Explaining Some Light Scattering Properties of Milk Using Representative Layer Theory

Abstract

Milk is an example of a strongly scattering material, as its white colour indicates. For non-scattering samples, the Beer-Lambert law can be used to compute an absorption coefficient for a material and this absorption coefficient can be used to calculate or predict the absorption for a sample of any thickness of that material. However, absorption coefficients calculated for scattering samples are less directly applicable to other samples of the same material, because the processes of absorption and scattering affect each other. To overcome this, “absorbance” for a scattering sample should not be defined as {log(1/ T)}, but as {-log( R+ T)} or {-log(1- A)}. Interactions between absorption and scattering can be understood through consideration of a layer of single particles, here termed a “representative layer”. A reasonable approximation for the “Beer's law absorbance” of a material is the {-log(1- A)} of the representative layer. Using the properties of the representative layer, the absorption and scattering properties of a sample can be understood based on the refractive index difference between the particles and the matrix, the size of the particles, the wavelength of the incident light, the concentration of the particles and the thickness of the sample. This review describes how the principles of representative layer theory can explain some of the light scattering properties of milk and examines several of the techniques used to separate the effects of absorption and scatter.