text: There is a simplified special case for the spectroscopic parameters of a sheet.  This sheet consists of three plane parallel layers (1:front surface, 2:interior, 3:rear surface) in which the surfaces both have the same remission fraction when illuminated from either direction, regardless of the relative refractive indices of the two media on either side of the surface. For the case of zero absorption in the interior, the total remission and transmission from the layer can be determined from the infinite series, where  is the remission from the surface:
  These formulas can be modified to account for absorption.  Alternatively, the spectroscopic parameters of a sheet (or slab) can be built up from the spectroscopic parameters of the individual pieces that compose the layer:  surface, interior, surface.  This can be done using an approach developed by Kubelka for treatment of inhomogeneous layers.  Using the example from the previous section: { , ,  } {, ,  }.
  We will assume the interior of the sheet is composed of a material that has Napierian absorption coefficient  of 0.5 cm, and the sheet is 1 mm thick ().  For this case, on a single trip through the interior, according to the Bouguer-Lambert law, , which according to our assumptions yields  and  .  Thus { , ,  }.
  Then one of Benford's equations can be applied.  If ,  and  are known for layer   and   and  are known for layer , the ART fractions for a sample composed of layer  and layer  are:
  :(The symbol  means the reflectance of layer  when the direction of illumination is antiparallel to that of the incident beam.  The difference in direction is important when dealing with inhomogeneous layers.  This consideration was added by Paul Kubelka in 1954.  He also pointed out that transmission was independent of the direction of illumination, but absorption and remission were not.)