Hmm, I think the potential mistake here has to be the assumption that a bullseye decklight is a perfect plano-convex lens. I don't think it is ... or at least I don't think it *should* be. As best as I can recall, the old original ones I've seen seemed to be a bit more complex with what looked like a Fresnel lens on the flatter side and a very pronounced central bump on the reverse (as opposed to a surface curved all the way across).
However, looking at the only (fairly small) pics I can find of modern versions, the bumped side does seem to be more like a plano-convex lens in shape however the "flat" side seems to be ridged or indented at the outer edge and the inner part, it seems to me, dished. That would possibly make the lens a positive meniscus, perhaps with an outer Fresnel element to capture more light and although such a lens would still focus light to a point, even mounted the right way up, (ignoring aberrations), as far as I can make out a meniscus lens could have a shorter focal length than a plano-convex lens so the light would diffuse better beyond the focal point (but I'm not sure about that from the info I've been able to glean from a fairly quick surf) Perhaps when mounted wrong way up, as they are on narrowboats, the focal length is longer? The right beast for the job would be a prismatic diffusion lens but maybe they weren't around in the olden days. I do know for certain that the suppliers of bullseye decklights very positively state that they should be mounted with the heavily curved side inwards or downwards and caution about the fire risk if they are mounted the wrong way round. Interesting little puzzle this! Cheers Bru > -----Original Message----- > From: [email protected] [mailto:[email protected]] > On Behalf Of [email protected] > Sent: 10 June 2010 17:25 > To: [email protected] > Subject: Re: [canals-list] Re: Roof lights and fires :-( > > [email protected] wrote: > > If there's any truth in that, then it's nothing to do with the basic > optics of how light passes through a plano-convex lens. A converging > lens is a converging lens in both directions. > > Further to what I wrote, I think I might have got to the bottom of > it... > > Whilst the basic principle above is fundamentally true - a plano-convex > lens converges light, whichever way round it is mounted - there is a > difference in terms of the abberrations. > > No lens focuses light perfectly: there will be second-order effects > such as spherical abberation (light rays passing near the edge of the > lens aren't quite in focus with those passing near the centre) and > chromatic aberration (different colours bend different amounts and > therefore focus at different distances). > > In the case of a plano-convex lens, spherical aberration is more > pronounced when the light passes from the flat side. It will still bend > all the light inwards towards the centre, but the increased amount of > aberration means that quite possibly it won't focus it sharply enough > to start a fire. > > Whether this also explains Bruce's experience that it diffuses through > the cabin better, I haven't worked out yet. > > See http://folk.uio.no/walmann/Publications/Master/node8.html for some > diagrams. > > Martin L > > > > ------------------------------------ > > Yahoo! Groups Links > > >
