You cannot make an accurate measurement without a good quality prism. While the prism has a relatively simple job in the process of electronic distance measurement &#; to reflect a signal &#; its ability to do this well depends on a number of factors. The quality of the materials used and the care taken in the production process have a huge impact on its accuracy and its ability to withstand harsh conditions on a job site. Here we look at the way reflectors in the Leica Geosystems Originals Accessories range are created to explain why choosing the right prism matters.
Click here to get more.
A good prism reflects back as much of the signal it receives as possible. To do this, it must be well-made, and well-protected so its accuracy isn&#;t affected by its environment. It should be able to cope with the distances that you need, for example, short or long distances require different coatings. And it needs to be so robust that it can drop from the height of a pole and still be as good as new. Leica Geosystems prisms are manufactured from glass of the highest quality, using sophisticated production techniques, strict assembly and quality control. Let&#;s look at the difference that makes to a prism&#;s accuracy and working life.
A prism is made from a glass block. The way that prism glass is manufactured plays a major role when reflecting signals. The accuracy of the cutting, grinding, polishing and mounting of the glass prisms directly influences how much EDM signal is returned from the reflector to the total station. The more accurate the corner angles and surfaces of the glass are ground, 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 miss the instrument and the distance cannot be measured.
How well a prism reflects the infrared EDM signal depends on the material it&#;s made of and its surface quality. Leica Geosystems&#; reflectors are coated with copper because it offers the best reflectivity across the range of EDM wavelengths used by Leica Geosystems total stations (660nm &#; 850nm), it&#;s a robust material and it doesn&#;t corrode, thanks to an appropriate lacquer layer on top of the copper coating. We also use an anti-reflex coating to prevent measurement errors at close range. Prisms without anti-reflex coating often cause errors at short distances because the front of a prism always directly reflects a certain percentage of a signal and could create a false reading.
Apart from the high-quality prism, an important factor when it comes to accurate surveying is what we call centering accuracy. Centering accuracy is the alignment and proper placement of the prism in the prism holder and the mounting stud. There is a high risk of measurement errors when original prism holders are not used, as these &#;Leica-like&#; holders are not configured according to Leica Geosystems standards.
Unless a prism unit is properly sealed, its surrounding environment can enter and destroy its ability to reflect, making it blind. Often prisms are left out in the field for a long time, especially if they are being used for monitoring, so they need to withstand harsh conditions. For example, one of our total station customers reported that their reflectors, which were attached to a railway line, couldn&#;t be measured to. We investigated and found that the reflectors were poor copies of Leica Geosystems Originals Accessories prisms and didn&#;t have the protective layers we use. Weeds and vegetation are prevented from growing in the area of railway lines by frequently spraying along the line with powerful weed-killers. The weedkiller 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 coating layer. An adhesive coating fixes the copper coating to the glass, a protective coating protects the copper layer, a lacquer seals all layers and finally, an antireflective layer in front adds an additional cover from environmental influences to the prism to ensure a long life.
Believe it or not, it takes six months to manufacture a 360° reflector from a glass block, a journey that includes 90 individual processes. The 360° prisms are designed for use with state-of-the-art robotic total stations which have automated technology that locks on to a prism without the operator at the pole needing to re-align it. Omni-directional, they consist of six triple prism-glass bodies, tightly assembled in a patented process.
Why does it take so long? Firstly, glass pyramids are created from a glass cube by cutting and then grinding the six sides of the cube to an angular accuracy of less than eight seconds. They are then polished to achieve a surface that is flat to a 2-digit nanometre &#; that&#;s one billionth of a metre &#; by hand, using optical contact bonding. Triangular pyramids of the same size are then ground from the cube and are subject to the same polishing. Next, the glass pyramids have layers of coating applied in a &#;clean-room&#; in a vacuum, under specific climatic conditions. They are then glued together using a slow-drying adhesive. The entire assembly process takes two weeks, during which the hardening process takes the most time. Next, the glass is mounted into the rubber, enhanced with a carbon or metal rod, and then becomes fully recognisable. Before a production lot leaves us, a sample is subject to a series of strict quality tests &#; one of which is the pole drop test.
It&#;s a fact of life that prisms will get dropped on the floor. Surveyors lean poles against walls, walk away and the pole falls over. It needs to survive intact and to make sure ours do, the 2m pole-drop test is one of the many quality tests at Leica Geosystems. The video below shows a drop test, which was conducted following our standard rules: the pole is 2m tall, it is allowed to fall from a vertical position and the fall is onto a hard wood surface placed on concrete. In the video three reflectors are tested: the Leica GRZ4 (360° reflector used for &#;normal&#; one-person survey work), the Leica GRZ122 (360° reflector strengthened for use as a SmartPole) and a copy of a Leica GRZ122. Watch the difference that Leica protection gives.
Using poor quality accessories can affect your work. A badly constructed prism will prevent you from getting accurate measurements, regardless of the quality or the cost of your total station. Even for surveying tasks that can tolerate a broader range of accuracy, in centimetres rather than millimetres, poor quality prisms won&#;t be useful for long because of their fragility.
Leica Geosystems customer service teams investigate any reports from customers that a Leica survey solution is not giving accurate results. We often find that the problem is being caused by third party accessories, many claiming to be &#;Leica-like&#;. This was the case for one of our customers who reported the 360° reflector they were using was not of the quality they expected from a Leica Geosystems accessory.
They said it was breaking apart when dropped from a height of only 1.3 metres. When we received the reflector for testing, we found it was a fake; the glass coatings were of poor quality and the glue was weak. Even the carbon rod that goes through the prism and is used for stabilisation was not built in, leading to this easy breaking. Unfortunately, the customer had bought from a seller on eBay and wasn&#;t able to get a refund. What seemed like a good deal was too good to be true &#; they lost money and valuable time on their project.
An original Leica Geosystems prism will always have a unique security code on the box that guarantees that what you&#;re buying is part of the Leica Geosystems complete solution. You can verify a product by entering its code online at the Leica Geosystems MyWorld portal or by scanning the QR code using any QR code reader. A genuine Leica Geosystems accessory comes with an exchange and replacement parts guarantee to give you peace of mind. For some products, we even guarantee to replace parts even if the product should be discontinued.
Find out here more information about Leica Geosystems prisms and reflectors.
All Leica Geosystems accessories follow high-quality manufacturing processes to ensure the best results in the field. Interested to know how the Leica Geosystems tripods are made and how they compare to non-original ones? Read here our blog about tripods and tribrachs!
Gerhard Soenser
Product Manager Accessories
Leica Geosystems AG
You will get efficient and thoughtful service from Hongsheng.
One of the most common images for any that has study optics is that of Sir Isaac Newton with a beam of white light going through a glass prism and a rainbow coming out on the other side. It is one of the most famous optical experiments, not only for its simplicity, but also because it helped Newton set the foundations for his corpuscular theory of light.
That small prism that Newton was using is just one of many applications that prisms have in many optical systems. In the article we will try to describe the four types of prisms: Dispersive (like the one Newton was using), Reflective, rotation and displacements. As well as some consideration on the manufacturing of prisms.
A dispersive prism is an optical element used to break up light into its different wavelength components &#; a phenomenon discovered by Sir Isaac Newton. By doing this, the prism separates light of varying wavelengths, with longer wavelengths (red) deflecting at a lesser angle than the shorter ones (violet). This phenomenon occurs because the prism&#;s refractive index varies by the wavelengths of the light.
The design of a dispersive prism is critical to its functionality and can significantly impact the quality of the light separation. This article will discuss essential parameters for designing a dispersive prism and provide guidelines for creating an effective and efficient device.
Figure 1. Solar light being disperse by a quartz prism. Image from KelvinsongDispersive prisms are used in a variety of scientific and technical applications, such as spectroscopy, where they are used to analyze the composition of materials based on their spectral signature. They are also used in optics and telecommunications, where they can be used to control the dispersion of light and correct chromatic aberration in lenses.
The dispersion of a prism is typically measured as the angular separation between two spectral lines of a particular wavelength. The following formula, which assumes a thin-prism, can be used to calculate the dispersion of a prism:
D = (n &#; 1) * A
where:
D = dispersion of the prism (in degrees)
n = refractive index of the prism material
A = apex angle of the prism (in degrees)
In conclusion, designing a dispersive prism involves considering several critical factors, including the choice of material, the shape of the prism, the angle of incidence, the size of the prism, and the coating. By carefully considering these factors, it is possible to design an effective and efficient dispersive prism that provides high-quality light separation.
Reflective prisms can be used in imaging systems. Due to the total internal reflection, light entering the prism can undergo multiple reflections until they reach an output face. It is possible to add a reflective surface so the prism behaves as a beam splitter. Reflective prisms are used to reduce the physical size of an optical system, to redirect the direction of light, and to reform the orientation of an image.
Reflective prisms present lower optical power losses than equivalent systems made with mirrors and are usually easier to align due to the fact that a single element is used instead of several.
Figure 1 shows one of the most common reflective prisms geometries. In general, if the number of reflective faces is even, we will be creating an upright image, while an even number of reflective surfaces will create an inverted image.
Different types of prisms and configurations
Right angle prisms are usually used to deviate the direction of light by 90-degrees. It&#;s possible to use a right angle prism in a Porro configuration when light is incident through the prism&#;s hypotenuse. Light will be deflected 180-degrees and flipped.
Dove prisms are right angle prisms with their top part removed. They can be used to invert images. It is possible to coat the side where light is reflected for optical sensing applications.
Right-angle roof prisms are usually used in binoculars or when a right angle deflection of an image is required. The image is deflected left-to-right not top-to-bottom.
A pentagonal roof prism, deviates the beam 90-degrees without deflection left-to-right or top-to-bottom.
Rhomboid Prisms create an output beam that is displaced from the input beam, but it doesn&#;t change the direction of the beam, nor does it invert the image.
Porro prisms (either stand-alone or in higher-degrees configurations) are usually used to change the orientation of an image. They are usually used as erectors in optical instruments like binoculars, telescopes, and microscopes where there are space restrictions. The degree of a porro system will depend on how many axes the image needs to be altered in
Wedge prisms: Have a shallow angle and can be used together for beam steering in a Risley prim pair
An interesting application of prisms is the change of the incident beam dimensions. This is caused exclusively by the geometry of the prism (e.g. the angle of the incident vs refracted faces), and not the focusing elements or collimating effects like in a lens. Anamorphic lenses are usually configured in pairs to keep the beam traveling along the optical axis.
The manufacturing of prisms usually involves several steps. Starting with the chosen glass, a series of cuts are done to form a basic prism shape. This stage usually ends in a rough draft of the final product. The prism will have the shape requested but of poor optical performance.
After that, a series of polishing and smoothing steps of the optical surfaces are needed. This can take several iterations depending on the optical tolerances requested by the client and their application. At this stage, antireflection coatings, filters, and metallic layers can be added to achieve the required performance.
A technician is in charge of supervising and evaluating each stage. Some prism geometries can be bought off-the-shelf but for specific applications or custom made optics, it usually requires a considerable amount of time for testing and manufacturing.
Please let us know in the comments if you have had the need to use custom prisms and what was your application.
If you want to learn more, please visit our website optical prism.