In photometry, illuminance's the total luminous flux incident on a surface, per unit area. It's a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance's the luminous flux per unit area emitted from a surface. Luminous emittance's also known as luminous exitance. In SI derived units, these are both measured in lux (lx) or lumens per square metre (cd·sr·m−2). In th… (More on Illuminance)