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Daylight contains much more than just visible light, however our eyes are only sensitive to a specific band of wavelengths. The intensity of visible light, corrected for the eyes' varying sensitivity to colour is measured in Lux. Wavelengths of daylight that the eye is not sensitive to do not contribute to a measurement of Lux.
The day-night cycle results from the earth's rotation. A consequence of this is a relatively quick decline of light at dusk. The proportion of visible light to other wavelengths varies not only on a diurnal cycle, but also seasonally and is dependent on prevailing weather conditions.
This means that only direct measurement of visible light level can accurately reflect the level of light that our eyes see as a result of daylight. This poses a problem when sensors used in PECUs have marked sensitivity to wavelengths of daylight outside of those visible to our eyes.
Although very accurate, some sensors may not be accurate at measuring Lux. A particular issue is Infra-Red (IR). Daylight contains significant infra-red waves however these are strongly affected by atmospheric conditions and can be highly attenuated in conditions where visible light is much less affected. As a result IR levels can be much lower than those anticipated by the level of visible light.
Many semiconductor sensors have marked sensitivity to IR as well as visible light; hence a PECU with significant IR sensitivity could switch on before reaching the target visible light level (in Lux). Sensors with IR sensitivity tend to underestimate equivalent visible light levels in daylight rather than over-estimating them, hence they always tend to operate in a ‘safe' manner.
Also see
To find out more about the Zodion Lighting Controls range of Photo Electric Control Units, please browse our catalogue pages.
Our PECUs range includes:
If you have any questions, please contact us on 01422 317 317 or fill in our contact form we will be delighted to answer any questions you might have on PECUs.
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