Re: Why Isn't Sapphire Important Optically?

It has uses in some optical designs as well as for windows. The three
Venus probes that were part of Pioneer Venus (1979) were 25 mm
diameter Sapphire windows. I worked on the windows and their mounting
while at Hughes Aircraft in Culver City CA.

I have seen Sapphire appear in other optical designs. It is mildly
bi-refringent but in some cases this is not a problem. Last I
remember, lens sizes were limited to < 7 inches because of crystal
growth size limitations. This may have changed.


jsavard@xxxxxxxxxxxxxxxxxxxxxxxxxx (John Savard) wrote:

I remember reading a posting asking about the optical properties of the
transparent armor material Aluminum Oxynitride.

Some optical design programs include it in their glass tables. Turns out
to have a very high index of refraction, and to sit at about the same
Abbe number as the flint/crown dividing line.

But sapphire has a similar index of refraction - and is also 'way below
ALON in dispersion.

It seems to be as close to a "perfect" optical material as there is; a
very high index of refraction, and very little dispersion of colors.

They make clear watch crystals from synthetic sapphire already, so there
is no technical barrier to making highly transparent pieces large enough
to be used for some optical systems.

Yes, that's for _expensive_ watches. Crystals start at $125 and go up.

Is it because optical design manages so well with regular glasses that
there is no longer any *urgent* need for exotic materials, once only
slightly exotic materials liberated us from the tyranny of the "old
glass line" (the one that led to the Petzval lens, because with
traditional glasses, you couldn't eliminate curvature of field, so you
had to give up, and use astigmatism to make the tangential field flat)?

Or is *birefringence* the killer?

Given the prices of some astronomical eyepieces these days, one would
have thought that $125 for a sapphire element would be a natural for
someone to try...

John Savard
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