A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextants with Polarizing filters
From: Bill B
Date: 2006 Jan 25, 21:29 -0500
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