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Prism Basics: Understanding Prism Constant

Leica GMP112 prism

A prism constant is a given property of the particular prism model that defines the relation of the distance measurement to the mechanical reference point of the prism (the holder). In principal, the prism constant is unvarying, and as long as it is considered, it should have no influence on the measurement. So why is it important?

Whenever measurements to prisms are made, independent of a distance or angle measurement, they must be referenced between two points: the standing axis on the instrument and the target point.

In order to guarantee a well-defined reference of the prism center to the designated point, it is important to consider the mechanical design of the prism body including the housing and mounting stud. Remember: prisms are still mainly made of glass, which has a different refraction index compared to air – the medium in which measurement signals normally spread out.

The most common definition of a prism constant is this: The propagation velocity of an electromagnetic wave as it enters a glass body is decelerated; therefore, the actual measured distance is extended. For all light beams perpendicular to the prism front, the length of the optical path is W = n x d.

d = the distance from the front surface of the prism to the corner point of the triple prism.

n = the index of refraction of the glass body.

W = the distance from the prism front surface to the theoretical reversal point.

Most prisms are mounted in a way to ensure a minimal effect on the angle and distance measurements in case the reflectors are not aligned perpendicular to the instrument’s line of sight.

Though a distance error from an inaccurate alignment of the prism reflector is normally negligible, an accurate alignment is still highly recommended in order to achieve a maximum degree of efficiency of the prism surface reflecting the incoming EDM signal.

So how does the prism constant affect your prism selection? Look for prisms with high-quality glass and precise assembly, along with an accurate determination of the refractive index. These prisms will have a highly reliable constant and negligible variations over time.