Acer S191HQL Service Manual page 16

1.5.2 Contrast ratio (CR):
1.5.2.1 General CR:
The values specified are at an approximate distance 50cm from the LCD surface at a viewing angle of  and 
equal to 0 °.
Contrast ratio(CR) is defined mathematically as :It is measured at center point(1)
Contrast ratio =
1.5.3 Response time:
Surface luminance with all white pixels
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Surface luminance with all black pixels
14