If the instrument supports an
      ambient measurement capability, then it will be used. If the
      insrument does not have an ambient mode, then an emissive
      measurement mode can be used, although typically many illuminants
      are too bright to directly point the instrument at. A work-around
      is to reflect the illuminant from a spectrally flat white surface.
      A good candidate for this is a piece of white, fine textured
      polystyrene foam. [The suitability of a reflector can be checked
      using 
spotread -S to
      check that the reflection characteristic is close to flat.]
      
      
 
  
   
    This is done by placing the paper
      such that it is uniformly illuminated with reasonable brightness,
      and then placing the instrument so that it receives the reflected
      light from the paper. This is typically achieved by placing the
      instrument close to the paper at about 45º, so that it's aperture
      has a clear view of the illuminated paper, but avoiding shadowing
      the region that is in view, and also avoiding measuring specular
      reflection if the paper is glossy.
      
      
