2015 CATALOG

PRISMS
Prism Offsets
PRISMbasics
`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.
Nodal Offsets
`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).
Line of
Sight
at -17.5 or -40 mm Offset
No movement
of the Nodal Point
Pivot Point
(Pole Center)
3
Some movement
of the Nodal Point
Distinct movement
of the Nodal Point
Line of
Sight
Line of
Sight
Pivot Point
(Pole Center)
Pivot Point
(Pole Center)
Non-Aligned at -30 mm Offset
Non-Aligned at 0 mm Offset
Prism Range
`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