handbook of modern sensors

420 14 Light Detectors
(A)
(B)
Fig. 14.13. Efficiency of a detector depends on its surface area a (A) or the area A of the
focusing system (B).
turn, is proportional to the receiver’s surface area. Figure 14.13B shows that the use
of a focusing lens can dramatically increase the area. The efficiency of a single lens
depends on its refractive index n. The overall improvement in the sensitivity can be
estimated by employing Eqs. (4.5) and (4.8) of Chapter 4:
[
k ≈ A ( ) ]
n − 1
2
1 − 2
, (14.15)
a n + 1
where A and a are effective areas of the lens and the sensing area of a photodetector,
respectively. For glasses and most plastics operating in the visible and near-infrared
spectral ranges, the equation can be simplified to
k ≈ 0.92 A a . (14.16)
It should be pointed out that the arbitrary placement of a lens may be more harmful
than helpful; that is, a lens system must be carefully planed to be effective. For
instance, many photodetectors have built-in lenses which are effective for parallel
rays. If an additional lens is introduced in front of such a detector, it will create
nonparallel rays at the input, resulting in the misalignment of the optical system and
poor performance. Thus, whenever additional optical devices need to be employed,
detector’s own optical properties must be considered.
14.4 Photoresistors
As a photodiode, a photoresistor is a photoconductive device. The most common materials
for its fabrication are cadmium sulfide 1 (CdS) and cadmium selenide (CdSe),
1 Information on CdS photoresistors is courtesy of Hamamatsu Photonics K.K.