Page 336 - Modern Optical Engineering The Design of Optical Systems

Optical System Layout 315

Figure 13.18 Radiometer with field lens to increase the
field of view with a small detector.

is when the image of the objective aperture is the same size as the
detector and

s A
1 ( )
s D
2
This arrangement not only makes a larger field angle possible, but
has the advantage of providing an even illumination over a large por-
tion of the detector surface. Most detectors vary in sensitivity from
point to point over their surface; with a field lens of focal length given by

s 1s 2
f
s s
1 2
the same area of the detector is illuminated regardless of where the
source is imaged in the field of view. Field lenses and immersion
lenses are frequently combined. Note that the insertion of a field lens
in a radiometer does not change the limitations of Eqs. 13.21 and
13.22; it simply permits the use of an objective system with a low
numerical aperture by raising the numerical aperture at the detector.
Another device to enlarge the field of view of a radiometer with a
small detector is the light pipe, or cone channel condenser. In Fig. 13.19,
a principal ray from the objective is shown being reflected from the walls
of a tapered light pipe. Note that without the light pipe, the ray would
completely miss the detector.

Figure 13.19 The action of a
reflecting light pipe in increasing
the field of view of a radiometer.