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TABLE 19.5 The Characteristics of a Number of Different Types of Light Source
Electrical
Description Size Input Light Output View Angle Spectral Type
Ultra-bright yellow LED 10 mm dia. 20 mA, 2.1 V 14 cd 4° Peak at 590 nm
Infrared GaAlAs LED 5 mm dia. 0.1 A, 1.9 V 16 mW sr −1 80° Peak at 880 nm
Infrared LED 5 mm dia. 0.1 A, 1.9 V 135 mW sr −1 8° Peak at 880 nm
Small filament lamp 11 mm dia. 6 V, 0.3 A 11 lm 360° Black body
Miniature fluorescent tube 300 × 16 mm dia. 8 W 480 lm 360° White
Standard fluorescent tube 1500 × 26 mm dia. 58 W 4800 lm 360° White
Tungsten halogen dichroic 51 mm dia. 12 V, 20 W 3300 cd 12° 3000 K
Tungsten halogen dichroic 51 mm dia. 12 V, 20 W 460 cd 36° 3000 K

FIGURE 19.101 A collection of light detectors is shown. Along the top row from left to right are four silicon
2
photodiodes with areas of 1, 5, 41.3, and 7.5 mm , the last with a photometric color correction filter. Along the
middle row from left to right are a CdS photoresistor, a pyroelectric detector, a phototransistor, and a quadrant silicon
photodiode containing four separate sensing elements. Along the bottom row from left to right are a 256-element
linear CCD, a 64-element charge integrating CMOS array, and a 16-element linear silicon photodiode in a 24-pin
d.i.l. package. The diode pitch is 1 mm.

emitting surface; lamp size and operating temperature. Table 19.5 lists characteristics of a number of
common types of light sources taken from the lamp suppliers data sheets.

Light Detectors
A light detector converts the radiant power it absorbs into a change of a device parameter such as
resistance, surface charge, current, or voltage. A number of light detectors are shown in Fig. 19.101. Some
signal conditioning electronics may also be needed to convert the basic output from the detector into a
more useful voltage signal, for example, for digitization by an analog-to-digital converter (ADC). This
may be integrated into the detector or require external components. Light detectors can be divided into
two main types, thermal or photon devices. In thermal detectors, the heating effect of the absorbed
radiation results in a change in a temperature dependent parameter, such as electrical resistance (in
bolometers) or thermoelectric emf (in thermopiles). The output of thermal detectors is usually propor-
tional to the radiant power absorbed in the detector, and provided the absorption efficiency is the same
at all wavelengths, the output is independent of wavelength. The most widely used type of thermal
detector is the pyroelectric detector, which is discussed in the next section. Photon detectors, in contrast
to thermal detectors, depend on the generation of free charge by the absorption of individual photons.
This photon-induced charge causes a change in device resistance, in the case of photoresistors, or an

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