Notes: All dimensionality is retained within the coefficients and
        constants of the formulas.  However, typically:

	Wavelengths is in microns.
	Refractive index is dimensionless.
	The thermal coefficient of refraction is in 10^(-6)/degree.
	Degrees are degrees C.

======================================================================
 Refractive Index Calculation
======================================================================

 A Sellmeier pair holds the coefficients for a single term:

			 B*lambda^2
		      ----------------
			lambda^2 - C
----------------------------------------------------------------------
 The Sellmeier formula is:

   n^2 - 1 = SUM( Sellmeier terms )

 whether the 3-term or 4-term variety is used.
----------------------------------------------------------------------
 Schott Technical Information note TI 19, 1988 ascribes a formula
 for the refractive index of air to Kohlrausch's "Praktische Physik",
 1968, Vol 1, page 408.

                         C*lambda^2       E*lambda^2
	n - 1 = B * ( A + -------------- + -------------- )
                        D*lambda^2 - 1   F*lambda^2 - 1

 In this formula the value of B is considered a non-variable, used to
 compensate for temperature and pressure variation:

                    C*lambda^2       E*lambda^2
	        A + -------------- + --------------
                  D*lambda^2 - 1   F*lambda^2 - 1             P
      n - 1 = ---------------------------------------     * -----
                        1 + At * (T - TO)                     P0

 The value of B would then be:

                       1             P
                ---------------- * -----
                1 + At * (T - TO)    P0

 for temperatures and pressures other than the reference.
----------------------------------------------------------------------
 The Schott formula is a double-ended truncated series.  The
 formula is:

	n^2 = SUM( terms )

 Only even powers of lambda are used, i.e.: the terms look like

		Ai * lambda^i

 where i is even.
----------------------------------------------------------------------
 The Conrady formula is:

	n = n0 + A/lambda + B/(lambda^3.5)
----------------------------------------------------------------------
 The Herzberger formula is:

	n = A + B*L + C*L^2 + D*lambda^2 + E*lambda^4 + F*lambda^6

 where:

	L = 1 / ( lambda^2 - 0.028 )	; 0.028 microns squared !
----------------------------------------------------------------------
 The Handbook of Optics formula one is:

	n^2 = A + B / ( lambda^2 - C ) - D*lambda^2
----------------------------------------------------------------------
 The Handbook of Optics formula two is:

	n^2 = A + B*lambda^2 / ( lambda^2 - C ) - D*lambda^2
----------------------------------------------------------------------
 The dispersion formula defines the type of dispersion formula used
 and the coefficients for that formula, as well as the minimum and
 maximum applicable wavelengths for the specific instance of that
 formula.

======================================================================
 Thermal Coefficient of Refractive Index Formulas
======================================================================

 The Sellmeier dndT formula is:

	dn/dT =
         ( n^2( lambda, T0 ) - 1 ) / ( 2 * n( lambda, T0 ) ) *
	   ( D0 + 2*D1*DeltaT + 3*D2*DeltaT^2 +
			(E0 + 2*E1*DeltaT)/(lambda^2 - lambdaTK^2) )

  where:

	DeltaT = T - T0
----------------------------------------------------------------------
 The Kohlrausch dndT formula is:

               -At * Bt * ( n( lambda, T, P ) - 1 )
	dn/dT =  ------------------------------------
                       1 + At * DeltaT

  where:

	DeltaT = T - T0

 As with the Kohlrausch dispersion formula, the name is taken from
 the Schott reference to "Praktische Physik".

 Note the implied dependence on pressure, as this formula is used
 for calculating the refractive index for air:

                           n( lambda, T0, P0 ) - 1      P
 n( lambda, T, P ) - 1 =  ------------------------- * -----
                               1 + At * DeltaT          P0

 where P0 is the reference pressure.  In this case, the pressure, P,
 could be considered to be in atmospheres, where an atmosphere is
 defined as 0.101325*10^(-6) Pascals.  The B coefficient here is
 included in the formula to allow for this pressure variation:

           -At * (P/P0) * ( n( lambda, T0, P0 ) - 1 )
   dn/dT = ------------------------------------------
                     (1 + At * DeltaT)^2

 by letting Bt = P/P0. This the type 2 dndT calculation.

 However, it is possible that the temperature compensating term
 has already been included in the calculation of the refractive
 index.  If this is the case, the type 1 dndT calculation is used:

           -At * (P/P0) * ( n( lambda, T, P0 ) - 1 )
   dn/dT = -----------------------------------------
                     1 + At * DeltaT

 Note that the pressure compensating term may be included in either
 the refractive index calculation or in the thermal coefficient, but
 not both.
----------------------------------------------------------------------
 The thermal coefficient formula defines the type of formula used
 and the coefficients for that formula.
----------------------------------------------------------------------