chAPter | 28 Optical Measurements 509 response accurately following a cosine law; that is, a cate- gory (2) measurement. Some "measurements" in category (1) are only required to determine the presence or absence of light, as in a very large number of industrial photoelectric controls and counters, burglar alarms, street-lighting controls, and so on. The only critical points here are that the detector should be arranged so that it receives only light from the intended source and that it shall have sufficient response speed. Photodiodes and phototransistors are often used and are robust, reliable, and cheap. Cadmium sulfide photoconductive cells are often used in lighting controls, but their long response time (about 1 s) restricts their use in many other applications. 28.5.1 Photometers A typical photometer head for measurements of illuminance (the amount of visible light per square meter falling on a plane) is shown in Figure 28.13. incident light falls on the opal glass cylinder A, and some reaches the detector surface C after passing through the filter layer B. The detector may be of either the photoconductive or photovoltaic type. The filter B is arranged to have a spectral transmission character- istic such that the detector-filter combination has a spectral instruments of this kind are available at a wide range of prices, depending on the accuracy demanded; the best instru- ments of this type can achieve an accuracy of 1 or 2 percent of the illuminance of normal light sources. Better accuracy can be achieved with the use of a dresler filter, which is built up from a mosaic of different filters rather than a single layer, but this is considerably more expensive. A widely used instrument is the Hagner photometer, which is a combined instrument for measuring both illuminances and luminances. The "illuminance" part is as described pre- viously, but the measuring head also incorporates a telescopic optical system focused on to a separate, internal detector. A beam divider enables the operator to look through the tele- scope and point the instrument at the surface whose lumi- nance is required; the internal detector output is indicated on a meter, also arranged to be within the field of view. The use of silicon detectors in photometers offers the possibility of measuring radiation of wavelengths above those of the visible spectrum, which is usually regarded as extending from 380 to 770 nm. Many silicon detectors will operate at wavelengths up to 1170 nm. in view of the inter- est in the 700­1100 nm region for the purposes of fiber optic communications, a variety of dual-function "photometer/ radiometer" instruments are available. Basically these are photometers that are equipped with two interchangeable