Contrast refers to the differences between luminances and is used as a factor or a ratio. The highest contrast ratio obtainable is the ratio between maximum and minimum luminance in an image. In plain language, it’s the difference between the maximum white and (minimum) black luminance level, expressed as a ratio between the two.
There are some essential points to understand about contrast:
Projector contrast ratio. The contrast figure in brochures is typically an on/off number (i.e. measurements taken at different times, representing the difference between a full white image and a black image). This can generate ratios such as 5,000:1 or greater, which do not relate to real life measurements at all. The only usable contrast data is the difference between the black levels and the white levels – on the same image. In this real life scenario you will rarely measure about 100:1. At time of writing, the authors have never measured over 200:1 contrast ratio (using proper ANSI methodology).
Image contrast ratio. Whatever the actual projector contrast ratio is, this will always be less when measured on screen.
Image contrast and ambient light. In all scenarios, excluding those which control ambient light totally (e.g. cinema), ambient light is a critical factor. How the screen type and technology can withstand the effects of ambient light is the key driver in achieving good contrast image ratios.
The image contrast ratio is the ratio between the brightest part (white) and the darkest part (black) in an image. Image brightness, or luminance, is measured in cd/m2 (nit) or Foot-Lamberts.
Contrast ratio = Brightest luminance / Darkest luminance
Example: White in an image is measured to 250 nits (Nit 1). Black is measured to 10 nits (Nit 2). The contrast ratio is 250/10=25. This is often presented 25:1 "twentyfive to one". Note that the contrast ratio doesn't say anything about the actual brightness of an image - just the ratio between white and black levels.