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Prism Basics: How Reflex and Anti-Reflex Coatings Affect Prism Reflection

surveying prisms

The degree of reflection of a surveying prism is defined as the ability of a material to reflect visible and infrared radiation. This capability depends on the material itself, as well as on the surface quality. To improve performance, reflex and anti-reflex coatings are often used. But what’s the difference between the coating types, and how do they affect prism reflection?

Reflex Coating

High-quality prisms are coated with copper because its degree of reflection is higher than 75% and it can be encapsulated within black epoxy to keep it from corroding. This guarantees a longer lifecycle of the prism glass body itself.

Anti-Reflex Coating

When measuring distance, a large percentage of the signal is returned through the prism, but besides the desired reflection through the prism body, the emitted EDM signal will also be reflected (typically 4%) by the front surface.

Besides the desired reflection through the prism body, the emitted EDM signal will also be reflected by the front surface of a prism

This part of the returned signal will disturb the desired signal because its travel time is shorter since it does not penetrate into the glass body of the prism.

In the above illustration, the signal reflected at the inner surface is in blue (about 4%), while the reflection caused at the front surface is shown in pink (less than 2%). The beam path of the expected incoming signal is in red and has the strongest intensity of about 70%.

This phenomenon can happen in close ranges assuming a very accurate alignment (the instrument’s line of sight must perpendicularly intersect the prism’s front surface). In this case, shorter distances are determined.

To avoid this phenomenon, a special layer of anti-reflex coating can be added to the front surface of the prism glass.

Not all prisms have this anti-reflex coating. If your prism does not have this coating – or if the coating is adjusted to the wrong wavelength – distance measurement errors of up to 3 mm can occur.

If you need help choosing the right prism or determining whether your prism is contributing to measurement errors, please contact us. Our knowledgeable experts will be glad to assist you.