Prism Research Report

ABSTRACT

This study presents an overview of the technical considerations involved with making a prism selection for deformation monitoring in a mining environment to ensure the highest quality results. We will investigate the unique design aspects which influence the performance of a prism and which prism attributes best suit the mining conditions.

INTRODUCTION

Prisms are an important part of the measuring process. Typically, spatial professionals widely accept that they are an accurate, reliable accessory without much consideration of the influence prism have on the quality of their measurements. Obtaining a high level of accuracy and reliability requires consideration of all possible effects on the measurements. A strong emphasis is placed on the specifications and accuracies of the total station, as it should be, but too often the considerations for prism selection are non-existent or fall back to a fiscal decision.

There are various applications where the required 3D accuracies are in the magnitude of centimetres but tasks such as deformation monitoring require much higher accuracies, in the order of millimetres. To achieve this level of accuracy and reliability we must look at the errors associated with prism selection and the associated effects in the mining environment.

REFLECTOR TYPES

Most spatial professionals have encountered three typical types of reflectors. Round prism types in varied diameters, 360° prisms and multiples of prism banks.

360° prisms have been developed for use with robotic total stations for increased tracking abilities but do not work well with stationary targets as with deformation monitoring where the EDM is used in a single burst. Multiple banks of prisms will increase the amount of signal available for retro reflection of the EDM, useful if you are only acquiring a distance, but multiple prisms disrupt the ability to make repeatable angular measurements resulting in scattered 3D accuracies. The best reflector type for deformation monitoring is a single 64mm ? round prism, aligned perpendicular to the total station. In scenarios where the reflector and/or total station is adversely affected by ambient light or condensation early in the morning, considerations should be made to include a hood on the prisms to increase repeatability of the measurements.

REFLECTOR ACCURACIES

When a prism is manufactured a piece of glass is ground to establish reflective angles. The quality of the grinding

plays a key role when reflecting signals, the more accurate and angle is achieved the better the signal will be

returned in the same direction. The intensity of the return signal is also stronger, a crucial point for the mining

environment given the climatic conditions.

The deviation between incoming and outgoing beams, known as beam deviation or the stated accuracy, has a critical influence on the measurement range.

Figure 2 depicts the effects of poor beam divergence from a prism with a greater accuracy of 2 arc seconds. The same amount of signal makes it too the prism as this is a function of the total station but none of the signal is returned or retro reflected from the prism to the receiver optics resulting in measurement failure.

To test and certify a prism at a stated accuracy you must test all six aspects of the glass reflection and qualify the largest rate of divergence as the prism beam divergence. If a prism is stated at 1.5” seconds then all six aspects have performed at or better than 1.5” of divergence.

Figure 3 shows the results from an Interferometer measurement taken to determine the homogeneity of all aspects of a prism.

REFELCTIVE COATINGS

The ability of a prism to return an emitted signal is greatly affected by the reflective qualities of medium or material.

While glass ground at angle provides a reflection, prisms benefit from a reflective coating such as gold silver or copper by as much as 30%. The application of this reflective coating enhances the ability of the prism to return a signal with sufficient intensity, hence increasing the operating range of the prism.

Figure 4 shows the reflective qualities of gold, silver, copper and aluminium coatings at varying wavelengths. Leica instruments used in the field testing of prisms operate between 0.66µm and 0.85µm frequencies. If you plot this wavelength on the chart you will see there is a minimal difference between the performance of gold, silver and copper, all operating at approximately 95%.

For ease of identification Pangea decided to utilise the copper coating which is clearly visible to the naked eye on the translucent glass. It is more difficult to confirm the presence of a silver coating.