OptLab-UW/SPX   Path Length Transformation

OptLab 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 Beer-Lambert law. However, it is necessary to take the surface reflectance value into consideration; this must be specified by the operator. OptLab allows 3 different principles to be used to enter this value:
–  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 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.

All parameters for thickness compensation – if activated – are elements of the OptLab method. The sample thickness can either be fixed or is requested by the software.