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 25, 08:32 +1100
From: Peter Fogg
Date: 2006 Jan 25, 08:32 +1100
> From: Frank Reed > ... the idea is to use crossed polarizing filters > which can be rotated to produce any desired amount of darkening > (within a certain range)... Sounds like a good idea. On the certain range: each polarising filter reduces the brightness of light passing through by about one and a half f/ stops, or 150% (this varies between manufacturers; 150/200%). So two used together would already have quite a significant minimal darkening effect, which could be halved by using only one. The maximum effect is complete opacity; no light gets through - try it with a couple of pairs of polarising sunglasses. A great advantage is that polarising filters significantly reduce glare, just what you want reduced when trying to pick out the horizon against sunlight glittering off the sea. I think polarising sunglasses are the best sunglasses for on-board use. Another reason is that by reducing reflections from the surface of the water they allow you to see more clearly below the surface. Just one head of coral unseen can ruin your whole day ...
