Contrast ratio is the difference between the brightest whites and the blackest blacks of any display. High contrast ratios deliver whiter whites and blacker blacks and a greater degree of gray values in between. If the contrast ratio is low, even if the image is bright, your image will look washed out.
Contrast Ratio on web design is of major importance due to Web Accesibility. You may fin the specification of W3C over here. This Guideline has three Conformance Level for testings:
- A (lowest), contrast ratio of
- AAA (highest)