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Extended Range Display Calibration

 
Author Steve

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#1 | Posted: 5 Feb 2022 15:25 
The recent addition to ColourSpace for accurate Extended Range LUT Generation has highlighted lack of true calibration knowledge and understanding within the wider calibration community.

We made the addition for accurate Extended Range LUT Generation as part of a larger upgrade to ColourSpace we are working on - specifically for PQ based HDR calibration.

See: https://www.lightillusion.com/forums/tips-tricks-17/limit-range-unique-approach-to-pq-hdr-calibration-778.html#msg4708

As part of this change we have, for the first time in any calibration system, correctly implemented Extended Range LUT Generation.

But, based on a number of comments we have seen on this change to ColourSpace, there appears to be a fairly serious lack of understanding within the calibration community.

Extended Range very specifically means a display that has 'super white', that goes above 100% white to 109%.

And that means an input signal range that goes from 16 (8 bit) for zero black to 255 for 109% super white, with 235 being 100% white.

When performing accurate calibration, the range 16-235 remains the target range, with 235/100% as the pivot point for the over-range super white.

Obviously, the pre-calibration EOTF will define what happens as display brightness goes above 100% white after calibration.

And that means the calibration LUT 'may' have to include extrapolated data, as the original profile may not extend to the final calibrated range of 109%

The pre-calibration EOTF of the display will define if extrapolated data is used, or not

This is totally correct, and is to be expected, and the LUT will be perfectly correct, based on the profile data.

The alternative is to treat 109% as the pivot point, which is how ColourSpace used to work (and can still be set to work), and is how other calibration systems work.

But, that will generate an inaccurate EOTF, as the pivot point for white is no longer 100%

And as all TV footage is Legal range, not Extended, calibration accuracy between 0% and 100% (16-235) is what matters.

Steve
Steve Shaw
Mob Boss at Light Illusion

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