Page 197 - Modern Optical Engineering The Design of Optical Systems

180 Chapter Nine

out by an internal diaphragm which is an accurate image of the objective
aperture. This “cold stop” is usually cooled and is located inside the
evacuated detector Dewar.
Since the stray radiation will appear to be coming from the wall, and
thus from outside the objective aperture, it will be imaged on the opaque
portion of a diaphragm which is located at an accurate image of the
objective aperture. Another glare stop could conceivably be located at
the exit pupil of this particular system, since it is real and accessible;
however, it would make visual use of the instrument quite inconvenient.
In most systems the aperture stop is located at or very near the
objective lens. This location gives the smallest possible diameter for
the objective, and since the objective is usually the most expensive
component (per inch of diameter), minimizing its diameter makes good
economic sense. In addition, there are often aberration considerations
which make this a desirable location. However, there are some systems,
such as scanners, where the need to minimize the size and weight of
the scanner mirror makes it necessary to put the stop or a pupil at the
scanner mirror rather than at the objective. This causes the objective
to be larger, more costly, and more difficult to design.
In an analogous manner, field stops for this system could be placed at
both internal images to further reduce stray radiation. The principle
here is straightforward. Once the primary field and aperture stops of
a system are determined, auxiliary stops may be located at images of
the primary stops to cut out glare. If the glare stops are accurately
located and are the same size as the images of the primary stops (or
slightly larger), they do not reduce the field or illumination, nor do
they introduce vignetting.
Baffles are often used to reduce the amount of radiation that
is reflected from walls, etc., in a system. Figure 9.6 shows a simple
radiometer consisting of a collector lens and a detector in a housing.
Assume that radiation from a powerful source (such as the sun) out-
side the field of view reflects from the inner walls of the mount on to the
detector and obscures the measurement of radiation from the desired
target, as shown in the upper half of the sketch. Under these condi-
tions, there is no possibility of using an internal glare stop (since there
is no internal image of the entrance pupil) and the internal walls of the
mount must be baffled as shown in the lower half of the sketch (although
an external hood or sunshade could also be used if circumstances
permit).
The key to the efficient use of baffles is to arrange them so that no
part of the detector can “see” a surface which is directly illuminated.
The method of laying out a set of baffles is illustrated in Fig. 9.7. The
dotted lines from the rim of the lens to the edge of the detector indicate the
necessary clearance space, into which the baffles cannot intrude without