Light lux
The sensitivity of photocells can be quoted in either of two ways: as the output at a given illumination, using illumination figures in units of lux, often 50 lux and 1000 lux; or as a figure of power falling on the cell per cm2 of sensitive area, a quantity known as irradiance. The lux figures for illumination are those obtained by using photometers, and a figure of 50 lux corresponds to a ‘normal’ domestic lighting level good enough for reading a newspaper. The level of illumination required for close inspection work and the reading of fine print is 1000 lux; on this scale, direct sunlight registers at about 100 000 lux. The use of mWjcm2 looks more comprehensible to anyone brought upwith electronics, but there is no simple direct con- version between power per cm2 and lux unless other quantities such as spectral composition of light are maintained constant. For the range of wavelengths used in photocells, however, you will often see the approximate figure of 1 mWjcm2 � 200 lux.
Another important point relating to the use of photocells is their wavelength at peak sensitivity. For many types of sensors, this may be biased to either the red or the violet end of the visible spectrum, and some sensors will have their peak response for invisible radiation either in the infrared or the ultraviolet. A few devices, notably some silicon photodiodes, have their peak sensitivity for the same colour as the peak sensitivity of the human eye. This makes the devices more suitable for use in automated processes that once involved visual inspection, but such replacements are not always successful. The reason is that although the peak sensitivity of the sensor may match that of the eye, the sensitivity at other colours may not follow the same pattern as that of the eye. In general, most sensors are more sensitive than the eye to colours at the extremes of the spectrum, and if a photosensor is to be used in applications such as colour matching, then filters will have to be placed in the light path to make the response curve over the whole spectrum match that of the eye more evenly. Figures 3.5 and 3.6 show some spectral response curves for selenium and for photoresistive cells, respectively.