ATM Re:Moon and Polarizing filters (longish)

[tmasnyk@surgery.bsd.uchicago.edu (Taras Masnyk)]

>
>what's the difference between polarized filters ands neutral filters.
>whats the differnce between a transmission neutral and a
>density neutral filter.. or are they one and the same thing?
>

"Moon filters" is a subset of neutral density filters that are used to
observe the moon :-)

Neutral density filters are called "neutral density" because they affect all
(visible) wavelengths of light equally (or nearly so); that is, there is no
appreciable color difference between the original scene and the scene viewed
through a neutral density filter.  This makes a "neutral density" filter  a
"transmission neutral" filter.  Compare this to colored filters which
strongly affect some wavelengths but not others.

Polarizing filters do not affect the wavelength of light, but do affect its
phase- a polarizing filter lets through only light of a certain phase.
Light 90 degrees out of phase from the filter is blocked completely.  This
is why polarized sunglasses cut down on glare- light from the sun is
unpolarized (all phases), but reflecting off the (relatively) shiny roadway
or windshield will polarize the light (the reflected beam is predominantly
of one phase- check physics texts to find out the details, I have long since
forgotten them).  The polarized glasses are oriented so that they generally
filter out the reflected light, as the glasses are 90 degrees out of phase
relative to the roadway glare.

This feature of polarizing lenses makes them useful as variable density
filters.  Take TWO polaroid filters, and rotate one with respect to the
other- the phase orientation of the filters will vary from completely in
phase to completely out-of-phase, leading to variable transmission from 100%
of the incident light (or nearly so) when the filters are in phase, to 0% of
the incident light (or nearly so) when the filters are exactly out of phase.
In practice, most polarizing filters I have seen also have some degree of a
neutral density effect to them (I believe this is in addition to the
polarizing effect, as polarizing itself doesn't cut out alot of light, but I
may be wrong), so actual transmission at in-phase is less than 100%.

For those interested in a little experiment without investing in a variable
polarized filter, go to your local drugstore and find the watch display.
Pick up a cheap LCD display watch (the display takes advantage of
polarization in creating the digits), and walk over to the sunglass
carousel.  Find a pair of polarized sunglasses or clip on lenses.  Place the
sunglasses over the display on the watch and rotate the glasses relative to
the display.  Watch in amazement as the display goes completely black!  You
have just oriented the polaroid element of the glasses and the watch at 90
degrees to each other!  Ain't science grand :-)

BTW I'm a little rusty on my nomeclature; in the preceding discussion i
mentioned light beams 90 degrees out of phase as being at opposite phase.
It may be that the correct term should be 180 degrees out of phase; if so
please make the appropriate substitution when reading the post.  In any case
I meant two beams of filter and beam completely out of phase with each other.

Clear skies,
Mud-Fud