Re: First-surface mirrors don't polarize, do they?


In article <46CECBA5.1090504@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Phil Hobbs <pcdh@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Cheap first surface mirrors are usually 'protected aluminum', which is
Al covered with a half-wavelength worth of _SiO_, which is a
nonstoichiometric, silicon-rich oxide whose index can range anywhere
from that of SiO2 (1.46) to about 1.9, depending on the deposition
conditions (which change the oxygen content of the film).

Aluminum-air surfaces are much better reflectors than aluminum-glass, so
protected aluminum mirrors have quite low efficiencies except where thin
film interference in the SiO helps. The thin film interference is
polarization sensitive off axis, of course, which makes protected Al
mirrors somewhat polarizing.

Better quality aluminum mirrors are usually 'enhanced aluminum', in
which the single SiO layer is replaced with a dielectric stack. With
more layers, the coating designer has more degrees of freedom, so the
off-axis and polarizing performance of enhanced Al is harder to know
from first principles.

Yikes. Neither of those sounds good for mirrors intended for use with
a variety of wavelengths, 200 to 2500nm in the case of this spectrometer.
I suppose actual silver would tarnish. Is it still used?

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