What is the difference between foot-candles(fc) and foot-lamberts(fL) or lux and candela/m² (cd/m²)?
A. Fc and lux are illuminance measurements (photometric flux per unit area) typically taken using a cosine corrector or a detector with a broad field of view. Illuminance varies in inverse proportion to the square of the distance. FL and cd/m2 are luminance measurements (flux density per solid angle) typically taken with a small aperture or small viewing area. Luminance measurements are independent of distance as the sampled area increases with distance.
