PerkinElmer Spotlight 400 Series, User Manual

Page: 92 / 120

92 . Spotlight 400 Series User’s Guide
Figure 36 Diffuse reflectance spectra of PMMA
Diffuse reflectance spectra of powders and pharmaceutical tablets can be collected in the
region 7800–4000 cm
−1
and to lower wavenumbers if required, using the Spotlight 400N
imaging system. These spectra generally do not suffer the non-linearity problems to the
same extent as the corresponding mid-infrared diffuse reflectance spectra in the
2000–1000 cm
−1
region.
Specular Reflectance
NOTE: Specular reflectance is mainly applicable to mid-infrared applications.
Specular reflectance is reflection in one direction (Figure 35). This is the type of reflection
that occurs from a smooth, polished surface.
Absorption information cannot be obtained directly from a specular reflectance spectrum of a
dielectric material, because the reflectance spectrum is governed by dispersion in the
refractive index. However, you can use the Kramers-Kronig integration to calculate the
absorbance spectrum from the specular reflectance spectrum. This integration is performed
by the Kramers-Kronig process in Spectrum.
Figure 37 Specular reflectance spectrum of a PMMA plate and the absorption
spectrum calculated from it with the Kramers-Kronig command