refractive index increment

in polymer chemistry
The change of the solution refractive index, n, with solute concentration, C, i.e. ∂ n ∂ C.
  1. The solute concentration is most frequently expressed in terms of mass concentration, molality or volume fraction. If expressed in terms of mass concentration or molality, the corresponding refractive index increments are referred to as specific or molalrefractive index increments, respectively.
  2. Following use of the full name, the abbreviated name refractive increment may be used.
Purple Book, p. 65