Curious_character
Flashlight Enthusiast
- Joined
- Nov 10, 2006
- Messages
- 1,211
This was written in reponse to a posting in the Cheers-and-Jeers section, but I thought it's more appropriate here. I hope it'll be helpful to some readers. I'd very much appreciate any corrections for any technical errors I've made -- as I say, I'm not really an expert at this.
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As far as I can tell (I'm not a light or optics expert), "throw" isn't a technical term like illuminance and luminous flux, but something used pretty casually in places like CPF. Here, it's used in at least a couple of ways. One is the brightness (illuminance) of the brightest part of the beam. Until I happened here, I just referred to that as being how "bright" a light was. "Brightness", though, doesn't seem to be a defined technical term either, so some people use it to mean the total amount of light produced by a flashlight. To those folks, a flashlight that lights a big area with a dim light can be "brighter" than a light sabre that'll make a hawk's eye glow back at a mile, if it puts out more total light. So we have "throw" as something that the light sabre has more of than the floodlight.
Total light output is measured in lumens, and there's only a single number for a given light at a given setting. This can be spread out in a flood or focused into a very sharp beam with a reflector or lens without changing the total amount of light or lumen value. The brightness or illuminance you can bring to bear on an object at some distance and in some direction is measured in lux. This is commonly normalized to a distance of one meter. Usually, a single value is given for a light, and this is the brightness or illuminance at the brightest part of the beam. That is, it's the light's maximum lux at one meter. So if we take two lights with the same total output (lumen value) and sharply focus one, it'll have a higher maximum lux value at a given distance.
Lux is often referred to as "throw", since it's a measure of how bright the brightest part of the beam is. But "throw" implies a distance, like how far you can throw a rock, not a brightness. Referring to lux at one meter as "throw" isn't consistent with this concept. My first encounter with this term was on Quickbeam's Flashight Reviews web site, where he used a consistent definition. (Since it's not a technical term, it's subject to being defined differently by different people.) His definition is this: It's the distance in meters at which the brightest part of the beam will produce an illuminance of one lux. That provides a rational way to compare how far away you can light things up with different lights. A light with a "throw" of 30 meters lights up a possum at 30 meters just as brightly (one lux, in fact) as a light with a "throw" of 15 meters does when the possum is 15 meters away. The possum is as bright at 20 meters with the 30-meter "throw" light as one at 10 meters with the 15 meter "throw" light. So it suits me as a logical definition which describes how far a flashlight can "throw" its light. One lux, by the way, is pretty dim, but you could read by it with dark-adapted eyes.
It's this definition that I used in my earlier posting. An important thing to keep in mind is that this "throw" distance isn't directly proportional to the lux at one meter which a light produces. The relationship is a square law -- the "throw" distance is proportional to the square root of the one-meter lux value (or the lux value at any fixed distance). So if one light has twice the lux value at one meter, the "throw" is only about 1.4 times as great. You need four times the lux value at a given distance to double the "throw".
Quickbeam chose one lux as the illumination level at the "throw" distance. This is a pretty arbitrary choice, and you could argue forever about how much illuminace you need to "see" or "light up" an object. The light, after all, doesn't travel out and abruptly stop at some brick-wall "throw" distance, but continues getting dimmer and dimmer and dimmer as the distance increases. In back-yard tests, one lux is a reasonably descriptive value in my opinion, and it has the advantage of simplicity -- by using one lux as the "throw" limit illuminance, the "throw" distance in meters becomes numerically simply the square root of the lux at one meter. So if you have 100 lux at one meter, the "throw" is 10 meters -- you can light up an object 10 meters away with an illuminace of one lux. The paragraph immediately above this one is true regardless of what illuminance value you declare to define the "throw" distance.
My thanks to Quickbeam for establishing this definition of an otherwise vaguely-defined term. It makes a useful and quantitative way to directly compare lights. Whenever I get the opportunity, I like to encourage folks to use this definition so we can all communicate better by meaning the same thing when we use the term.
[My apology to Quickbeam, whom I mistakenly referred to as Quicksilver in the original posting. It's now been edited to correct the error.]
c_c
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As far as I can tell (I'm not a light or optics expert), "throw" isn't a technical term like illuminance and luminous flux, but something used pretty casually in places like CPF. Here, it's used in at least a couple of ways. One is the brightness (illuminance) of the brightest part of the beam. Until I happened here, I just referred to that as being how "bright" a light was. "Brightness", though, doesn't seem to be a defined technical term either, so some people use it to mean the total amount of light produced by a flashlight. To those folks, a flashlight that lights a big area with a dim light can be "brighter" than a light sabre that'll make a hawk's eye glow back at a mile, if it puts out more total light. So we have "throw" as something that the light sabre has more of than the floodlight.
Total light output is measured in lumens, and there's only a single number for a given light at a given setting. This can be spread out in a flood or focused into a very sharp beam with a reflector or lens without changing the total amount of light or lumen value. The brightness or illuminance you can bring to bear on an object at some distance and in some direction is measured in lux. This is commonly normalized to a distance of one meter. Usually, a single value is given for a light, and this is the brightness or illuminance at the brightest part of the beam. That is, it's the light's maximum lux at one meter. So if we take two lights with the same total output (lumen value) and sharply focus one, it'll have a higher maximum lux value at a given distance.
Lux is often referred to as "throw", since it's a measure of how bright the brightest part of the beam is. But "throw" implies a distance, like how far you can throw a rock, not a brightness. Referring to lux at one meter as "throw" isn't consistent with this concept. My first encounter with this term was on Quickbeam's Flashight Reviews web site, where he used a consistent definition. (Since it's not a technical term, it's subject to being defined differently by different people.) His definition is this: It's the distance in meters at which the brightest part of the beam will produce an illuminance of one lux. That provides a rational way to compare how far away you can light things up with different lights. A light with a "throw" of 30 meters lights up a possum at 30 meters just as brightly (one lux, in fact) as a light with a "throw" of 15 meters does when the possum is 15 meters away. The possum is as bright at 20 meters with the 30-meter "throw" light as one at 10 meters with the 15 meter "throw" light. So it suits me as a logical definition which describes how far a flashlight can "throw" its light. One lux, by the way, is pretty dim, but you could read by it with dark-adapted eyes.
It's this definition that I used in my earlier posting. An important thing to keep in mind is that this "throw" distance isn't directly proportional to the lux at one meter which a light produces. The relationship is a square law -- the "throw" distance is proportional to the square root of the one-meter lux value (or the lux value at any fixed distance). So if one light has twice the lux value at one meter, the "throw" is only about 1.4 times as great. You need four times the lux value at a given distance to double the "throw".
Quickbeam chose one lux as the illumination level at the "throw" distance. This is a pretty arbitrary choice, and you could argue forever about how much illuminace you need to "see" or "light up" an object. The light, after all, doesn't travel out and abruptly stop at some brick-wall "throw" distance, but continues getting dimmer and dimmer and dimmer as the distance increases. In back-yard tests, one lux is a reasonably descriptive value in my opinion, and it has the advantage of simplicity -- by using one lux as the "throw" limit illuminance, the "throw" distance in meters becomes numerically simply the square root of the lux at one meter. So if you have 100 lux at one meter, the "throw" is 10 meters -- you can light up an object 10 meters away with an illuminace of one lux. The paragraph immediately above this one is true regardless of what illuminance value you declare to define the "throw" distance.
My thanks to Quickbeam for establishing this definition of an otherwise vaguely-defined term. It makes a useful and quantitative way to directly compare lights. Whenever I get the opportunity, I like to encourage folks to use this definition so we can all communicate better by meaning the same thing when we use the term.
[My apology to Quickbeam, whom I mistakenly referred to as Quicksilver in the original posting. It's now been edited to correct the error.]
c_c
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(more something for the laser ppl among us 
