OptLab-SPX calculates a number of different parameters from a transmission or a reflection spectrum. If the parameters of different materials with varying thickness are to be compared, it is necessary to transform the sample spectra to a nominal thickness.
The transmission spectrum of a non-turbid sample consists of the pure material transmission values and the reflectance values at the two surfaces. The transmission values can be adapted to a nominal thickness according to the Beers law.
However, it is necessary to take the surface reflectance value into consideration; this must be specified by the operator. OptLab-SPX allows 3 different principles to enter the surface reflectance data:
– Entry of a single reflectance value for the surface reflection. A single value does not take into account the wavelength dependency of the reflectance. Generally, however, the accuracy of this procedure is sufficient, since the reflectance value only influences the calculation indirectly.
– If the reflectance value is not available, the refractive index can be entered instead. OptLab-SPX generates the reflectance value on this basis for normal incidence. Nevertheless, this principle likewise does not take the wavelength dependency of the reflectance value into account.
– Entry of a data file containing the reflectance spectrum for the full wavelength range covered by the sample spectrum. This option assures highest accuracy for the thickness correction and is especially relevant if the sample spectrum spans a wide wavelength range, so that wavelength independence of the reflectance data cannot be assumed.
The exact calculation procedure is derived from the norm EN 410.
OptLab-SPX window for thickness correction
All parameters for thickness compensation – if activated – are elements of the OptLab-SPX method. The sample thickness can either be fixed or is requested by the software.