Abbe refractometer measures a material’s refractive index through determining the critical angle αc under total internal reflection conditions.
As shown in above figure, B is the specimen with a refractive index (nB) to be tested. nA is the well-known refractive index of the measurement prism. Thus the refractive index nB of the sample can be easily calculated by the following equation under total internal reflection condition.
ac can be measured by CCD calculated based on the designed optical geometry. nA is known. Therefore nB can be measured and read out through modern display. Example is shown in the following figure.
Refractive index is wavelength dependent; typically it decreases with increasing incident light wavelength. Thus the refractive index is measured with monochromatic light. The common practice is to use the sodium D-line wavelength 589 nm. The refractive index measured at this wavelength is usually denoted by nD. Other available filters in the market have the wavelength of 450nm, 680nm, 1100nm, 1550nm etc. The temperature is usually 20 °C or 25 °C for the measurement.