Contrast is the difference between the light and dark areas on your sign. Positive contrast (light border or text on a dark background) is easier to read than negative contrast (dark border or text on a light background).
Luminance is the intensity of light emitted from the surface of your sign. Sign text is considered to be optimally contrasted when its measured brightness is between four to ten times the brightness of its background material.
Be aware that certain sign codes limit the light output allowable in on-premise signage. This philosophy (known increasingly as the "Dark Skies" movement) is often used by communities concerned with energy conservation. You and your sign professional should be aware of whether your code has such restrictions.
