Posted on March 31, 2018 By Eric Pfoutz
BenQ HT2550 Projector Review – Advanced Calibration: CMS Calibration, Calibration Charts
This pages focuses on calibrating the CMS – the color management system of the BenQ HT2550 projector.
Essentially the calibration separately calibrates each of the primary and secondary colors – Red, Green, Blue, Cyan, Yellow and Magenta. What is this about? In a perfect world, a projected pure red would be just that, it wouldn’t shift slightly to orange, purple, magenta, etc But reality is that the primaries and secondaries are not “on the money” As a result that affects all the color. I sometimes refer to calibrating the CMS “the last 10%” but it could be far more significant on some projectors, if they are off more than average before any calibration. -art
Delta E is a metric for understanding how the human eye perceives color difference. The term delta comes from mathematics, meaning change in a variable or function. The suffix E references the German word Empfindung, which broadly means sensation. Simply put, look at Delta E as a measure of grayscale/color accuracy. 3 and under is considered ‘Excellent’ and imperceptible by the human eye.
The default CMS looked to be hand adjusted at the factory because the values for all six colors were not at their centered (default) positions as most projectors are. As I mentioned on the calibration page, setting the projector to a 2.2 gamma improved the gamut performance over a 2.4. However, all of the colors still have room for improvement. The 3 secondary colors (cyan, magenta & yellow) needed brightness, saturation and hue adjustments. The 3 primary colors mostly need brightness & hue adjustments.
Vivid TV’s color gamut was not as good as Cinemas, as the types of errors seen in Cinema mode are more pronounced here. I did keep Brilliant Color on with Vivid TV but that does not account for the more sever errors I encountered. Obviously, BenQ has tweaked Vivid TV beyond the user adjustments. The trade-off in using Vivid TV mode is the increased lumen output. 1394 vs 851 you get with Cinema Mode & better color accuracy. I felt the trade-off was worth it because we are trying to make the image as bright as possible but still watchable (i.e. sports, daytime viewing).
BenQ advertises the HT2550 as reproducing the 96% of the REC.709 color gamut which I’d say is an accurate statement. What I found odd however that there is a Color Gamut adjustment in the user menu with REC.2020, DCI-P3, REC.709 and Auto selections. So why would they include REC.2020 & DCI-P3 if the projector only does REC.709? Now if they were a hold-over from other BenQ projectors like their HT8050 then I would think they would just be grayed out. But they do seem to work to some degree but performance is mostly poor. More on that but first I was curious why have a REC.2020 selection when there are no displays (projector or flat panel) that can even come close to REC.2020. Most 4k/HDR displays fail to reach a 100% of the DCI-P3 color gamut with all 6 colors.
When I fed the HT2050 a 4k/HDR signal using Auto as my Color Gamut choice, it was the same as if I’d selected DCI-P3, now that does make sense. When I chose REC.2020 the measured color gamut was slightly different than DCI-P3 but no bigger or better. So, I left Color Gamut on auto (as most users will) and set my calibration software to measure DCI-P3 inside the REC.2020 container. This is common practice for all 4K/HDR displays.
As I mentioned earlier performance was poor and calibration could only make small improvements, mainly to the hue of all 6 colors. Blue was the only color that covered 100% of the DCI-P3 color gamut. Magenta came close at 90% with the rest ranging between 60% to 80%. As I mentioned on the calibration page, HDR performance seem to vary a lot from scene to scene and flesh tones tended to take on a muddy quality. Setting HDR Brightness to 0 or +1 (-1 measured flatter) seemed to help flesh tones but at the expense of washing out the overall image.
Cinema Mode (user 1) Pre-Calibration CIE 1976 Saturation Sweeps (target REC.709)
Cinema Mode (user 1) Post-Calibration CIE 1976 Saturation Sweeps (target REC.709)
Vivid TV Mode Pre-Calibration CIE 1976 Saturation Sweeps (target REC.709)
User 2 Mode Pre-Calibration P3 Saturation Sweeps inside BT.2020
User 2 Mode Post-Calibration P3 Saturation Sweeps inside BT.2020
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