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In the vicinity of an absorption band the refractive index varies considerably, an effect known as
anomalous dispersion. So a reflection spectrum is formed because the refractive index changes around
an absorption band.
Figure 7: Spectrum exhibiting anomalous dispersion
Kramers-Kronig Transform
In order to produce a spectrum that is corrected for this anomalous dispersion, a correction known as a
Kramers-Kronig transformation may be performed on the spectrum. This transformation attempts to
produce a spectrum that does not contain any derivative peak shapes. The raw data and the result of a
Kramers-Kronig transform are shown below.
Figure 8: Reflectance spectrum of Poly (Methyl Methacrylate)
Absorption
Absorption Wavelength
Band
Band
Re
frac
tiv
e I
nd
ex
