A substantially neutral filter across the white-light spectrum is obtained
by minimizing the wavelength-dependent effects of filtering components
and of reflective coatings on surfaces. The incoming light is captured by
a first prism and directed toward the interface formed by the optical
contact of a second prism with a face of the first prism, where the light
is partially reflected internally and partially transmitted into the
second prism. The reflected light is further reflected internally and
directed toward the image surface out of the first prism. The fraction of
light transmitted into the second prism is reflected back and redirected
out of the system toward the incoming beam. The amount of energy directed
toward the image plane is controlled by the judicious selection of the
refractive indices in the two materials at the contacting interface of
the first and second prisms. The second prism consists of a
retro-reflector element adapted to reflect substantially all light
transmitted at the interface with the first prism back along the optical
path of the incoming beam.