EDM Orange Surveying Reflector Prism 64mm Diamater 0 Constant
Model: AK18-1
AK18-1 Specifications:
Prism Constant: 0mm/-30mm
Type: Single Prism
Effective Diameter: 64mm
Weight: 1Kg
Used For: Total Station (Sokkia, Nikon, Trimble, Pentax, Topcon etc.)
Package: Soft Carrying Bag
Target Plate: Metal
Category: Optics Instruments Accessory
Thread Type: 5/8x11 female thread
Surveying Prism Applications
Optical Survey prisms are a specially designed retro reflector, specifically a corner reflector, that is used to reflect the Electronic Distance Measurement (EDM) beam from a total station. A survey prism reflects the EDM beam back to its source with both a wide angle of incidence and with high precision. Prisms reduce the scatter of the beam as it is reflected back to the total which allows for both a more accurate measurement and a longer range that the measurement can be made.
Survey prisms are used by surveyors and engineers to measure the change in position of a target that is assumed to be moving. Survey prisms are used for a wide range of monitoring applications including:
- Rail Monitoring
- Settlement Monitoring
- Displacement Monitoring
- Deformation Monitoring
- Convergence Monitoring
Survey Prism Offsets
Retro-reflectors (or corner cube prisms) used with electronic distance measuring (EDM) instruments have an offset because 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 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 on a surveying prism 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 surveying prism has a nodal offset. Surveying 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.
For high accuracy surveying it is recommended to use a round prism instead of a 360 degree prism if you are using a robotic total station. A Nodal Point prism should be used for high accuracy traversing and layout work.
Prism Measuring Accuracy
The range of a prism results from, among other things, its coating and the glass geometry. Measurement errors occur frequently at close ranges when prisms without anti-reflex coating are used, as the front of a prism always directly reflects a certain percentage of a signal. Errors occur in the use of prisms without copper coating on the back side when moisture droplets accumulate or the prism becomes foggy.
High standard quality prisms have a special coating on the reflective surfaces – the Anti-Reflex Coating, and a copper coating on the reverse side. Without these, the range of distance measuring, ATR and Powersearch would be reduced by up to 30%. The workmanship and the durability of the copper coating are decisive for a long life.
The glass dimensions, the position in the holder and with it the areal orientation, are important for measuring accuracy.
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