A measure of the flux reflected or emitted from a surface. It has English units of footlamberts and metric units of Candelas/square meter. One definition of a footlambert outlines a perfect diffuser uniformly illuminated on one side by one footcandle to have a luminance or brightness of one footlambert on the other side. This perfect diffuser has no transmission loss and diffuses the light with a complete cosine characteristic in all directions. Such a surface is describes as Lambertian.

While such a diffuser does not exist, its characteristics give rise to a convenient means of relating illumination and luminance. Photometers with sensors or attachments calibrated in luminance units of footlamberts or candelas/square meter have a fixed field of view ranging from 1/4° to 15° or larger and are pointed at the surface to be measured. If the surface has a uniform brightness, the reading is independent of the distance from the surface, providing that the entire field of view is filled.

K-Light Laboratories owns a series of Pritchard Photometers with a variety of appertures and optics to accomodate almost any brighness reading application. Please call for a quotation on your particular need.