plastic strips for encoders

Many people have written about using the US Digital plastic strips for
encoders.
Has anyone calculated the thermal effects of the plastic?
The CTE of plastics is so much larger than other engineering materials that
even a few degrees F over a foot will make a very inaccurate measurement.
When I get to my office I'll look up the exact numbers, but they are
freightening.
If the strip was glued to a piece of glass it would work better, but might
not fit in the reader.
If the encoders use transmissive optic a metal backing wouldn't work.

Elliot Burke
HighTide Instruments