The Fresnel rhomb converts the linearly polarized light to circularly polarized light. Rotation of the linear polarizer 45 to the appropriate Fresnel rhomb axis induces the generation of left or right polarized light.

Figure 4. Apparatus for Measuring Circular Dichroism

A modern form of the CD spectrometer is shown diagrammatically in figure 5.

Figure 5. The Basic CD Spectrometer

Referring to figure 5, light from a broad emission source is passed through a chopper to provide an AC output from the sensor. This procedure is carried out because small AC signals are easier to amplify than small DC signals and also reduces the noise level and consequently increases the sensitivity.. The light is then passed through a monochrometer that allows light of a specific wavelength to be selected. The selected monochromatic light passes through a filter and then through a polarizer that is usually a Rochon prism. The light then passes through a photo elastic modulator (called a Pockels Cell). An explanation of the function of the Pockels Cell is as follows.

The alternate production of left or right circularly polarized light is called polarization modulation. A beam of linearly polarized light can be considered as the resultant of two orthogonal linear light beams each in phase with one another. Consider a block of isotropic fused silica that is rendered bi-refringent by pressure exerted along the (x) or (y) axis as shown in figure 6.