Live SupportMON-FRI 8AM-5PM EST

1-888-368-5461

 

  • My Cart

    Your cart is empty

    You have no items in your shopping cart.

Prism Basics: How Prism Glass Affects Signal Accuracy

surveying prism

The grinding of a prism glass plays a major role when reflecting signals. The more accurately the glass is ground (corner angles and surfaces), the more accurate and intense the signal. How can you tell whether your prism meets the basic standard?

The deviation between incoming and outgoing beams has a critical influence on the measurement range. The returning signal goes along a deviant angle. The figure below shows the deviation of the reflected signal after leaving the prism towards the instrument.

The distance measurement is based on the detection of the phase difference (or time of flight) between incoming and outgoing signals.
 
The distance measurement is based on the detection of the phase difference (or time of flight) between incoming and outgoing signals.

Typically, surveying prisms have a beam deviation of a few arc seconds. Prisms from major manufacturers like Leica Geosystems, Chicago Steel Tape (CST), and SECO have been checked after assembly to see whether the beam deviation falls below the threshold of one arc second and are certified.

prism beam deviation

The above example shows a test measurement of the beam deviation for a round prism taken with an interferometer. Interferometer measurements determine phase in homogeneity, how well polished a particular prism is.

In this example, the star-shaped pattern represents a slightly higher deviation than average caused by the prism’s edges. The round prism tested above has a maximum deviation of 0.8 arc seconds. This means that in every sixth of the prism’s glass body, the direction of the incoming beam differs less than 0.8 arc seconds compared to the outgoing beam.

If the glass edges are excluded, then the average values are far below one arc second.

Most prisms today are standardized, so there is not a big separation of quality between brands. If you have any questions about your prism performance or need help selecting a prism, please contact us.