2.7 Characteristics of Optical Detectors

2.7 Characteristics of Optical Detectors

An element which converts the electro-magnetic energy to an electric signal is called a detector. There are various types of detectors with respect to the detecting wavelength.

Figure 2.7.1 shows three types of detectors; photo emission type, optical excitation type and thermal effect type. Photo tube and photo multiplier tubes are the examples of the photo emission type which has sensitivity in the region from ultra violet to visible light. Figure 2.7.2 shows the response sensitivity of several photo tubes.

Photodiode, phototransistor, photo conductive detectors and linear array sensors (see 2.11), are examples of optical excitation types, which have sensitivity in the infrared region. Photo diode detectors utilize electric voltage from the excitation of electrons, while photo transistor and photo conductive detector utilize electric current. Table 2.7.1 shows the characteristics of these optical detectors with respect to type, temperature, range of wavelength, peak wavelength, sensitivity in term of D* and response time.

Thermocouple barometers and pyroelectric barometers are examples of the thermal effect type, which has sensitivity from near infrared to far infrared regions. However the response is not very high because of the thermal effect. Figure 2.7.3 shows the detectivity of the pyroelectric barometer.

Detectivity denoted as D* (termed D star) is usually related to the sensitivity, expressed as NEP (noise equivalent power). D* is used for comparison between different detectors. NEP is defined as the signal input identical to the noise output. NEP depends on the type of detector, surface of detector or band of frequency. D* is inversely proportional to NEP, and is given as follows.

D* : (Ad f ) / NEP
D* : detectivity (cm Hz / W)
NEP : noise equivalent power (W)
Ad : surface area of detector ( cm )
f : band of frequency ( Hz )