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Spectral Response

The spectral response describes the sensitivity of the photosensor to optical radiation of different wavelengths. This is important because only a small part of the optical radiation spectrum is visible.

The photocells used in photosensors are sensitive to a wider range of wavelengths than what the human eye sees. In other words, photocells respond to portions of the ultraviolet (UV) and infrared (IR) spectrum as well as the visible spectrum. Filters limit the sensitivity to UV and IR radiation. These filters are typically incorporated into the photocell element, although filtering can be done anywhere in the optical path of the photosensor.

For a system that has only one type of light source (for example a windowless interior office with fluorescent lighting), an exact spectral match between the photosensor's spectral response and the human eye response is not as important. A simple multiplying factor can be applied to the photosensor signal to make it correspond to the light level. For a system with two spectrally different light sources (for example daylight and electric light), a simple multiplying factor is not sufficient because the spectrum at the photosensor changes as the relative contribution of each source changes. With only two sources, the multiplying factor changes in a systematic way as the light level changes, so it is possible to account for this changing factor in the photosensor control algorithm. Both the open-loop and closed-loop proportional control algorithms can do this. With more than two spectrally different light sources (for example, adding incandescent task lighting to daylight and fluorescent lighting) no known method of correction exists to correct for a less than ideal spectral response.

More information on light and optical radiation.

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