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FIGURE 6.29 A thin segment of a hemispherical concave refl ector would
form an image from a 180° strip fi eld of view onto optical fi bers.

the bidirectional reflectance distribution functions of aerosols, imag-
ing spectrometry, oceanography, and exploration of the planets.
The imaging optics would be a segment of concave hemispherical
reflecting surfaces placed slightly off center (Fig. 6.29). Like other
reflecting optics, it would be achromatic. The unique optical configu-
ration would practically eliminate geometric distortion of the image.
The optical structure could be fabricated and athermalized fairly eas-
ily in that it could be machined out of one or a few pieces of metal,
and the spherical reflecting surface could be finished by diamond
turning. In comparison, a camera sensor with a fish-eye lens, which
provides a nearly hemispherical field of view, exhibits distortion,
chromatism, and poor athermalization. The image would be formed
on a thin semicircular strip at half the radius of the sphere. A coherent
bundle of optical fibers would collect the light from this strip and
transfer the image to a linear or rectangular array of photodetectors
or to the entrance slit of an image spectrograph. Provided that the
input ends of the fibers were properly aimed, the cones of acceptance
of the fibers would act as aperture stops. Typically, the resulting width
of the effective aperture of the camera sensor would be about one-
third the focal length (f/3).