The sensitivity of an ellipsometer is such that a change if film thickness of a few Angstroms is usually easy to detect. A schematic diagram of a nulling ellipsometer with the quarter-wave plate placed before the light is reflected from the sample. Figure 1 shows a schematic diagram of a basic nulling ellipsometer with a quarter-wave plate positioned before reflection. Here L represents an unpolarized monochromatic light source that is highly collimated with a small beam divergence. A low power Helium-Neon laser serves this purpose quite nicely.
Some lasers produce linearly polarized light.
When a polarized laser is used the light is circularly polarized just after it emerges from the laser using a quarter-wave plate. The polarizer follows the light source. This is a high quality Glan-Thompson prism mounted in a holder that can be rotated about an axis that is parallel to the direction of the light beam. The compensator follows the polarizer. A high quality compensator is the heart of a good nulling ellipsometer.