1. Measurement of the Refractive Index of Highly Turbid Media
Samir Bali, Department of Physics, Miami University Oxford, OH
Turbid media refers to disordered media such as colloidal suspensions
that generate extensive multiple scattering of the incident light.
Turbidity is described by the attenuation coefficient  (see inset on right).
Examples of - values for visible/near-IR frequencies:
Milk 40 cm-1 (skim milk) <  < 125 cm-1 (whole milk)
Cream 500 cm-1 (‘half & half’) <  < 1200 cm-1 (whipping)
Biotissue, Crude Petroleum 800 cm-1 <  < 1500 cm-1
The attenuation coefficient  is directly related to the imaginary part of the
refractive index, and is determined by a transmission measurement as
indicated in the inset on right. The real part of the refractive index is the
“usual refractive index” arising from the bending of light at the surface, and
Is determined by measuring the critical angle for total internal reflection (TIR).
The problem is that for highly turbid media a) the critical angle does not exist, and b) almost no light transmits
through samples as thin as ~ 0.1mm. Based on a new theoretical model we developed for TIR we made a first
real-time simultaneous measurement of the real and imaginary parts of the refractive index of a turbid medium.
Key concept in Total Internal Reflection (TIR)
Our next goal is to apply our new understanding of TIR
and refractive index measurement in turbid media toward
a) detection of the onset of asphaltene precipitation
in undiluted petroleum fluids
and
b) a laser-based study of the early detection of
wax formation in fuels at low temperatures.
In TIR the reflected ray penetrates into the medium up to ~100 nm and scatters while inside. No penetration occurs in ordinary reflection.
Thus, in TIR Ir < Ii, yielding convenient measurement of the attenuation coefficient  in addition to the “usual” (i.e., real part of the) refractive index.
turbid medium
~ nm
Incident ray, Ii
Incident
Ray, Ii
TIR ray, Ir