Item Brochure

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PRISMS PRISMS AND ASSEMBLIES
` Corner cube prisms (or retroreflectors) used with electronic distance measuring (EDM) instruments have an offset due to the
fact that the transmitted light beam from the EDM takes longer to enter and exit the prism. This longer time translates to a
longer distance measured. This distance is corrected by using an ‘offset’ and/or positioning the prism in relationship to the
plumb line of the prism holder. The offset is determined by multiplying the height of the prism by the refractive index of the
glass used. Common offsets are 0, -17.5 mm, -30 mm, -34 mm, and -40 mm.
` A nodal offset is defined by the relationship of the prism holder and prism that places the optical center of the prism on the
plumb line of the holder. This offset is the most advantageous at close range work when the apex or center of the prism is
used as a pointing target for vertical and horizontal angle measurement. Every prism has a nodal offset. Prisms and holders
that are not in the nodal position however, can cause angular errors when using the prism apex as a target if it is not perfectly
pointed toward the measuring instrument.
` To reduce pointing errors, several SECO prism assemblies are designed with a
nodal point alignment at -17.5 (for 25 mm prisms) or -40 mm offset (for 62 mm
prisms). See illustration at right.
` Commonly, the best reading of the beam is achieved when the prism face is
perpendicular to the incoming beam for a parallel return. Unfortunately, ideal
alignment is not always achieved in the field.
` To compensate for pointing errors, nodal point alignment minimizes the error
associated with misalignments (two illustrations below).
Nodal Offsets
Prism Range
Prism Holder Accuracy
Prism Offsets
` Corner cube prisms redirect the measuring beam back to the EDM for signal processing.
There are two main factors for good range measurement: prism diameter and beam deviation.
If the beam deviation is not accurate enough, the returned beam will miss the EDM entirely.
This occurs mainly near the maximum range of the EDM (check with your manufacturer).
SECO standard prisms are <5 seconds. Higher accuracy prisms are also available.
` Prism accuracy is determined by the physical location of the prism in its canister and then to its
holder. SECO prism accuracy is 1 mm or less.
` SECO uses a ZYGO GPI-XP/D interferometer to test and confirm the beam deviation of prisms
to ensure that they meet specifications. Certificates of compliance are available upon request.
Come see us for more details.
at -17.5 or -40 mm Offset
No movement
of the Nodal Point
Pivot Point
(Pole Center)
Line of
Sight
Non-Aligned at 0 mm Offset
Distinct movement
of the Nodal Point
Pivot Point
(Pole Center)
Line of
Sight
Non-Aligned at -30 mm Offset
Some movement
of the Nodal Point
Pivot Point
(Pole Center)
Line of
Sight
PRISM
basics