Fundamentals of reflection spectroscopy

There are two kinds of reflection to be distinguished specular reflection and diffuse reflection. 

If an electromagnetic wave in a vacuum, is incident on a medium having a refractive index n and a plane interface, at an angle a to the normal, then the wave will be partly or wholly reflected at an angle a to ad on the opposite side of the normal, where a = a (Fig. 1). However, it is also possible that part of the light penetrates into the medium and while doing so is refracted towards the normal and at an angle to the normal. Thus the index of refraction 

The specular reflection will not be dealt with here in detail, since it is unsuitable for the evaluation of reagent carriers. 

If it is no longer possible to differentiate between the phenomena of specular reflection, refraction and diffraction because the diameter d of the sample to be assayed is disproportionately greater than the wavelength X of the light (i.e. the electromagnetic radiation), the reflection must be considered to be diffuse. 

The diffuse reflection capacity depends on the properties of scatter and absorption of the particular sample. The property of scatter can be expressed by means of a coefficient of scatter S which is related to the unit of layer thickness, whereas the absorption capacity can be expressed by a coefficient 

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