The whole discipline of optical measurement techniques can be roughly subdivided into the two areas of photometry and radiometry. Whereas the central problem of photometry is the determination of optical quantities closely related to the sensitivity of the human eye, radiometry deals with the measurement of physical radiations emitted by light sources or impinging on a particular surface. Photometry is in fact the measurement of light, which is defined as electromagnetic radiation which is detectable by the human eye. It is thus restricted to the wavelength range from about 380 to 780 nanometers. Photometry can be consider similar to radiometry except that every measurement is weighted by the spectral response of the eye. The only real difference between radiometry and photometry is that radiometry includes the entire optical radiation spectrum, while photometry is limited to the visible spectrum as defined by the response of the eye. Gigahertz-Optik manufactures light detectors for use in combination with optometers and accessories to form complete photometric measurement systems.
A photometric detector head should have (following DIN 5032 and CIE 69):
- a linear relation between photocurrent and absolute physical quantity, - a good V(λ) adjustement, - an appropriate cosine function.
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