Generally you cannot.  Incident light will be transmitted, absorbed, and
reflected.  You have knowledge of only one of the three components.  You could
null the reflection with appropriate polarization at the Brewster's angle, but
this requires prior knowledge of the film characteristics (complex permittivity
and thickness), which you do not seem to have.

I am unaware of any other mathematical tricks that involve the combination of
incident angles and/or polarizations to establish the absorbance spectra.
Perhaps others are.

> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On Behalf Of D. Zhou
> Sent: Friday, June 01, 2007 3:00 AM
> To: General MEMS discussion
> Subject: [mems-talk] Absorbance measurement of multi-layer structures
>
> Dear all,
>
> I am now working on the design and characterisation of multi-layer
> structures for efficient mid- or far-infrared radiation absorbers. I not
> sure how to measure the absorption of the fabricated structure, for
> example, a metal/Si/metal sandwich structure. In our group we can only
> conduct transmission measurement. So how can we extract absorbance from
> the transmission data? Please give me some hints if you have any experience
> in this. Thanks very much.

--
Raj Gupta, Ph.D.
http://terahz.org