CCT increasing with decreasing IRE; assumed whitepoint" error

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  • #11631

    Mark Appleton
    Participant
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    I’ve enclosed an HTML report of a recent calibration run.

    As can be seen in the colour temperature graph, the CCT doesn’t stay at 6500K for the whole greyscale; it increases as the IRE level decreases. This isn’t an error – DispCAL has set these increased colour temperatures as targets (the larger circles in the graph). Why does it do this, and can I prevent it from happening and get 6500K for the whole range?

    Also, I am having problems with the “assumed whitepoint” setting – is there any way to fix it to 6500K? I always specify D65 when beginning a run, but it after the run it can be pretty much anything ,and it is entirely beyond my control!

    • This topic was modified 5 years, 11 months ago by Mark Appleton.
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    #11647

    Florian Höch
    Administrator
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    Hi,

    this is expected. The whitepoint hue shifts over to the native black point hue as the neutral values get darker, which is usually desirable because it maintains contrast on (already contrast challenged) LCD displays. The only way to have the whitepoint hue across the whole range is to correct the black point (by setting black point correction to a above zero value on the calibration tab, and/or alternatively moving the cross-over rate from whitepoint hue to black point hue to be closer to black – see the documentation), which in turn makes it lighter, which in turn is probably not what you want.

    Also, I am having problems with the “assumed whitepoint” setting – is there any way to fix it to 6500K?

    The assumed whitepoint is based on the actual measured whitepoint at the time the report was created (rounded in 100K increments, e.g. 6450K gets rounded up to 6500K, 6510K gets rounded down to 6500K etc). This is useful because it allows you to evaluate whitepoint drift independent from color drift, and is a better way of quantizing white point drift because most importance lies in the proximity to a daylight or blackbody locus, not necessarily a difference of a few (or, in fact, several hundred) Kelvin of color temperature difference (which is in and of itself a much less useful metric) to any given target.

    #11655

    Mark Appleton
    Participant
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    I think I understand – since the black CCT is locked, the greyscale has to have a gradual decrease toward white to avoid jarring changes? Would it be possible to do what we do with TVs with ten-point greyscale adjustment – that is, just allow a blue-coloured black at 10% and get all the other levels to D65?  Indeed, if there’s a way to set the “lower than this IRE, don’t care about D65/above this IRE, prioritze D65” cutoff I would love to know about it:).

    As far as the assumed whitepoint goes, I’ll just have to make sure I’m within 50K of D65 in future!

    M.

    • This reply was modified 5 years, 11 months ago by Mark Appleton.
    #11661

    Florian Höch
    Administrator
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    allow a blue-coloured black at 10% and get all the other levels to D65?

    That’s what the calibration black point correction rate control is for (advanced option, see the documentation).

    #11668

    Mark Appleton
    Participant
    • Offline

    allow a blue-coloured black at 10% and get all the other levels to D65?

    That’s what the calibration black point correction rate control is for (advanced option, see the documentation).

    Aha – you really did think of everything!

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