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    Re: Lights,Leds and scopes etc.
    From: George Huxtable
    Date: 2003 Oct 15, 01:38 +0100

    Dave Weilacher asked-
    >>Does anyone have a 'workable' (as opposed to accurate) formula for
    >>converting between lumens, candlepower, and watts (lighting)?
    >>Is candlepower and candela synonimous?
    >>Is it true that a WW2 pilot could see a candle burning in a window at 8
    Rino van Dam responded-
    >Found this on the 'Net:
    >Perry's handbook of Chemical Engineering - Conversion tables 1-7
    >to convert from            to          multiply by
    >Candle power(spherical)  lumens          12.556
    >lumens                    watts         .001496
    >result  .0188 watts/CP
    Comment from George.
    Yes, it seems to me that the candela is just a sanitised definition of the
    candle-power. So in the colregs, Annex I. 8 (b), a light for which the
    required visibilty is 2 miles, therefore requiring a luminous intensity of
    4.3 candelas, only needs to appear as bright as four candles would (and a
    bit). Not a lot, is it?
    As for the Watts required to produce a candlepower (candela), I think Rino
    van Dam has provided an answer to a different question than Dave was
    asking. I have found some information in a book by Houston, a 1938 ed. of
    which the first ed. was 1915, so not very up to date. In that he says that
    the tungsten filament lamp (of those days) requires obout 1 or 1.25 Watts
    per candle, far more than Rino's quoted figure, by a fraction of over 60!
    Why? Well of the power that went into the lamp, only a tiny fraction of it
    was emitted as radiant energy. And only a small fraction of that energy was
    in the visible spectrum, to be useful to the eye. I think Rino's figure
    must relate a candlepower to the amount of radiant energy PUT OUT by the
    lamp, whereas Dave is presumably asking how much total energy has to be PUT
    INTO the lamp to make it as bright as a candle; a very different matter.
    The above figures relate to a light-source which is emitting equally in all
    directions. The LED's used for the application appear to give a lot of
    light, in the direction of their axis, because their output is tightly
    focussed into a narrow beam. Which is why many such lamps are needed, to
    fill in any gaps over the required arc.
    However, photometry isn't my subject, so the views above are expressed with
    some diffidence. I could be wrong.
    As for what a World-war 2 pilot could see: when I was a kid of 6 in
    Liverpool and our streets were being blitzed, we were told a similar story,
    to ensure that we blacked out all our windows efficiently. To that extent,
    the story certainly worked, but whether it was true, I just couldn't say.
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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