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, 00:53 -0500
From: Bill B
Date: 2006 Jan 25, 00:53 -0500
George First a primer for those without your scientific/engineering background: A polarizing filter is akin to a picket fence. A series of fine lines that allow light vibrating on one plane to pass through. Using the fence analogy, a telephone pole (vertical) aligned with the fence can be seen in its entirety through the gaps in the fence (given sufficient distance). With a curb (horizontal) only the portions not blocked by the fence can be observed. How much affect a polarizing filter can have depends on how "scattered" the light source is. A north sky (in the northern hemisphere) is reflected light, so very scattered. In this case the filter may reduce the sky by 2 f stops (1/4 of light transmitted). In other cases, where the light operates closer to one plane, only 1 f stop loss (1/2 the light). NOTE: f-stops are based on the square root of 2. Hence 1, 1.4, 2, 2.8, 4, 5.6, 8, 11.2, 16, 22, 32 .... Each stop admits 1/2 the light of the previous stop. Photo texts will tell you (in theory) that a polarizing filter will not help reduce glare from a mirror or other highly-reflective surface. In practice this is not true. A lot depends on what the surface is reflecting. On average, a polarizing filter will reduce light by 1.5 f-stops (1 to 2 stops range depending on light source.) Now, if we stack 2 filters (picket fences) together, with each axis aligned, we have a combined loss of 1-2 f stops + 1-2 f stops (depending on light source). Therefore a loss of 2-4 f stops. Given your analytical mind, and to paraphrase your observation, "If the sun's rays are assumed as parallel, and I nail 2 identical fences together, why would the second fence cause anymore light loss than the first fence." In a perfect world, you would be correct. But alas... Back to the pragmatic world. As we begin to rotate one filter (picket fence) relative to the other, the "apertures" that allows light to pass through get smaller and smaller. Finally, at 90 degrees rotation, all we are left with are square areas whose sides are equal to the distance between the "slats" transmitting light. Not a bunch of aligned photons given the spacing between "slats" on a polarizing filter. Nonetheless, not 0 light transmission. From a practitioner's viewpoint, I have stacked polarizing filters to photograph a waterfall of rocky stream bed for a "dream like" quality where the moving water becomes "fog." Adjusted filters for approx. 1/30 second exposure. Then adjust for reciprocity failure (which truly is a dog chasing its own tail). Then bracket, bracket, bracket exposures. Or for copy-stand work of glossy flats, polarize each light source, and add a polarizing filter over the lens. Speaking to Mikes point on IE change, "It is important to note however that even good polarizers change the index error when used. I know that the manuals say that this can happen with any shades but I have only ever measured it with polarisers - maybe it has to do with the two pieces of glass in each filter. In any event IE must be measured with the filters in place - orientation does not seem to matter." Yes, the pragmatic problems of stacking filters. Myriad. Are the filters exactly parallel to each other and the ray path? If not, different refraction indexes of colored glasses. All but the new "ideal glass" transmits and reflects, so ghosts (flare) from light bouncing between filters. Stacked filters allow the air between the filters to heat up... Hope that helps, Bill > Some things I don't understand about Peter Fogg's recent message are > > 1. How can you reduce the brightness of light by more than 100%? If you have > reduced it by 100%, > then you have already achieved complete darkness. > > 2. Yes, if you introduce one polarising filter, you will approximately halve > the light intensity, > for initially unpolarised light, because you are excluding the 50% of the > light that is polarised at > right angles to the direction of the filter that lets light through. Plus a > slight further > reduction, to allow for losses in the plastic material and in surface > reflections. > > But then, if you add a second filter, that's polarised in the same direction > as the first, that > won't halve the light intensity again, because now all the light entering it > is polarised in the > preferred direction. This second filter will therefore cause a slight > reduction in intensity, as > before, because of further losses in that extra layer of plastic and further > reflections, but > nothing like the reduction that the first filter caused. That's a deduction > based on theoretical > reasoning, not on any practical tests by me, so it may possibly be > contradicted by experiment. I > would be interested to learn if it is.