Illuminance can be measured with the help of a photometer, which comprises a photo-electric sensor or cell and a display unit. The sensor is designed to filter the incident rays of light so that the sensitivity of the photometer corresponds to that of the human eye (Vλ-Vl analysis; spectral luminous efficiency of the human eye).
Illuminance photometers are categorised according to the quality classes L, A, B and C pursuant to DIN 5032, Part 7. Illuminance photometry can also be used to measure luminous intensity (luminous intensity distribution curves) and luminance.
When using such equipment it is important to ensure that the reading is corrected to take account of the angle of incidence of the light, as illuminance is a factor of the cosine of that angle (cosine-corrected reading) or that the angle of incidence is always 90°.