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issues with laser measurement of distance through hole


New member
Nov 10, 2021

this is my first post and I think this is the best thread to ask this question, apologise in advance if that's not the case.

I'm using a (cheap) laser tape measures to get the distance from the laser to a flat flat surface. I can obviously get this nominal distance easily and without any problems. However, I need to make the laser beam go through a disc of cardboard with a hole in it before measuring the distance to the target surface. In the actual set-up this disc will be an insulating material that minimises the mix of relatively cold and hot air. What happens is that I get different measurements between the nominal distance and the distance measured when the laser goes through the hole (about 5mm difference over 400mm). All the measurements through the hole are always smaller by about 5mm than the nominal distance. Holes smaller than 20mm diam give me this discrepancy, above 20mm there is no difference (perhaps ±1mm)

The schematic of the set-up is:
laser ---------------------------------------- flat surface
With hole:
laser ---- disc of cardboard with hole ----- flat surface

I suspect that there is laser dispersion when going/coming back through the hole and that's why my measurements differ. I painted black the cardboard disc (edges of the hole and top and bottom surfaces) to maximise laser absorption using graphite; however, this didn't help and I still get different measurements when going through the hole. I've also tried a black permanent pen to paint it black but I didn't expect to get great results -and obviously I didn't get them- since the ink is somewhat glossy.

I'd really appreciate any suggestions and help, especially if they are relatively simple solutions.

Thanks a lot for your time
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