that the gamma curve measured over a Display Port is quite different than the one over VGA (ie analogue). The VGA one is squashed downward, so that the upper 10-15% of voltage levels all produce the same (max) luminance. The overall number of luminance steps available is probably the same, so long as you have a high bit-depth device like BITS Plus Plus to drive the VGA signal.
The panel claims to have 10-bit resolution, but as you would expect, it's not quite that; as with all LCDs, there is a significant loss of resolution on the low luminance end, such that you may need to increment 5-10 DAC values before you get an actual luminance change. Roughly speaking, the display is more like 9.7 bits.
Reported by AlanRobinson.


Revision [1649]

Edited on 2011-01-14 19:35:19 by AlanRobinson
Additions:
One strange 'issue' to know about is that the gamma curve measured over a Display Port is quite different than the one over VGA (ie analogue). The VGA one is squashed downward, so that the upper 10-15% of voltage levels all produce the same (max) luminance. The overall number of luminance steps available is probably the same, so long as you have a high bit-depth device like BITS Plus Plus to drive the VGA signal.
Deletions:
One strange 'issue' to know about is that the gamma curve measured over a Display Port is quite different than the one over VGA (ie analogue). The VGA one is squashed downward, so that the upper 10-15% of voltage levels all produce the same (max) luminance. The overall number of luminance steps available is probably the same, so long as you have a high bit-depth device like BITS++ to drive the VGA signal.


Revision [1648]

The oldest known version of this page was created on 2011-01-14 19:34:58 by AlanRobinson
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