P3D Re: Circular or Round?

[Brian Reynolds <reynolds@xxxxxxxxx>]

Michael Kersenbrock wrote:
Bob_Maxey wrote:
> > What's the difference, and forgive me, I am not sure I understand
> > what you mean. Are you referring to a circular polarizer as in
> > circular shape or a circular polarizer as in a linear polarizer
> > with the combined retarder to make it a circular polarizer - a
> > specific kind of polarizer filter.
> 
> A filter that does circular polarization.  I understand these are
> made for cameras nowdays because linear polarized ones can cause
> problems with modern camera operation (don't recall if it was with
> autofocus or fancy metering schemes).
> 
> I have no idea if it'd be as effective to get the "polarizer effect"
> inasmuch I've not used the circulars before, only the linears (and
> then only in 2D).  However it'd solve the problem of buying small
> sized filters (for price) and having to turn them in precise unison
> -- they'd only have to be mounted whichever way and left stationary.
> Probably couldn't get them in series V format however.  :-(
> 

This is another of my pet peeves, so I'm going to go into some detail
about it.  Further disscusion (beyond correcting any stupid mistakes I
make) should probably go to tech-3d.  Note that it's been a long time
since I took electromagnetic theory, so I might have gotten the
"handedness" of the diagram wrong.

Light can be thought of as an electromagentic wave (it is also thought
of as a particle (or photon), but that's not important for this
problem).  One component is an electric field, the other is a magnetic
field.  They are always at right angles to each other and the light
wave travels perpendicular to both fields.  WARNING: ugly ASCII art
follows.

    E
    |_M
    \
     d

    fig. 1

In figure 1 the vertical electric field E is perpendicular to the
horizontal magnetic field M and they are travelling along direction d
(which should be perpendicular to the surface of the screen).
Normally all of the E fields in a beam of light will be at random
orientations with respect to each other.

If you have a beam of light where all the E fields are arranged the
same (e.g., all pointing up, or all pointing down and to the left, or
whatever) the beam is said to be linearly polarized.  Because the M
field is always perpendicular to the E field, the M fields will also
be aligned with each other.

A light beam reflecting off certain materials (e.g., water, metal)
will become linearly polarized.  A polarizing filter (which contains a
single polarizing surface) is oriented so that it is 45 degrees from
the angle of polarization of the incoming light.  This will cut the
intensity of the linearly polarized line in half, thereby reducing
glare.

The EM fields can also rotate about the d direction.  If all the E
fields in a beam rotate in the same direction (clockwise or
counter-clockwise) the beam is said to be circularly polarized.

Many modern cameras use off the film (OTF) metering.  The camera's
electronics measure the light reflected off the film during the
exposure in order to determine the exposure for the photo.  This light
becomes linearly polarized when it reflects off the film.  Using a
linear polarizing filter with such a metering system will give
inaccurate results because the combined polarizing results of the
filter and the reflection will lower the amount of light reaching the
metering system, but not the light reaching the film.  To correct for
this camera companies created a special polarizing filter.  The first
part of the filter is a linear polarizing filter, the second part
takes the polarized light (from which the glare has been reduced) and
randomizes it again so that it is no longer linearly polarized.  This
light can then reflect off the film and be read correctly by the
camera's metering system.  Unfortunately even thought the light from
this special polarizing filter is not circularly polarized, the camera
companies decided to call it a circular polarizing filter.

-- 
Brian Reynolds     | "Dee Dee!  Don't touch that button!"
reynolds@xxxxxxxxx | "Oooh!"
NAR# 54438         |    -- Dexter and Dee Dee
                   |       "Dexter's Laboratory"


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