NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextants with Polarizing filters
From: Peter Fogg
Date: 2006 Jan 27, 11:42 +1100
From: Peter Fogg
Date: 2006 Jan 27, 11:42 +1100
George contends: > Reduction due to applying a perfect polariser to unpolarised light - 1 > stop (as it should be > theoretically). Presumably this comes from the idea that 50% of the diffused light is being passed, and presumably what George means by a "perfect polariser" is one without inherent density (including surface reflections, etc). Of course this perfect thing only exists in the imagination. In practice single polarizing filters lose between one and a half and two f/ stops. An exception may be when they are faced with light that is already polarized (which happens in nature, and also when accepting light from the first polarizing filter) and then they lose about a stop. While confirmable by experiment (as I did yesterday) it is such an established fact that polarizing filters often come with this printed information (1.5 to 2 stops) provided by the manufacturer, as indeed normally do all photographic filters - it may be important to know in advance the density loss of a filter you plan to make an exposure through. > Additional light loss due to passage of polarised light through a perfect > aligned polariser - 0 > stop. > Additional light loss at each passage through a real polarising filter, > due to its imperfections - > 0.5 stop. As outlined above, I think this idea bears further examination before being accepted. > Not significantly. You could shine light through the first polaroid ... As a keen amateur of matters pedantic, George, I'm sure you will be happy for me to point out that Polaroid is a registered trademark of the company founded by the remarkable Dr Land. On the other hand, you are welcome to spell polarizing anyway you like, although I bet you pronounce it with a 'z', and not polariceing? > And it's got nothing at all to do with navigation. Sorry about that. Wrong again! (said he cheerfully). Polarization seems to have an application in sextant filters - remember? Also Polaroid sunglasses (and those made by other manufacturers) have an important navigational role to play when worn on-board by navigators, for a variety of reasons including the reduction of glare and their ability to reduce reflections from the surface of water.