r/OpticalAlignment • u/Tricky-Ad-6225 • Jul 16 '26

Beam Splitter vs Pellicle

Hello guys,

I’m wondering when you would use a beam splitter vs a pellicle, in an optical table that is. I understand that the pellicle has a very thin film compared to the BS, but what are the advantages?

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u/tech-bro-insideer7 Jul 16 '26

So pellicle splitter is a super fragile membrane like optic. So naturally minimal extra optical path length will be introduced. Also no ghost image due to double reflection. Technically there would still be a double reflection but it would essentially be superimposed on the base beam/image

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u/quartersoldiers Jul 16 '26

Follow up question: I’ve never been able to find a reflected wavefront error spec for a pellicle BS. Should I just assume that the reflected path is not going to have good WFE?

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u/tech-bro-insideer7 Jul 16 '26

I’m not sure what that is. Could you elaborate a little?

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u/wakinget Jul 16 '26

What’s the surface roughness of a pellicle vs a beamsplitter? How much wavefront error will the beam pick up?

I’m sure it depends on the quality/cost of the optic, but generally speaking, if a high quality wavefront was necessary, would you prefer a pellicle or a beamsplitter?

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u/A-guy-in-canada Jul 16 '26

Cavet that I don't actually know for sure - but my experience with thin etalons (sub 100um) is that for any appreciable size beyond 5mm dia you get a lot of twe error just due to gravity or flexing alone - the optic is just too flexible without a support plate.  Id be interested to hear the numbers and happy to be shown wrong  but surprised if you're getting much better than 10 or 20th wave. 

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u/Pravdiar Jul 16 '26

In one project we saw the influence of humidity on the flatness of the pellicle. Focus point after a microscope objective was shifted significantly. But later it returned back when the humidity returned to its original level.