Datacolor User to User Support Group.

--- In colorvision_group@yahoogroups.com, "esimanor" <esimanor@...> 
wrote:

> 
> If contrast was to control that curve then it should be called 
Gamma, 
> really.... no?
> The terms brightness and contrast are confusing, even more when they 
> relate to an LCD display. Strictly speaking and since the luminance 
> of your display depends partly on your own subjective assesment and 
> partly on the amount of ambiant light, these two "knobs" are there 
to 
> allow you to set your pedestal (black level, usualy called 
> brightness) and your white point luminance (usualy called contrast). 
> This rule applies well on a CRT, as for LCD's I am not so sure.

In truth I am right and you are partly right. When adjusting the 
contrast of a photographic image you adjust the centre of an S curve 
but the curve in question, the contrast on a CRT or LCD, is a straight 
line and so adjusting the contrast of an image with a fixed black 
point does indeed affect the luminance of white.

Contrast is definitely not gamma. Gamma is a curve that corrects 
linear digital images to account for the non linearity of visual 
perception. In fact the term I think originated in the compensation 
applied to the analogue video signal driving the grid of CRT's to 
correct for the non linear transfer function between grid voltage and 
anode current. This of course is long before the days of computer 
monitors.


> 
> > Presumably your client's screen is not the end use for your 
images. 
> If 
> > they are going to be ending up as print then the illumination 
> levels 
> > are meaningless anyway.
> 
> Well this is true but if you adjust your pictures for a screen whose 
> over all luminance is widely different from the one on which your 
> pictures will be seen then your pictures will appear widely too 
> bright or dark on that same display, which indeed does not imply 
> anything as for further uses. 

I don't believe that an observer is capable of making absolute 
luminance judgements. The contrast should be adjusted for the comfort 
of the viewer and to keep the CRT or LCD within it's linear range of 
reproduction. You may not realise it but if you try to drive your CRT 
too hard to get a bright picture in a room of high illumination 
current limiter circuits in the monitor will act to limit the 
brightness and protect the tube. When that happens it will affect the 
accuracy of the image.
All you should have to do in profiling your display is accurately 
reproduce colour and greyscale divisions and make sure that all 
gradations between black and maximum white are discernable. The Spyder 
can do that.