Item Brochure

26
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
PRISMS
` 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.
product shipped from the US warehouse only