Figure 2 shows an illustration of the principles used in the transparent detection system of a hand-held refractometer. The far left of the illustration shows the workings of light refraction inside a prism. Two dotted lines in the prism portray the way light is refracted out of the prism through a sample solution, with one angle showing small refraction (line b) and the other showing large refraction (line a). The top line that radiates out of the prism represents a small refraction of light due to a solution with a higher salt content, while the bottom line represents a large refraction of light due to a lower salt content in the solution.

As the light travels through the lens (drawn just to the right of the prism in the illustration) it is radiated further to a scale face, which catches the light from the two different refraction angles and measures the level of refraction for the solutions. The line marked "a" shows the large refraction is caught near the bottom of the scale face, while line "b" shows the small refraction of light that is caught near the top of the scale face.

Just to the right of the scale face and to the far right of the illustration are two drawings that each show a field of the refraction of light on an arbitrary scale from 0 to 20. The refractions are measured from the boundary lines between the light and dark markings on the scale. The first field, representing the measurement found for a large refraction, shows the boundary line falling just above 0. The second field, representing the measurement found for a small refraction, shows the boundary line placed directly below the 20.