Page 504 - The Mechatronics Handbook

0066_frame_C19 Page 126 Wednesday, January 9, 2002 5:32 PM

TABLE 19.6 The Characteristics of a Number of Different Types of Light Detector
Response Acceptance
Description Active Region Response Spectral Response Dark Current Time Angle
−1
Medium 41.3 mm 2 0.5 A W 800 nm peak, range 4 nA 25 ns NA
area silicon peak 350–1100 nm
photodiode
Ultra high 0.5 mm 2 0.35 A W −1 800 nm peak, range 10 nA 1 ns NA
speed silicon 400–1000 nm
photodiode
Filtered silicon 7.5 mm 2 7 nA lux −1 560 nm peak, range 2 nA 3.5 µs 100°
photodiode 460–750 nm
16 photodiode Each diode 0.6 A W −1 900 nm peak, range 0.1 nA 4 ns NA
array on 0.66 mm 2 400–1100 nm
1 mm pitch
Silicon 0.7 mm 2 9 µA lux −1 880 nm peak, range 0.3 µA 15 µs 30°
phototransistor 450–1100 nm
Silicon 0.7 mm 2 2 µA lux −1 880 nm peak, range 0.3 µA 15 µs 80°
phototransistor 450–1100 nm
CdS 6.3 mm dia. 9 kΩ at 10 lux, 530 nm peak NA 100 ms NA
photoconductor 400 Ω at
1000 lux

Incident
FIGURE 19.105 A simple phototransistor light detec-
radiation
tor circuit is shown. Photon-generated current flowing +V
in the base-collector diode may be amplified several hun-
Base not
dred times by transistor action. Although the photon- connected
generated current is much larger than in an equivalent Output
voltage
photodiode, response time of the phototransistor is R L
much longer.

In this photoconductive mode, the current through the photodiode varies linearly with light irradiance.
The dark current i 0 varies rapidly with temperature and limits the sensitivity of the device but the photo-
conductive mode generally has faster response, better stability, and wider dynamic range than the photovoltaic
−1
mode. The responsivity K D of the detector is defined by the relation i p = K D P D and is less than 1 A W for
a silicon diode. In the ideal case, K D varies linearly with wavelength, according to Eq. (19.80), up to the
threshold value set by the energy gap. Photodiodes are available with a wide variety of characteristics
differing in sensitivity (area), speed of response, spectral response, and acceptance angle. They are available
with single devices or multiple devices (quad, linear array) in a single package.
The output signals from photodiodes needs amplification for many applications. This may be provided
by a separate amplifier or by providing internal gain as in the phototransistor. This is constructed so that
radiation can fall on the base region of the transistor and the resulting base current is then internally
amplified. Often there is no external connection to the base and the amplified photocurrent is monitored
using the simple circuit shown in Fig. 19.105. A typical phototransistor has a responsivity several hundred
times higher than that of a photodiode but the frequency response is relatively poor. Phototransistors
are often integrated with a spectrally matched LED into a single sensor package to act as a proximity
sensor, as in end-of-tape sensors, coin detectors, and level sensors. For reference, the characteristics of
several different types of discrete light detector are listed in Table 19.6.
By fabricating many small light detectors in a closely spaced array, it is possible to measure light
intensity at an array of points over a region. This is ideal for electronic imaging applications involving
video and still cameras. Image sensors designed for this purpose are discussed in the section titled ‘‘Image
Sensors,” but first it is useful to consider the formation of the images which the detectors sense.

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