Illuminance is the luminous power incident per unit area of surface. It is consequently a measurement of the amount of light from a source which strikes the surface of an object. It is typically measured in lux or foot-candles. (One lux represents one lumen per square metre.) Two other terms derived from this basic measurement are horizontal illuminance and vertical illuminance. Horizontal illuminance is a measurement of the the amount of light striking a horizontal surface, such a floor or desk, and vertical illuminance is a measurement of the amount of light from a source striking a vertical surface such as a cliff or a wall. A typical office has an illuminance of 300 to 500 lux (or 28 to 46.5 foot-candles).
Luminance is a measurement of the amount of light leaving a surface in a particular direction, and can be thought of as the measured brightness of a surface as seen by the eye. Luminance is expressed in candelas per square metre (or per square foot) - ie Cd/m2 or Cd/ft2. A typical computer monitor has a luminance of about 100 or 150 Cd/m2. A figure of about 120 Cd/m2 is often found to be comfortable for normal interior daylight viewing conditions.
The difference between illuminance and luminance is more simply explained by example. If a common light source is used to illuminate two equidistant areas of a vertical wall, one painted white and the other matte black, the illuminance of each will be precisely the same. However, the white area of the wall will exhibit a higher luminance value because it reflects more light and hence appears much brighter to the eye.