574
R. W. Kessler and W. Kessler
Photons which are directly (specular) reflected from the surface of the pellet can
hardly uptake the chemical information “red” as no or very little penetration of
the photon takes place, and the probability of absorption (= inelastic interaction) is
therefore low. However, the spatial information is high as the reflected photon carries
more or less the exact spatial information with high resolution. On the other hand,
if the pellet is illuminated, e.g., at 0 angle degree with parallel polarization and the
scattered photon is detected also at 0 degree but measured with a crossed polarizer,
the probability is high to detect only diffuse reflected photons. This means, these
photons have migrated deeply into the system and have thus a high chance to carry
the chemical information “red.” However, due to the photon diffusion into the pellet
and statistical scatter several times, the photon loses its spatial coordinates and thus
a blurred picture is observed from the diffusely scattered photons.
Figure 23.10 left and right side shows the result of the 2 different optical setups.
A more grayish color can be seen in the setup measuring the specular reflectance
(left). And a more reddish color is visible in case of the setup measuring the diffuse
reflectance (right), but this view is a bit blurred. More details of the pellets and the
spectra in the UV–Vis and NIR region are discussed in [51].
Due to a scratch on the surface, the photons are specular reflected off axis
compared to the measurement optical axis. Thus, the intensity of the reflected light,
which is captured by the detector, decreases, and the scratch appears a little bit darker.
When measured with mixed polarization to identify, e.g., the active ingredient of a
tablet, the lower intensity may then be misinterpreted as a region of higher absorbance
and therewith an increase in concentration of the API. The bottom images can be
viewed by red-green glasses in three dimensions. The specular reflectance picture
(left) shows clear structures, whereas the diffuse reflectance picture (right) is blurred
and shows the three-dimensional photon diffusion.
Another possibility to exclude the specular reflectance is to illuminate the sample
with a so-called Lambertian illuminator (see discussion below).
23.5.2.2 Multiple Scattering and Penetration Depth
The probability of photons penetrating an absorbing material is dominated by the
absorption and the scattering [1]. The prerequisite for scattering to occur is a difference in the refractive indices of the material and its surroundings of at least n =
0.1. In air, this difference is so great that most of the organic and inorganic powders
scatter strongly. In organic media, this is not always the case, as the differences in
the refractive indices become smaller.
The penetration depth depends primarily on the scattering coefficient S and the
absorption coefficient K of the sample [1, 40, 47, 52–54]. The scattering is determined
by the size, packing density, and relative refractive index (i.e., indirectly, e.g., also
by the water content) of the scattering particles. The absorption of visible light is
caused by chromophoric groups, while in the near-infrared, OH and CH harmonics
are mainly responsible for the scattering.
R. W. Kessler and W. Kessler
Photons which are directly (specular) reflected from the surface of the pellet can
hardly uptake the chemical information “red” as no or very little penetration of
the photon takes place, and the probability of absorption (= inelastic interaction) is
therefore low. However, the spatial information is high as the reflected photon carries
more or less the exact spatial information with high resolution. On the other hand,
if the pellet is illuminated, e.g., at 0 angle degree with parallel polarization and the
scattered photon is detected also at 0 degree but measured with a crossed polarizer,
the probability is high to detect only diffuse reflected photons. This means, these
photons have migrated deeply into the system and have thus a high chance to carry
the chemical information “red.” However, due to the photon diffusion into the pellet
and statistical scatter several times, the photon loses its spatial coordinates and thus
a blurred picture is observed from the diffusely scattered photons.
Figure 23.10 left and right side shows the result of the 2 different optical setups.
A more grayish color can be seen in the setup measuring the specular reflectance
(left). And a more reddish color is visible in case of the setup measuring the diffuse
reflectance (right), but this view is a bit blurred. More details of the pellets and the
spectra in the UV–Vis and NIR region are discussed in [51].
Due to a scratch on the surface, the photons are specular reflected off axis
compared to the measurement optical axis. Thus, the intensity of the reflected light,
which is captured by the detector, decreases, and the scratch appears a little bit darker.
When measured with mixed polarization to identify, e.g., the active ingredient of a
tablet, the lower intensity may then be misinterpreted as a region of higher absorbance
and therewith an increase in concentration of the API. The bottom images can be
viewed by red-green glasses in three dimensions. The specular reflectance picture
(left) shows clear structures, whereas the diffuse reflectance picture (right) is blurred
and shows the three-dimensional photon diffusion.
Another possibility to exclude the specular reflectance is to illuminate the sample
with a so-called Lambertian illuminator (see discussion below).
23.5.2.2 Multiple Scattering and Penetration Depth
The probability of photons penetrating an absorbing material is dominated by the
absorption and the scattering [1]. The prerequisite for scattering to occur is a difference in the refractive indices of the material and its surroundings of at least n =
0.1. In air, this difference is so great that most of the organic and inorganic powders
scatter strongly. In organic media, this is not always the case, as the differences in
the refractive indices become smaller.
The penetration depth depends primarily on the scattering coefficient S and the
absorption coefficient K of the sample [1, 40, 47, 52–54]. The scattering is determined
by the size, packing density, and relative refractive index (i.e., indirectly, e.g., also
by the water content) of the scattering particles. The absorption of visible light is
caused by chromophoric groups, while in the near-infrared, OH and CH harmonics
are mainly responsible for the scattering.