Refractive Index Measurement
Light is electromagnetic radiation of wavelength about 450–700 nm, in which region the human eye is sensitive and refractive index measurements are commonly made. The refractive index or indices of a substance describe an important part of its interaction with electromagnetic radiation. Refractive index is a dimensionless quantity, real for transparent materials and complex if there is absorption. It generally depends on the direction of light relative to axes of the material; such substances, for example, many crystals, are anisotropic and possess more than one refractive index. In general, the index is a tensor (3 × 3 matrix) with up to nine components. Many substances are isotropic, for example, liquids, glasses, and other noncrystalline materials, and one refractive index is sufficient. Some substances are optically inhomogeneous and possess refractive index fluctuations over distances comparable in size with a wavelength. They scatter light and appear milky or turbid. Many foodstuffs and drinks are optically heterogeneous. Refractometry is, fortunately, a robust technique in the face of material complexities. It has widespread application to many industries and materials. This chapter treats the refractive index as the principal measurand; the complications of real materials may require extra attention. Excluded ones are those strongly absorbing materials such as metals in which absorption are more important than refraction. The appropriate measurement technique is then ellipsometry, in which the amplitude and phase of the reflected light are measured, as the polarization and the angle of incidence is varied. Some refractometric methods are essentially ellipsometric, but for transparent samples transmission methods are also important. Refractive index measurements of high precision, for example, 10−4–10−5, are possible with relatively simple techniques. Although ocular instruments remain popular, semiconductor technology has been applied to refractive index measurement in recent years, improving the measurement speed and precision. One of the limits to improvement in precision is the sample’s absorption or heterogeneity, a factor considered herein. Several online instruments exist, and the application of refractive index measurements to complex industrial fluids has been recently described . Fiber-optic methods do not yet possess the precision of conventional methods but offer the possibility of remote sensing.