Hi there,

An ellipsometer is the best option. Basically the ellipsometer is the method
used to measure the optical properties for unknown thin film but coated on
some substrate: glass or PMMA or silicon or any nontransparent substrate.

Even if you have a transparent substrate, no problem. If you have the film
on both sides, just slightly scratch of the thin film in one side using some
soft sheet.  Have you heard of this emery sheet for polishing metal
substrate?  You can use that, and give a black sketching but fully black, so
that when polarising light enters into the film it will reflect back from
each interface with different change in angle of the phase.  The ellipsometer
measures thickness like that.  For unknown thin film coatings it is this
cauchy model as suggested by MIKE.  You directly get the values in graph.
THIS IS ONE CASE but you clearly say it is an infrared light source.

Measuring a plastic like PMMA is hard.  I worked on an ellipsometer and
was not able to find and fit a model for PMMA.

>From your mail it says thin plastic film.  What is this thin film
transparent plastic?

Thickness and refractive index are inversely proportional so you have to
know one to find the other... so you have to know something.

regards,
righeira




On Wed, Apr 6, 2011 at 6:45 PM, Xiaohui Lin wrote:

> We have got a plastic transparent film and we would like to measure its
> optical properties.
>
> Could anyone suggest a way or equipment that can be used to determine its
> refractive index for 1.55um light?
>
> Will ellipsometer do? My understanding is that you have to know the
> material
> and choose model when dealing with ellipsometer.
>
> Any suggestion is appreciated.
>
> Xavier Lin