I would like to do a deep dive into the subject of prisms.
Do we have any resident experts on here on the subject?
Nodal vs non-nodal, 360's vs circular, mini vs full size, prism centering accuracy specs, is ATR affected by prism selection, has anyone directly compared knock offs to the genuine counterpart, how is centering error tested, why do some manufactures quote angular accuracy for their 360s and ignore centering and others quote centering and ignore angular, what exactly is "beam deviation" and how does that affect accuracy?
I'm in meetings right now, but here's a good paper that's been floating around for a while:
Leica has a white paper on the subject. They provide data only on their own products, but they do provide a lot of food for thought.
You could get some readings from the metrology side of the house. They don’t use the term prism but SMR. In Williamsburg VA there is a place that does a lot of this they write the software for spatial analyzer. There is a man’s there I cannot for the life of me remember his name but is a LSA genius. I had all of my smr’s aka prisms calibrated and certified through there program when I was doing metrology measurements. I had him take 3 seco prisms two brand new and one that was used and had a bubble in it like most do in every day surveying trucks. Some would be surprised at the effects of a cloudy or slight deformity in an everyday prism has on the true constant or precision. Even things not seen by the naked eye. I had a prism smr that I could not use on certain jobs because it was not good enough and it looked brand new. I could not see anything wrong with it but through the testing it showed an issue. Nothing we as surveyors would really care about but it was enough I couldn’t use it to align certain items.
Nice.
Very interesting, what are we talking here? 1mm? 3mm? More?
is ATR affected by prism selection, has anyone directly compared knock offs to the genuine counterpart,
When running in passive mode, I can't tell any difference between the typical prisms I use, at least not for tracking purposes. The instrument tracks a standard 62mm diameter prism as well as it tracks a peanut or mini prism.
I have compared the generic Seco (and other) brand prisms to the high-accuracy traverse kit prisms, and there is definitely a difference. It's not always apparent, but if you're setting up on very tight control and swap between the two you'll absolutely see the effects of that less-precise centering and generic coatings (see below for more on that).
why do some manufactures quote angular accuracy for their 360s and ignore centering and others quote centering and ignore angular,
360 prisms are rarely a single prism, but almost always an array of multiple prisms, either spaced out like for Trimble or aligned right next to each other like for Leica. I don't know for sure, but strongly suspect that if there are multiple prisms involved, it's hard to apply the required tests for centering accuracy, and easier to just test angular accuracy and quote that instead. Especially since 360s are not intended for high-accuracy work.
One thing I did not see mentioned was copper vs silver and anti-reflective coating. Here's this skinny on that:
https://geospatial.trimble.com/en/resources/blog/why-silver-vs-copper-prism-coating-matters
Long story short, there's a reason manufacturers want you to use their prisms, because they are optimized for the EDM wavelengths that their instruments use.
"Nodal vs non-nodal" - Nodal prisms have the yoke mounting point at the optical center of the prism. For a standard size 62mm prism this usually means a prism constant of -40mm. The standard Leica circular prism has an almost nodal constant of -34.4mm. My favorite Leica GMP101 mini prism has a nodal constant of -16.9. Different diameter prisms will have a different prism constant to achieve a nodal mounting point. Nodal prism mounting means that (within reason) the distance measurement and the target lock will stay on point even if the prism is not aimed directly at the total station. Non-nodal prisms usually have the yoke mounting point behind the optical center of the prism, a little bit on a standard size -30mm prism and a lot on a 0mm prism. If a non-nodal prism is not carefully pointed at the total station, the distance measurement can be affected a little, and the automatic target lock (angular) measurement can be affected significantly. If an instrument operator of a manual total station is taking their sight on a target plate rather than the prism glass, the angular error is moot. Many firms in the past would standardize all their prisms on 0mm constant to avoid blunders in switching between prisms. Today with easy prism selection on the data collector screen, and increased reliance on automatic target lock, everyone should be using nodal prisms.
"360’s vs circular" - As the above quoted studies show, 360 prisms are less accurate. So the choice is easy, use a 360 for topo and mapping shots, use a circular for everything important.
"mini vs full size" - The main reason to use a bigger diameter prism is increased range. If your total station working range with a mini is enough for your jobsite, then put the full size prisms in the closet and don't look back. I have never had a site in New England where I needed a full size prism, and have not used one in years. I have tested mini and full size prisms with various standardized simulated obstructions and different total stations to determine what combination would be best for topo in the brush. The full size prisms only help get light around twigs with older non-laser edms. The small beam laser edm in every current total station only effectively uses the center 20mm of the prism at reasonable ranges, so the larger size is no benefit.
"prism
centering accuracy specs" - They are what they are, not sure why this is a question.
"is ATR affected by prism selection" - The ATR sensor is looking for a reflection of a dot of light, centering the aiming point on that dot image. The ATR does not see the target plate or the prism housing. If the prism is a 360 with multiple pieces of glass, they rotate off-center as the prism is turned, and the dot reflection will not be on point. Likewise, if a non-nodal circular prism is used (especially a 0 offset), the dot image will move off point as the prism is rotated. If the prism is very far away, the reflection might be too small for the imager to see it, requiring a bigger prism.
"has anyone
directly compared knock offs to the genuine counterpart" - I have, and it is easy to do yourself. Set up in your parking lot on good tripods, take some shots, keep notes and swap prisms. It is recommended to check every new prism compared to your known set. I have found some to be perfect and some to be off up to 0.02'. The bad ones get dedicated to forest topo where they get knocked around anyway.
"how is
centering error tested" - In a stable set-up, shoot the prism, then flip and rotate it 180 and shoot it again. The centering error is half the difference. You might have to remove the target plate, also some prism mounts can't be flipped.
"why do some manufactures quote angular accuracy
for their 360s and ignore centering and others quote centering and
ignore angular" - Not sure, but it doesn't matter anyway, see "360 vs circular"
"what exactly is “beam deviation” and how does that
affect accuracy" - Beam deviation is how close to 180 degrees is the reflection of the EDM light beam sent back to the total station. It does not affect accuracy, but it does affect range. If the reflected light beam is far enough off-axis it will miss the total station lens and the EDM will not be able to read the distance. This is only really important at extreme ranges. I have used all kinds of poor quality glass and it has never been an issue.
It depends really. Everything had to calibrated for what I was doing. What I can tell you is I remember the two good decor prisms a different offset was given to us. Which was in the 1 to 2 mm range. So a -30mm changed a bit. Not I was not very concerned about the seco prisms I was not using those for the tight work but I had to check the box. I want to say the one seco was closer to a cm out. Now the smr’s they had to be tight. I ended up having one set on my desk as a paper weight as it would not meet specs. Those things are not cheap either. Now the amount of error on it for us would have been negligible. But for alignment and certification of a machine like item it was not.
I use to remember the formula and routine to do a 3 legged peg test to check prism constants in the field. I think we did like 10 shots each to all prisms in each truck we traversed with part of our quarterly calibration and check routines or before a big traverse. Something along the lines of PC prism constant = AB -PC AC -Pc etc. any way I can’t remember the whole formula but we had broken it down algebraicly I believe to its simplest form for how we did it. If I remember we set the total station to 0 offset or it’s equivalent just to measure that. It has been a while. Glad this thread was posted it made me remember I had forgotten all about that thing. Guess I will have to look it up. . Of course the prisms depending on which model one buys from a manufacturer makes a difference in quality and precision for sure. Just like the different specs on a tribrach etc. everything from torque wrenches to any measurement equipment got certified and checked every so often. A whole department was in just making sure we stayed in compliance. I did the second just for giggles as we had found some old yet never or hardly used old survey equipment. I had 10 red old EDMs. Some had the new smell still. Tripods from regular like todays to screw on transit ones. They might have chunked an old military one that might be made into a lamp one day but I know nothing of that lol.
I'm late to this party, but I'll offer this little writeup that I did a while back, trying to understand prisms.
https://www.dropbox.com/s/01lrznaq7ve4ed3/PrismNotes.pdf?dl=0
My set-up has Sokkia Prisms and I have never liked the Standard -30mm Sokkia type. I would like to know why this was so prevalent. I previously had Leica which for their standard round prism is nodal. I even have one old example just sitting on my desk. It makes me feel good just looking at it! Being well designed in so many ways! Its such a shame, the way that the manufacturers calculate the constants is different. Its maddening, a bit like E and N convention compared to N and E., or even encountering a Staff with the E in a different Position.
When it comes to Mini Prisms I had the Leica nodal Mini Prism, even though I had to change the Prism constant. I made sure I had the PC configured to be on my display. You can't always get to so you can point the prism that well when you are checking bolts in concrete formwork. Nowadays ball prisms seem to be the go to for this.
One other thing that I would like to mention is my Sokkia APT1 Prism. Mine has some damage to the corner of one of the prisms which looks horrible and under careful experiment on a tribrach throws the distance out sideways by about 3mm. I was in the habbit of taking the prism off for some measurements, and unsurprisingly it has fallen off a couple of times! If laid down on a flat surface the glass is only about 3mm away from the surface, so a piece of chip from a road can easily touch the glass. I plan to glue a large rubber washer to the top and bottom to protect it. I have seen a better protected Leica Prism.
Has any one else damaged a prism like this.
I had a couple of "Swiss style" GRZ4 knock-offs come apart after a pole fall. (Yes, I said a couple and it happened within two weeks off each other.) I surprised as I had seen other prisms survive more, in fact the first time I ever saw a Trimble MT1000 the salesperson threw the pole down to a concrete floor to prove how resilient the set up was. There was surprisingly little glue holding the glass to the plastic in the ones that came apart on me. I glued them back together and continued using them. There was a small nick in the glass of one of them but considering I was doing "pole work" I never thought to check the value versus other reflectors.
I worked on a project with a gentlemen a couple of years back. He purchased a handful of Leica mini 360 (GRZ101) knock-offs from Amazon. He set them up on a tribrach and rotated them as the station tracked them. The difference between the real deal and all of the lookalikes was negligible.
I haven't given into the temptation for off brand accessories myself (yet) but it was interesting to see.
Tested at relatively short range under good conditions ..... Try it again at the limits.
