Contrast ratio, in general, is the ratio between bright and dark spots in an image, used most notably in three point lighting situations. The difference in lighting between the key light half of the face and the fill light half is compared in this format: key+fill light : fill light alone. Typical ratios for television productions are 2:1 for flat lighting to 4:1 for a more chiaroscuro look. A typical sunny day has a contrast ratio of 9:1, which may need additional fill lighting.
Contrast ratio also refers to a property of monitors that defines the ratio between the deepest blacks and brightest whites. There are a few different techniques to measure this ratio, including the full on/full off method and the ANSI contrast test. Unfortunately, this measurement lacks standardization, so manufacturers can boast inflated ratios that don’t translate in real world situations.