Wayne J. Cosshall wrote:

"Wayne J. Cosshall" wrote in message
u...

I've been experimenting with using a circular polarizing filter when
shooting digital infrared images:
http://www.dimagemaker.com/article.php?articleID=1053
Wayne


A few remarks, FWTW. There are two different influences affecting the
final results in your IR pictures. A circular polarizer consists of two
filters: a linear polarizer followed by a quarter-wave retarder. Neither
of these is fully achromatic, that is, they only have a limited
wavelength range over which they are fully effective. Outside its range
the linear polarizer will have decreasing effect in polarizing light,
and outside its range the retarder will convert less of the lineraly
polarized light to circular. Both tend to start failing rather badly as
you go into the IR, so I would expect diminished affect by rotating the
filter in the IR. How much depends on the specific polarizer and could
change with maker.

For my research I had achromatic polarizers and retarders fabricated
that worked from the UV to the IR, but they were very expensive. Normal
ones made for cameras start losing there effectiveness in the blue and
the red, only working well in between.

Joe