DIYbanter - View Single Post - Bit of a Con Really - Follow-up ...

Bob Larter

wrote:
William Sommerwerck wrote:
I was about to jump on that, but it's basically correct. However,
you'd want the backlight to be "reasonably close", so you didn't
have to push any channel to its limits of adjustment.
... not really. The backlight on this monitor is far removed from the
colour temp its operating at, and all is well. When its far removed it
does affect contrast ratio a bit.
I have to disagree. Suppose the backlight doesn't produce sufficient blue
for the desired color temperature. You can compensate by displaying the blue
pixels at a higher luminance level. But you can't go higher than 100% -- the
lightest (highest) level the LCD can transmit. That level might not be
enough to match the green and red levels.

indeed, but you'd have to have a huge mismatch between backlight CCT
and displayed image CCT for that problem to occur. A 15,000K backlight
with a 5000K display works just fine.

There's no such thing as a 5000K LCD display, sans backlight. Until you
put light through it, an LCD doesn't have a colour temperature at all.

A roughly similar situation occurs with color-negative film. If you expose
daylight-balanced film at 2800K, the blue layer might be unacceptably
underexposed, and no amount of additional blue-layer exposure during
printing will restore the lost shadow detail. Ditto for exposing 3200K film
under daylight, except the error is on the side of overexposure.

yes that happens with film, but nothing like it happens with an LCD
display. What happens is that if your image is far removed from the
backlight in terms of CCT, then one of the RGB LCD colour channels
operates over part of its potential range, not the full range. So for
example on this display the B pixels might never exceed 50% light
transmission. It doesnt cause a problem.

Yes, it does. It drastically reduces the number of transmission levels
available to the pixels of that colour, causing posterisation & colour
distortion. The more out of whack the colour temperature of the
backlight is in comparison to the desired colour temperature of the
image, the worse the problem will get.
Unlike CRTs, the transmission value of an LCD pixel isn't infinitely
variable, they only work in steps.

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