text: Under these criteria, we can propose a model for the fractions of incident light that are absorbed (), remitted (), and transmitted () by one representative layer.
  in which:
  •   is the fraction of cross-sectional surface area that is occupied by particles of type .
  •   is the effective absorption coefficient for particles of type .
  •   is the remission coefficient for particles of type .
  •   is the thickness of a particle of type  in the direction of the incident beam.
  • The summation is carried out over all of the distinct “types” of particle.
  In effect, represents the fraction of light that will interact with a particle of type , and and  quantify the likelihood of that interaction resulting in absorption and remission, respectively.
  Surface area fractions and volume fractions for each type of particle can be defined as follows:
  in which:
  •   is the mass fraction of particles of type i in the sample.
  •   is the fraction of occupied volume composed of particles of type i.
  •   is the fraction of particle surface area that is composed of particles of type i.
  •   is the fraction of total volume composed of particles of type i.
  •   is the fraction of cross-sectional surface area that is composed of particles of type i.
  •   is the density of particles of type i.
  •   is the void fraction of the sample.
  This is a logical way of relating the spectroscopic behavior of a “representative layer” to the properties of the individual particles that make up the layer.  The values of the absorption and remission coefficients represent a challenge in this modeling approach. Absorption is calculated from the fraction of light striking each type of particle and a “Beer’s law”-type calculation of the absorption by each type of particle, so the values of used should ideally model the ability of the particle to absorb light, independent of other processes (scattering, remission) that also occur.  We referred to this as the absorbing power in the section above.