Why choosing the right prism is so important?

An accurate measurement cannot be made without a good quality prism.

While the prism has a relatively simple job in the process of electronic distance measurement (reflecting a signal), its ability to do it well depends on several factors. The quality of the materials used and the care that is put into the production process have a major impact on its precision and its ability to withstand the harsh conditions of the job site. Below we look at how reflectors from the Leica Geosystems range of original accessories are created to explain why choosing the right prism is important.

What makes a prism better than other prisms?

A good prism reflects the largest amount of signal it receives. To do this, it must be well made and well protected so that its accuracy is not affected by its surroundings. You need to be able to cope with the distances you need, for example short or long distances require different coatings. And it must be so robust that it can be dropped from the height of a pole and still be as good as new. Leica Geosystems prisms are manufactured from the highest quality glass, using sophisticated production techniques, strict assembly and quality control. Let’s look at the difference that makes the accuracy and life of a prism.

1. Prism alignment to avoid beam drift
A prism is made from a block of glass. The way prismatic glass is made plays an important role in reflecting signals. The precision of cutting, grinding, polishing, and mounting the glass prisms directly influences the amount of EDM signal that is returned from the reflector to the total station. The more precise the angles of the corners and surfaces of the glass, the better the signal will be returned in the same direction and the better its intensity. If the angle of the prism is only a few seconds out of specification, the returned signal will not be picked up by the instrument and the distance cannot be measured.

2. Special coatings to improve performance: copper and anti-reflective
How well a prism reflects the infrared EDM signal depends on the material it is made of and the quality of its surface. Leica Geosystems reflectors are coated with copper because it offers the best reflectivity in the entire range of EDM wavelengths used by Leica Geosystems total stations (660nm – 850nm), it is a robust material and it does not corrode, thanks to a coating of suitable lacquer on top of the copper coating. We also use an anti-reflective coating to prevent measurement errors at close range. Prisms without an antireflection coating often cause errors at close ranges because the front of a prism always directly reflects a certain percentage of a signal and could create a false reading.

3. Centering accuracy
Apart from high-quality prism, an important factor when it comes to accurate surveying is what we call centering accuracy. Centering accuracy is the proper alignment and placement of the prism on the prism bracket and mounting bolt. There is a high risk of measurement errors when the original prism mounts are not used, as these “Leica-like” mounts are not configured according to Leica Geosystems standards.

4. Waterproofing to prevent “reflector blindness”
Unless a prism unit is properly sealed, the surrounding environment can enter and destroy its reflective ability, blinding it. Prisms are often left in the field for a long time, especially if they are used for monitoring, so they have to withstand tough conditions. For example, one of our total station customers reported that his reflectors, which were attached to a railway line, could not be measured. We investigated and found that the reflectors were poor copies of original Leica Geosystems accessory prisms and did not have the protective coatings we use. Weeds and vegetation are prevented from growing in the area of the railway tracks by frequent spraying along the line with strong herbicides. The herbicide had eaten away at the prisms and killed their reflectivity.
A Leica Geosystems prism actually includes three layers of protection, in addition to its copper clad layer. An adhesive layer fixes the copper layer to the glass, a protective layer protects the copper layer, a lacquer seals all layers, and finally, an anti-reflective layer on the front adds additional cover from environmental influences to the prism to ensure a long life. .

Manufacturing of a Leica 360° reflector: 90 processes over six months

Believe it or not, it takes six months to make a 360° reflector from a block of glass, a journey that includes 90 individual processes. The 360° prisms are designed for use with state-of-the-art robotic total stations that have automated technology that locks into a prism without the need for realignment by the operator on the pole. Omnidirectional, they consist of six glass bodies of three prisms, firmly assembled in a proprietary process.

Why it takes so long? First, glass pyramids are created from a glass cube by cutting and then crushing all six sides of the cube with an angular precision of less than eight seconds. They are then polished to a surface that is flat to a 2-digit nanometer, ie one billionth of a meter, by hand using optical contact bonding. Triangular pyramids of the same size are ground from the cube and undergo the same polishing. The glass pyramids then have coating layers applied in a vacuum “clean room”, under specific climatic conditions. They are then glued on with a slow-drying adhesive. The entire assembly process takes two weeks, during which the hardening process takes the longest. Next, the glass is mounted to the rubber, enhanced with a carbon or metal rod, and then it becomes fully recognizable. Before a production batch leaves us, a sample undergoes a series of stringent quality tests, one of which is the pole drop test.

The 2 m prism drop prism test

It is a reality that the prisms fall to the ground. The surveyors lean the poles against the walls, walk away, and the pole falls over. It needs to survive intact and to make sure ours does, the 2m pole drop test is one of many quality tests at Leica Geosystems. The video below shows a drop test, which was performed following our standard rules: the pole is 2m tall, it is allowed to fall from a vertical position, and the drop is onto a hardwood surface laid on concrete. Three reflectors are tested in the video: the Leica GRZ4 (360° reflector used for “normal” one person survey work), the Leica GRZ122 (360° reflector reinforced for use as a SmartPole) and a copy of a Leica GRZ122 . See the difference that Leica protection offers.

Risks of using a poor prism and how to spot a fake

The use of poor quality accessories can affect your work. A poorly constructed prism will prevent you from getting accurate measurements, regardless of the quality or cost of your total station. Even for surveying tasks that can tolerate a wider range of accuracy, in centimeters rather than millimeters, poor quality prisms won’t be useful for long due to their brittleness.

Leica Geosystems customer service teams investigate any customer reports that a Leica survey solution is not delivering accurate results. We often find that the problem is caused by third-party accessories, many of which claim to be “Leica-like”. This was the case with one of our customers who reported that the 360° reflector they were using was not of the quality they expected from a Leica Geosystems accessory. Leica Geosystems Original Accessory QR Code

They said it was breaking when it fell from a height of only 1.3 meters. When we received the reflector for testing, we found it to be a fake; the glass liners were of poor quality and the glue was weak. Even the carbon rod that goes through the prism and is used for stabilization was not built in, making it easy to break. Unfortunately, the customer had purchased from an eBay seller and was unable to obtain a refund. What seemed like a good deal was too good to be true: they lost valuable time and money on their project.

An original Leica Geosystems prism will always have a unique security code on the box ensuring that what you are buying is part of the complete Leica Geosystems solution. You can verify a product by entering its code online in the Leica Geosystems MyWorld portal or by scanning the QR code with any QR code reader. A genuine Leica Geosystems accessory comes with an exchange guarantee and replacement parts to give you peace of mind. For some products, we even guarantee parts replacement even if the product has to be discontinued.

Author: Gerhard Soenser

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