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*To*: quattro@coimbra.ans.net*Subject*: Candela (candle), lux, lumen, etc*From*: "Andrei Broder" <broder@pa.dec.com>*Date*: Mon, 05 Feb 96 00:47:45 -0800*Sender*: quattro-owner@coimbra.ans.net

Light flux is measured in lumens. Light sources are often labeled with an output rating in lumens. I posted a while ago a way to convert wattage for various bulbs into lumens. (Warning: knowing the output is not enough since you don't know where the lumens go. For DOT approved lamps they certainly don't go where you want them :-) Lumen is actually a "derived unit". The basic international unit, measures luminous intensity and is called candela. Candles and candlepower are the same thing but these are deprecated names. It tells how much flux is flowing through a solid angle, which is measured in steradians. A point source that has intensity of one candle puts out a lumen per steradian. A unit area, all at unit distance from a point covers exactly a steradian. Illumination (illuminance) is the area density of incident luminous flux: how many lumens per unit area. A lux is one lumen per one square meter. Illumination from a point source falls off as the square of the distance. So if you divide the intensity of a point source in candles by the distance from it in meters squared, you have the illumination in lux at that distance. (Remember this assumes a single point source in a sphere - no reflectors, lenses, etc. What we have are often multiple complex sources.) Read the rec.photo FAQ for a good intro to all this. I cribbed from it. Ideally what you would like to know when buying a headlight would be a diagram that shows the illumination (in lux) at various distance and at a various heights. I never seen a catalog that shows this clearly. Hella has some nice diagrams but they don't give a lux figure. PIAA gives you a candlepower (candela) figure, presumably they integrate the flux outside the headlamp. The person asking about this units was interested in the rear fog light. The short answer is: I don't know. The long answer is that it is a 21W standard bulb. I would guess it puts out around 13 lm/watt so let's say you get 250 lm at the source. You lose at lot (75% ?) because of the horizontal position of the filament and through absorptions in the reflector and through the lens. So my guesstimate is that you can approximate the illuminance close to the axis as if the source has a 750 candela intensity (300 * 4 * pi /4). I'd be surprised if I am wrong by more than an order of magnitude :-) - Andrei

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