Language of Light

3.Instrumentation

3.2 PHOTOMETER

3.2.3.3 LUMINOUS FLUX METER

Luminous flux measurement is to determine the total visible energy emitted by a light source. An integrating sphere is often used to converge all the power emitted by the source to the detector head.

The integrating sphere has to be large enough to encompass the light source being measured, and as a general rule, the larger the sphere, the smaller the errors in measuring luminous flux for different light sources. As a rough example, calibrating a 1.5m tubular lamp in a 2.5m diameter sphere against a small incandescent standard will produce half the error that would result from calibration the same lamp in a 2m sphere. Calibration of such integrating sphere can be carried out by means of transfer lamp standards which are traceable to recognised national standards. A good quality integrating sphere which postulates the performance of an ideally spherical, evenly coated interior requires a huge investment and usually have to be customised to the light measurement application. Hence, the existence of a general purpose luminous flux meter is very limited.