2. Measurement of Light with Integral Detectors
Spectroradiometry – the measurement of radiation intensity as a function of wavelength – is the only way to provide full spectral information about optical radiation emitted by a light source or radiation which impinges on a surface. Until quite recently, spectroradiometers tended to be highly sophisticated optical measurement devices requiring relatively complicated calibration, operation and maintenance. Modern designs such as the Gigahertz-Optik MSC15, BTS256-LED Tester and BTS2048-VL-TEC make spectroradiometric measurements much more straightforward. However, for many applications (e. g. UV light measurement) integral detectors offer an economical and user friendly alternative. The term “integral” describes the fact that the output signal of an integral detector is proportional to the wavelength integral over the measured quantity’s spectral distribution which is then multiplied with the detector’s spectral sensitivity. Such detectors are applied in cases where determining the exact spectral distribution of the measured quantity is not necessary. Ideally, such cases require a detector that is specially designed to match a certain predefined spectral sensitivity function. As an example, the spectral sensitivity of photometric detectors is matched to the CIE spectral luminous sensitivity function V(λ) whereas detectors for solar UV irradiance, which is potentially harmful to the human skin, are matched to the CIE erythema action spectrum. Since integral detectors provide just a single output signal (usually voltage or photocurrent), they are much easier to characterize than spectroradiometers. The main parameters determining the usability and quality of an integral detector are: