Figure 3 shows an illustration of the principles used in the reflective detection system of a digital refractometer. This illustration shows how sources of light entering from different angles of the prism are reflected out of the prism when making contact with the sample solution. Each angle of light (A, B, and C) is shown entering and exiting the prism with a solid line that has an arrowhead pointing into the prism.

The drawing represents the incident light angle and the angle of reflection of light B with a dashed line.

The illustration also shows a gray dotted line that represents the boundary line between the light and dark fields used to measure the solution's refractive index.