Saturday, May 23, 2015

Blinded Me With Science

Just figured out why my cast acrylic eye pieces are so hard to see through, even when I do get a decent surface.

Cast acrylic has an index of refraction higher than that of air!

The outer surface of the helmet is curved, therefor the front surface of the eye pieces describes an arc. And because they were cast in an open mold, the rear is flat. That makes them a lens.

It is always sort of cool when some of the basic physics of the universe pops up in the middle of a construction project like this.

I've been having a lot of trouble getting a good surface anyhow. Went progressively up to wet-sanding with 2000 grit, and followed with Novus #2, but it still leaves a cloudy surface. I don't think the casting resin is quite compatible with the resin polish. (I'd previously tried using a Dremel polishing wheel at low speed but it still gouged holes in the plastic).

This is all surface effect, which is why flame polishing and MEK softening methods (sort of) work for this sort of thing. Which means the surface is a lot more clear when wet. And, as it turns out, when OILED. A little drop of sewing machine oil brought it...well, far from optical quality, but enough to confirm there was a major lensing issue that made it impossible to see anything clearly through the eyepieces.

So I guess I'm back to trying to carve, bend, and shape 1/8" to 1/4" cast acrylic sheet. Which I'll have to do at TechShop because it will be a messy bit of grinding and filing and sanding.

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