The question actually needs to be asked in several parts to better understand how a display behaves in terms of luminance display range.
1. How bright is a full field black on the display? This gives you an idea of how much background light "noise" is always overlaid on the picture. CRT based RPTV's should be very low (good) on this measure. Any light pollution from poor performance on this parameter will tend to make dark scenes less convincing and less transparent. The effect of too much light leakage is a veiled appearance to dark scenes.
2. What is the ratio of light between the black and white rectangles of a an ANSI contrast test pattern? The higher the number here the better. This differs from the first parameter because it is also affected by light scatter in the display and is a RATIO. This value alone does not directly address the degree of brightness of black. A higher contrast number here is desirable.
3. What is the peak white for both 1% and ANSI test patterns. While the ANSI peak white is a better estimate of average light output capability, the smaller areal peak white is also important to know because that affects perceived brilliance of the picture. Digital projection should have very similar values for large area (ANSI) and small area peak whites. CRT's can exhibit quite a difference between the two. This difference is one reason a CRT with low total light output can appear to produce a more brilliant image than an equal light output display with constant peak white.
4. How does the gamma curve of the display match the transfer characteristic of a gamma 2.5 CRT? If the display does not gamma match correctly, midtones will be misrepresented as too bright or too dark. Information about this characteristic of displays is rarely available but can completely alter the correct representation of luminance values in an image. If gamma is wrong, there will always be some portion of the picture which is displayed as too dark or too light relative to what was intended.
These numbers are all hopefully take with the display operating inside the cutoff range wherein all gradations of gray can be represented. In other words, one could cheat and crank up the contrast beyond clipping levels to achieve an artificially high contrast ratio.
So who polices the measurement of these parameters?????? Do you believe the numbers you get?
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Guy Kuo
www.ovationsw.com
Ovation Software, the Home of AVIA DVD
