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    Re: Sextants with Polarizing filters
    From: Bill B
    Date: 2006 Jan 25, 21:29 -0500

    George wrote:
    
    "If you look at a scene through a single polaroid, its brightess is reduced,
    usually to about half the original, but it depends on how polarised the
    light was in the original scene. Now add another similar polaroid. First
    find the angle at which the two polarisation planes are crossed, by turning
    one until you get blackout. Now turn one through 90 degrees, until they are
    aligned, and are transnitting maximal light. Now remove one of the
    polaroids. Does the brightness increase significantly when you do that? I
    predict that there won't be a noticeable increase. Because the
    second polaroid, when its axis is aligned with the first, didn't give rise
    to a significant reduction in brightness, compared with what is seen through
    a single polaroid, the light already being polarised in the right plane.
    Bill may even be able to measure it with a light-meter. Of course, some
    reduction, perhaps 10% or so, is expected, simply because any optical
    component you put into a light path, even clear glass, loses some light by
    surface reflections."
    
    George
    
    Sorry you do not endorse my paraphrase ;-) Googled filters today, and a
    maker of fine optics, Schneider, used the same analogy (albeit a better
    explanation--a picket fence:
    
    http://www.schneideroptics.com/info/white_papers/true-pol.pdf
    
    "HOW POLARIZERS WORK
    
    Ed.  Rope hung from tree limb
    
    Imagine that you could shake the rope in all directions at any given time,
    and you can picture the behavior of unpolarized light. Now, picture that the
    rope you are holding passes through a picket fence on its way to the tree.
    This picket fence has slots that run in a vertical direction. If you now
    shake the rope in all directions (up/down, side-to-side, in between) only
    the vibrations that are parallel to the slots will pass through them. The
    rope (light) vibrations from the side-to-side motion (horizontally polarized
    light waves) will hit the fence slots and not pass through. However, the
    vertically polarized light waves will pass through the fence, and are passed
    on to the tree.
    
    Light waves move and travel very similarly to the rope. If you have a
    polarizing filter with many fence-like slots (called palings) and close
    spaces between the palings, you could control (or we could say reject)
    almost all light that is not parallel to the palings. If you had a filter
    with few palings and large spaces between them, you can have partially
    polarized light pass through along with the polarized light. That
    would reject only a small amount of glare, and that glare would end up on
    the film. The width and quality of the palings determines the quality and
    performance of the filter."
    
    As per your request, did a quick series of tests using a tripod mounted
    Minolta digital flash meter, pointed at a flat white interior wall
    illuminated by an incandescent bulb.
    
    Baseline reading, no filter    f5.6  .9
    With Vivitar 77mm filter       f4.0  .4
    With Hoya 67mm filter          f4.0  .6
    With Nikon 52mm filter         f4.0  .5
    77mm and 67mm combined         f4.0  .0
    
    Also tested using an Nikon F3 with 100mm lens.  Same results (but less
    precise as I was reading f stops off the barrel.
    
    Nominally 1.5 stops loss per filter, 2 stops with filters combined and
    aligned.
    
    Note: these are traditional, linear polarizing filters, not the "circular"
    type designed for zoom/auto-focus lenses whose front element may rotated as
    the lens is focused/zoomed.
    
    I quickly reversed the order of the filters to see if that appreciably
    affected the stacking loss. I did not.
    
    The results are clearly well below my predicted addition of filter factors,
    and above your 10% estimate.  Sorry about the fuzzy thinking on the affects
    being additive--never had a reason two align filtered light sources and/or
    lens filters.  That would sort of the defeat the purpose(s) of using the
    filters in the first place. 
    
    Questions that come to mind would include:
    
    What affect will paling difference between manufactures have? (If the
    pickets and spacing of the two "fences" are are not identical.)
    
    Is the light coming from the first filter "totally" polarized?
    
    Even if the light from the first filter is "totally" polarized, does it get
    diffused kicking around between the facing filter surfaces?
    
    What would be the result from two identical filters, placed surface to
    surface, with palings aligned via microscope.
    
    If 2 fences were not identical, or not perfectly aligned, could a rope that
    made it through the first gap between the slats whap into a slat on the
    second fence?
    
    Beyond your transmission/reflectance observation, will placing slats and
    openings by its very nature reduce transmission?  I am out of my depth on
    this one, not knowing the scale of light waves vs. openings and spacing of
    the openings.  Perhaps one of our physics mavens can address that?
    
    Will try to do some daylight experiments tomorrow to move on the the total
    loss by offsetting the filters by 90d.
    
    Bill
    
    
    

       
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