Spectral-Valued IoR for Clear Resins

General Discussion
1

Hi,

I’m looking for the per-wavelength index of refraction for the Formlabs clear resin(s). I found the value “1.5304” quoted in an older thread, but the wavelength this was measured at is not listed, nor is how the value changes with respect to wavelength.

I’d like to ask if someone has better data for this. A Sellmeier fit would be ideal, but any standard formula or measured data would be useful too. (Note: I already asked Formlabs support; they have not yet measured per-wavelength IOR, and I’m asking here at their suggestion.)

If no-one has measured it yet, we’d be happy to try it ourselves for the visible spectrum and share our results. It would be helpful to have a nicely-prepared sample (a short, rectangular tile (say 1cm⨯5cm⨯10cm) with a smooth, flat, transparent surface would give most-accurate results). Perhaps someone could send us such a sample?

Thanks,
Ian Mallett

2

Ian,

Just saw this. Did somebody send you a sample?

Do you guys know of somebody that can help me design a light pipe for the innards of my next project ?

Cheers
Dave

3

I don’t, and no; we haven’t received a sample from anyone yet. If anyone’s thinking about it, a semicircle (radius 3+ cm) extruded 1 cm thick would be better and use much less volume.

4

How well would it have to be polished, and are you willing to polish a sample yourself?

5

Crystal clarity. I’m uncomfortable with finishing a sample myself, as I never have before. In-particular, if the finishing involves curing a thin layer of resin over the surface to get a smooth exterior, then I can’t do that (don’t have the resin nor the UV).

Here’s a part design (units = cm) which uses even less volume. The curved surface and especially the long flat side would need to be perfectly smooth and transparent.

The plan is to use six wavelengths (as provided by six already-ordered lasers) to measure the critical angle on the flat surface from a beam incident through the curved side. This gives a value for the IOR at that wavelength, which allows for a Sellmeier equation fit.

6

The part design might be too small to handle and finish easily, even though it does take up a tiny volume; the rectangular prism or the extruded semicircle might be easier to polish, though I haven’t polished a part to optical clarity either, so I don’t know exactly what to expect. I don’t know if I’ll be able to send you samples, but I’m curious enough to at least run a print or two in Clear v4, and try some polishing if I have time.

7

Oops, I missed your note about the scale on that part design and printed it in mm units instead of cm. It looks a lot more reasonable scaled up by 10. I’ll print some while I see about polishing the other samples.

8

Okay, keep me posted! We’ve got 4 lasers, and several more on the way (so at least 8 wavelengths now!). I learned about a way to calibrate and measure their peak wavelengths as well, hopefully to at least a few ångströms precision (so this may give a few more (close) spectral lines to sample).

9

[ I got an email notification of a reply, but it appeared to be missing here. I gave it a few days to see if it would resolve, but it didn’t. Hence, here’s the message (Ike on 2018-03-05): ]

Hi there,

I’m sorry this has been taking so long! A while ago, I managed to print clear test parts in a few geometries: the flat slab, semicircle, and quarter-circle with a protrusion, as well as some equilateral triangle prisms.

I tried clear-coating them with a Krylon clear coat, but that didn’t work terribly well, so I think I’m going to try manually polishing them or curing a thin liquid layer of resin on them next to try and achieve smoothness and clarity.

I don’t know how soon I’ll be able to do those things.

Another option is I could just print a bunch more parts and send them to you, if you wanted to experiment with the clear-coat and abrasive polishing methods.

I’m curious how good the surfaces really need to be; with lasers, maybe you’ll be able to distinguish the proper direction of the beam from the light scattered from surface imperfections?

Regards,
Ike

I recommend curing a thin liquid layer on the surface. As long as any air bubbles can be removed, that should be best. The trouble with a coat of something else is that the refractive index of the coating will probably be different.

Re: precision, the way the test works is you see whether the laser is totally-internally-reflected or if it escapes. If the surface is roughened, then the beam is “blurry” and some portion of the beam does both. As I mentioned, I don’t have the tooling to try finishing any test objects myself.

I ordered a test print of a prism-like object a while back. I was not blown away by the quality, and indeed the surface appears to have been finished using some kind of sanding technique. Nevertheless, I believe it would be possible to use an equivalent-quality part to get a reasonable estimate. As a reasonable heuristic, if you can see objects fairly clearly through it, then the imaging quality is acceptable:

20180131_125410

10

To be fair 50 bucks worth of polishing papers / mineral oil and a flat work surface are all you need and probably all anyone will be using short of actually curing liquid resin on the surface… but then the surface won’t be flatter than a hand-sanded parts. With very fine polishing paper you can attain very good surface sate, it just takes elbow grease and time.

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