Description
    ===========
      From [CARLSONETAL1992]:
 
      The Photometric Calibration Target (PCT) and an associated optical
      element, the Photometric Calibration Mirror (PCM) together form a
      source of diffusely reflected solar radiation which can be used by
      the remote sensing experiments for intra- and inter-instrument
      comparisons. These two elements are mounted on the Science Boom; the
      mirror reflects solar radiation onto the diffusing target surface
      which is placed outboard from the mirror and in a position that can
      be viewed from the scan platform. In order that the target be
      illuminated over the nominal range of solar cone angles, the mirror
      is convex, with a radius of curvature of 46 cm and located 53 cm
      from the target. The reflecting surface is vacuum deposited
      aluminum, with a protective overcoat formed by its natural oxide.
      The target surface is similar to the Voyager diffuser plate,
      consisting of sand blasted aluminum. The combination produces a
      spectrally gray diffuse surface, with an effective albedo of roughly
      0.05.  They were calibrated over the spectral range of 0.3 to 5.2
      micrometers and for a variety of incidence angles.