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Optical System Layout 311

Figure 13.15 (a) A Fresnel lens is shown with the equivalent lens from
which it is derived. Each annular zone of the Fresnel lens has the same
surface slope as the corresponding zone of the lens. (b) The split-prism
rangefinder of a 35-mm SLR camera splits an out-of-focus image in two
by means of oppositely oriented wedge prisms in its central zone. If the
image is focused on the wedge surface, it is not deviated or split. The area
surrounding the split prism is comprised of tiny pyramidal prisms which
break up an out-of-focus image and exaggerate its blur. The outer zone is
a Fresnel lens acting as a field lens for the camera viewfinder.

13.7 Radiometer and Detector Optics
A radiometer is a device for measuring the radiation from a source.
In a simple form, it may consist of an objective lens (or mirror) which
collects the radiation from the source and images it on the sensitive
surface of a detector capable of converting the incident radiation into
an electrical signal. A “chopper,” which may be as simple as a minia-
ture fan blade, is usually interposed in front of the detector to provide
an alternating signal for the benefit of the electronic circuitry which
must amplify and process the detector output.