Application Notes:

Liquid Prism Measurements

Recently there has been interest in measuring the refractive index of liquids. Interest is driven by the semiconductor industry where liquid immersion lithography has become a popular way of printing smaller features on silicon wafers, and by the biological
community interested in knowing the index of various aqueous fluids for studies of biological processes at the liquid-solid interface.

While it is possible to pour liquid into a cup and measure the reflected light from the surface as if the liquid were a solid piece of glass, there are some important complications when measuring liquids. For example, sample alignment can be more difficult for
liquids, and external vibrations can cause the surface to ripple. These complications introduce uncertainty in the measured angle of incidence and loss of light at the
detector due to scattering from a rippled surface. Also, since only a surface reflection is measured, the refractive index is generally only good to 2 or 3 decimal places since light does not pass through a bulk quantity of the liquid. This lack of optical path length also results in lower sensitivity to small k-values which determine absorption in the liquid.

Very accurate index measurements of “bulk” materials like glass and liquids are done using the prism minimum deviation technique. Figure 1 shows the principle of the minimum deviation method. Light is passed through a prism placed on a rotation stage. The stage is rotated until the angles of the entrance and exiting beams are equal. This is the minimum deviation condition. This technique is often used by glass manufacturers to determine the refractive index of various glass prisms. With accurate control of temperature, minimum deviation measurements can be accurate to better than 5 decimal places. Measuring the
intensity of light passing through the prism at two different path lengths allows for accurate determination of the extinction coefficient, k.

Figure 1. Geometry of prism minimum deviation measurement. Fluid is placed in a hollow prism and the deviation angle of the transmitted beam is measured.

Figure 3. Refractive index of water at 21.5° C measured on the VUV-VASE. NIST reference values from Burnett are also shown. Excellent agreement was achieved with the NISTvalues.