Re: [APML] CRT vs Flat Panel for display of astrophotos

[Dave Hutchison <davehutch@flash.net>]

Contrast ratio is measured as follows:

Luminance at peak white over Luminance at black.  It does not account for number
of bits used to represent peak in/out.  The reason why you can get so many
values for this is because there are different conditions under which the
measurement is taken.  One way to measure is All white over all dark.  Another
method is to take the brightest white over the darkest black.  Then there is
ANSI where a checkerboard pattern is placed on the screen.  The white squares
are averaged over the black squares.  Each of these gives a different result
and are used for various marketing advantage.

You may want to check out the site:
http://www.poynton.com/

Charles has lots of good info on gamma and brightness/contrast with respect to
displays.  The problem can get quite complex.

Dave Hutchison


Quoting Rick Kellogg <rkellog1@twcny.rr.com>:

> >Contrast ratio of 600 etc. <
> (I'm about to say something controversial, please keep responses civil)
> 
> I too have no idea exactly how contrast ratio is measured or what it means.
> But what I took it to mean, is the ratio of the brightest (non-saturating)
> picture value to the smallest discernable difference in picture value (on a
> linear scale).  This is the same definition that most use for dynamic range.
> 
> Now a say your display is set for a gamma of 2.2 (sRGB) with 8 bits/pixel,
> then the light from the brightest pixel value can be represented by:
> 
> gain*255^2.2 + bias + ambient light reflected off display.
> 
> The dimmest pixel by:
> 
> gain*1^2.2 + bias + ambient light reflected off display.
> 
> If the bias and ambient light are small compared gain*1, then the dynamic
> range (or contrast ratio) is:
> 
> 255^2.2 =  ~200k.
> 
> Now all this does is say how much dynamic range is in the number system,
> using 8 bits/pixel, gamma 2.2.  It says nothing about how much dynamic range
> is in your display.   It also ignores that there are 3, 8 bit numbers/pixel
> (one for each color).
> 
> But I would point out that high end broadcast digital VTRs use 10
> bits/pixel, not 8, so as not to see banding in subtle color shades. So while
> I don't know how good CRTs actually are, I would bet they are greater than
> 600:1.
> 
> 
> Rick Kellogg
> 
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