For once I'm forced to agree with Adrian (damn it) Bullseye portholes should indeed be installed "bump" inwards whence they act as a diffuser. T'other way round they will focus sunlight to a point with potentially disastrous consequences
Bullseyes were originally intended to be installed in the decks and occasionally sides of ships etc. to light cabins. When they were fitted into the roofs of working narrowboats the right way up it is said that the boatmen got fed up with bumping heads on the, er, bump, so turned them upside down not realising that this would defeat the object of the exercise with potentially disastrous results. I've seen the charring of cabin woodwork more than once and also seen the effect of a correctly installed bullseye - much better lighting in the cabin The standard boatman trick, and as far as I'm aware it does actually work although I can't explain the physics, is to paint a black dot in the centre of the outside flat surface of the lens. As I say, I don't know why it works but empirically it seems to do the trick. Of course, installed wrong way up the bulleye is worse than useless at lighting the cabin interior (you'd probably be better off with a plain glass porthole!) but who cares about practicalities, it's all about tradition! Bru > -----Original Message----- > From: [email protected] [mailto:[email protected]] > On Behalf Of [email protected] > Sent: 10 June 2010 10:10 > To: [email protected] > Subject: Re: [canals-list] Re: Roof lights and fires :-( > > [email protected] wrote: > > Trevor Sheppard > > <[email protected]> wrote: > > [regarding focusing of light by bullseye rooflights] > > > I've heard tell that this problem can occur only if the bullseye is > > incorrectly installed, which is a common mistake. > > AIUI, the bullseye should be installed with the curved (convex) side > > *down*. This lights the interior better, by dispersing the light. > If > > the bullseye is installed the other way up, it concentrates the light > > (and heat) on a point in the interior. > > I've heard that too, but I'm unconvinced. My memory from studying > physics (quite as long time ago, admittedly) is that the paths taken by > light rays passing through a lens are 'reversible'; in other words if a > lens converges light passing through it in one direction, then it will > also converge light passing through it in the opposite direction. > > A plano-convex lens (which is what a bullseye approximates to) will > therefore concentrate the light (and heat) to a point in the interior > of the boat, whichever way up it is installed. The focal point might > well end up a different distance from the roof of the boat (anyone care > to do the sums?) and at low sun angles there might be a difference in > the amount of light that it 'catches'), but the basic principle will > remain the same. > > Or have I remembered it wrongly? > > Martin L > > > > ------------------------------------ > > Yahoo! Groups Links > > >
