Electronics, Lighting, & Gadgets - Are lumens cumulative?

I haven't found a good answer to this anywhere --

Coming from the music world, I've known that sound pressure levels from multiple sources do not add up in a linear fashion. That is, if one instrument can put out 100 dB and is joined by another, the result is not 200 dB (thank goodness!).

I understand that lighting measurements are different, and that besides lumens, we've also got candela, foot-candles, lux, etc.

So, do two 600-lumen lights really add up to 1200 lumens? Or do they still add up while measurements of luminance do not add up the same way?

I'm having difficulty finding answers elsewhere because hardly anyone else seems to care about adding multiple light sources -- they usually just describe single sources. There was a similar question posted on a physics forum somewhere from a year ago that hasn't gotten any response, either.

Technically, in the audio world 3db is considered twice as loud...

As for the lumens, I think that if you point them in different places then you definitely can add the lumens up. If you point them on top of each other I think it's pretty much a wash.

I think the ideal setup for me would be one light as a flood and one as a spot. Then they'd be covering different areas and add up.

There may be issues with constructive/destructive interference, but I was under the impression that the lumens were summed as long as they all focus on the same area. For example, 4 spotlights @ 200 lumen each will measure as 200 lumens if the beams don't overlap. However, if they are all pointed at one spot, measuring the lumens at that point should give ~800 lumen. This is just an educated guess, and I'm sure someone with more knowledge will chip in shortly.

Technically, in the audio world 3db is considered twice as loud...

Right, that's part of my questioning...

As for the lumens, I think that if you point them in different places then you definitely can add the lumens up. If you point them on top of each other I think it's pretty much a wash.

Okay -- then:

For example, 4 spotlights @ 200 lumen each will measure as 200 lumens if the beams don't overlap. However, if they are all pointed at one spot, measuring the lumens at that point should give ~800 lumen.

See what I mean? Directly conflicting thoughts. ;)

This is just an educated guess, and I'm sure someone with more knowledge will chip in shortly.

+1 (especially about finding someone with more knowledge :D )

Lumens do add. Sound pressure adds as well, it's just reported on a logrithmic scale so the value doesn't add (dynes add and convert to dB). Two 600 lumen lights, on at the same time, make 1200 lumens worth of light.

The problem with lumen measurements for bike lighting is it only tells half the story. The other half is where you put all those fancy lumens. Spray them out in all directions and 1200 lumens worth of light doesn't mean squat. The night will just be a little less black. Focus them on the road ahead and it's a ton of light. What needs to be considered is lumen output AND beam pattern as it's only the lumens that fall where you want to look that are important.

There may be issues with constructive/destructive interference, but I was under the impression that the lumens were summed as long as they all focus on the same area.

This isn't too much of consideration with non-coherent (ex: stuff that isn't a laser) light.

Twice as many lumens doesn't help you see twice as far, though, right? Or does that get into a logarithmic-type scale, too?

Say that one light can be useful for seeing objects out to 100 feet; would two lights work for 200 feet?

There may be issues with constructive/destructive interference, but I was under the impression that the lumens were summed as long as they all focus on the same area. For example, 4 spotlights @ 200 lumen each will measure as 200 lumens if the beams don't overlap. However, if they are all pointed at one spot, measuring the lumens at that point should give ~800 lumen. This is just an educated guess, and I'm sure someone with more knowledge will chip in shortly.

I'd say you are pretty close. Lumens are a measure of the luminous flux or the perceived power of light output. If all the lights are focused on the same spot, it will look almost like one beam of 800 lumen. In practice, however, I've found that a single source putting out 800 lumen is brighter than 4 sources putting out 200 lumen each. I think it has more to do with beam placement...it's hard to focus 4 beams onto the same place...then total output.

Wikipedia to the rescue!

http://en.wikipedia.org/wiki/Lumen_%28unit%29

The lumen can be thought of casually as a measure of the total "amount" of visible light emitted. So, lumens add together regardless of whether they are projected into the same space or not. If you've got three emitters beaming out light from approximately the same place, you could add them together.

Candlepower, OTOH, is a measure of luminous intensity based on the sensitivity of the human eye.

All of these are not to be confused with luminance, which also takes the surface area into account.

Note that the 60W halogen bulb used in those 15 million candlepower spotlights only outputs 1100 lumens!

Wikipedia to the rescue!

http://en.wikipedia.org/wiki/Lumen_%28unit%29

Yeah, I found that, and I read through all the other light-measuring-unit articles that Wiki has, too.

I'm beginning to think that the lumen isn't exactly a useful unit of measuring light output, especially when running UFO-like clusters that we do. If four 200-lumen lights (or eight 100-lumen lights) don't shine as far as a single one putting out 800 lumens, then it's not really safe to assume that you'll see twice as far by merely doubling the number of lights.

Twice as many lumens doesn't help you see twice as far, though, right? Or does that get into a logarithmic-type scale, too?

Depends on the optics. If you have a bare lightbulb then it spreads its lumens out in a sphere. Let's say that sphere is 1 foot in diameter and the lightbulb puts out 100 lumens. The surface of the sphere is 3.14 (pi) times the diameter squared. 3.14x1x1. So 100 lumens gets spread out over 3.14 square feet. About 31.8 lumens per square foot.

Now, double the size of the sphere. 3.14x2x2=12.56 square feet. The same 100 lumens makes for 7.9 lumens per square foot.

Doubling the distance from 1 foot to 2 feet makes your lumens per square foot go from 31.8 to 7.9. That's a BIG differene. And that's why beam pattern is important. This example is one where there are no optics, there is no beam pattern. Concentrating the beam reduces how much area these lumens get spread over as the distance increases. There's no simple answer to the question "does doubling the lumens double the distance" since it's so dependant on beam pattern. I'd rather have a weak source and good optics than a good source and poor optics.

Say that the beam pattern is the same, then; that'll be another controlled variable.

If so, then would a single 800-lumen lamp help the rider see farther than four 200-lumen lamps?

Taken another way -- how many 200-L lamps would it take to equal the "reach" of a single 800?

Say that the beam pattern is the same, then; that'll be another controlled variable.

If so, then would a single 800-lumen lamp help the rider see farther than four 200-lumen lamps?

Taken another way -- how many 200-L lamps would it take to equal the "reach" of a single 800?

Well, four 200 lm lamps focused onto the same area as the 800 lm lamp would illuminate that area the same amount. A single 200 lm lamp focused onto that area would be one-quarter as bright. That brightness (or luminance, to be more accurate) may be washed out by other lights, or may be below the threshold of your eye's light-gathering capability.

Let's eliminate the variable of where they're aimed, then.

Let's eliminate the variable of where they're aimed, then.

Given equal optics, four 200 lumen lamps pointed at the same point will be exactly as bright as one 800 lumen lamp.

The problem is you can only point four 200 lumen lamps at exactly the same point as a single 800 at one fixed distance. Beyond that the beams are converging or diverging. So at twice the distance those four lamps are spread out over a wider field than the single 800. Still, that assumes equal optics which is a HUGE assumption (typically the optical needs for a 200 lumen source are very different from those of an 800 lumen source).

In general though I'd rather have 4 200 lumen lamps than a single 800 lumen since the 4 lamps will allow me to select better beam placement. For a ride on a straight, fast downhill I can array them to light up the road in front of me AND down the road ahead. For a single track mountain bike ride I can array them around me in the near field so I get a wide picture of the close terrain.

Many bike lights have full power and half power modes. Going from half to full power, you can see that the beam is noticeably brighter, but it doesn't appear to be twice as bright. Your eye senses brightness logarithmically, similar to hearing.

It's good that it works this way, since daylight can be up to 100,000 lumens per square meter(lux), and moonlight is about .25 lumens per square meter. per Wikipedia. (http://en.wikipedia.org/wiki/Lux)

Let's eliminate the variable of where they're aimed, then.

I've been trying to come up with a way of explaining this and keep coming up short:mad: But, with a lot of hand waving and seat of my pants science, I'm going to try again.

Let's start with 200 lamps of 1 lumen each with a suitable reflector for focusing them. Turn them on and shine them on a target a given distance away and you'll have 200 lumens (the intensity of the light that we perceive). But each lamp is going to be a different distance from the center of the light beam array where the light is generated. The distance from the target is going to vary for lamp to lamp. The intensity of the light on the target is going to be a certain value for a lamp at the middle of the array and different for a lamp a the outside of the array (assuming a flat target). Since intensity follow an inverse square, the distance from the target is going to have a significant effect on the amount of light reaching the target.

Now take a source of light that puts out 200 lumen. The distance to the target is the same but the intensity of the light hitting the target is much greater because the light travels the same distance from a single source. Move the target and the amount of light still drops by the square of the distance but you only have one source of light rather than 200. The light from the array is going to be much, much dimmer.

There are going to be other factors involved too. The light is traveling through air which scatters and diffracts it and absorbs little bits of it. The more individual sources you use the more the light is subjected to these factors. You can think of it as a single high intensity source will hold together longer than multiple low intensity sources.

From the standpoint of how much light we perceive, lumens is a pretty good measure of light. Adding them together from multiple sources is probably correct for specific distances but once the distance changes...which happens all the time while moving...multiple sources aren't going to perform the same way as a single high intensity source.

Given equal optics, four 200 lumen lamps pointed at the same point will be exactly as bright as one 800 lumen lamp.

The problem is you can only point four 200 lumen lamps at exactly the same point as a single 800 at one fixed distance. Beyond that the beams are converging or diverging. So at twice the distance those four lamps are spread out over a wider field than the single 800. Still, that assumes equal optics which is a HUGE assumption (typically the optical needs for a 200 lumen source are very different from those of an 800 lumen source).

The ~3-4 inch separation of an array of four lamps will make little difference with regards to convergence, especially if you arranged the four lamps in a square pattern.

My Dinotte headlight has 50, 100 and 200 lumen settings with the same beam. I'm comfortable riding 8-10 mph at 50 lumens, 14-15 mph at 100, and 18-20 at 200 lumens. I can see details farther ahead with more light. So adding lumens really does help a lot. I guess this sortof contradicts my previous post. The higher setting doesn't look that much brighter, but it has a big effect on my riding speeds.

I was riding with a rider that had a HID light. Those are usually in the 600-800 lumen range. It had an even, wide beam that really lit the road and the sides, too. But my 175 lumen Fenix spot beam could easily reach farther than the HID, in a much smaller area.

I've been trying to come up with a way of explaining this and keep coming up short:mad: But, with a lot of hand waving and seat of my pants science, I'm going to try again.

Hey, I appreciate the effort; my original questions were about lumens, not optics, unequal distances, etc. If I could test them all in a lab, I'd still try to eliminate all those variables.

Hey, I appreciate the effort; my original questions were about lumens, not optics, unequal distances, etc. If I could test them all in a lab, I'd still try to eliminate all those variables.

I think we have to look at this as an optic problem. When you make the light, the source sits at the focal point of the mirror system. Light coming from the focal point is going to diverge as it travels away from the source. A stronger source is going to diverge less for a longer distance than a weaker source. If you take 200 lumen sources and shine them at the same thing you'll have two weak sources, each diverging as the inverse square of the distance. The amount of light hitting the target is going to be some fraction of a single 400 lumen source.

I can't do the optics (it's been a long time since physics) nor can I back it up but I can tell you from experience that 2 weak sources don't equal a single strong one. You can gang a bunch of low intensity lights together (like lots of LEDs are doing now) and add the lumens to get the 'same' output but it's not going to have the same distance and intensity as a single source of the same output.

My Dinotte headlight has 50, 100 and 200 lumen settings with the same beam. I'm comfortable riding 8-10 mph at 50 lumens, 14-15 mph at 100, and 18-20 at 200 lumens. I can see details farther ahead with more light. So adding lumens really does help a lot. I guess this sortof contradicts my previous post. The higher setting doesn't look that much brighter, but it has a big effect on my riding speeds.

I was riding with a rider that had a HID light. Those are usually in the 600-800 lumen range. It had an even, wide beam that really lit the road and the sides, too. But my 175 lumen Fenix spot beam could easily reach farther than the HID, in a much smaller area.

A question you can answer since you have variable light output settings (None of my lights do): I know you put out less light but does the distance that beam is thrown out in front of you vary by much? In other words, you should just get a less intense circle of light but the same distance from the bike at the different settings, correct?

A question you can answer since you have variable light output settings (None of my lights do): I know you put out less light but does the distance that beam is thrown out in front of you vary by much? In other words, you should just get a less intense circle of light but the same distance from the bike at the different settings, correct?

Yes, the light pattern is the same.

At dimmer settings, I would usually point it down, closer to the bike, since the beam spreads out more when it's far away.

Unless I expect to need the longer run times that I get on medium or low, I always run the lights at high power.

Yes, the light pattern is the same.

At dimmer settings, I would usually point it down, closer to the bike, since the beam spreads out more when it's far away.

Unless I expect to need the longer run times that I get on medium or low, I always run the lights at high power.

That's why I suspect that adding lumens by using multiple sources is an optics problem. Your optics are the same and gives the same pattern and distance with just a lower intensity. Add another source of light and your brightness will go up but the distance and pattern won't change, BarracksSi.

IMHO, how much light you need depends on the ambient light level. On a sufficiently dark road, I can see perfectly fine a long distance ahead of me with a poor 3 W bulb. 20 mph would be no problem at all for me.

However, on a road with lights nearby, or with a lot of oncoming traffic, your eyes will adapt to a different light level, and the light from your bulb is no longer enough to provide sufficient illumination.

Remember that the light that reaches the ground from your lamp decreases exponentially with distance, but ALSO that the light reflected from the ground does so too on its way back to you and your eyes. Narrowing the beam or increasing the area intensity by increasing light output, will both increase the amount of light reaching the ground, and the intensity of the light reflected back to you.

Another issue is how well you are seen by others. For an incandescent bulb in a parabolic reflector, bulb output divided by reflector cross-sectional area should give the apparent intensity to an observer.
A regular car headlight uses a 55 W halogen bulb and a typical reflector diameter might be around 15 cm. My 3 W lamps are about 4 cm in diameter. The car's reflector then has an area 14 times that of my lamps.

3 W * 14 = 42 (apparent watts)

That means that the apparent intensity (brightness) of my bike lamp should be close to that of a car's headlight. This assumes that the light output of the bulbs are equal in terms of visible light photons/watt, which probably isn't the case. But I couldn't find any data on that through a quick search.

My experience, though, is that a 3 W generator-powered bicycle lamp is nearly as bright to the eye as the headlight of a regular car.

That's why I suspect that adding lumens by using multiple sources is an optics problem. Your optics is the same and gives the same pattern and distance with just a lower intensity. Add another source of light and your brightness will go up but the distance and pattern won't change, BarracksSi.

Higher intensity should give better distance, though, at least enough to be noticeable by the eye. I've also got a Dinotte headlight, and between its high & low settings, it's easier to see farther when it's set at its brightest.

My understanding of Lumen is total light output measured from a light source, be it single or cluster. Using that definition I would say yes, Lumens is cumulative. I believe this is the value most suppliers will use for their products, as it is easier to draw a direct comparison across the board. So in the example previously given, 3x200L's would be the equivalent of a 1x600L, using that definition.

http://www.led.net/images/147-4.gif


If you are talking about End Lumens, that is a different measurement, and is dependant on many factors such as those previously mentioned IE, lenses, reflectors, source convergence, surroundings..etc. All those variables could increase or decrease the total end lumens depending on design and set up.

http://www.led.net/images/147-5.gif


Reference (http://www.led.net/pages/tech4.htm)

I guess the reason I was thinking that they weren't cumulative is that I can turn on my ~100 lumen LED light and it looks bright. Then I can turn on my MR16 and not even see the LED anymore because the MR16 over powers it. However, I can point the LED to a spot where the MR16 doesn't light up that well and the perceived brightness of the combo is increased...

Okay -- who's got a light meter and too many headlights?

I've been trying to come up with a way of explaining this and keep coming up short:mad: But, with a lot of hand waving and seat of my pants science, I'm going to try again.


You guys are talking at cross purposes a bit because he's a theoretician and you're an empiricist. ;) In *theory*, 4 200 lumen sources would be the same as an 800 lumen source. In *practice*, as you know, you're not going to be able to get the things focused on exactly the same beam pattern.

But the guy's asking the question in fairly general terms - it's a lot closer to additive than it is to logarithmic, which is really what he wanted to know, so good enough. I would also say that the less focused the beam pattern, the easier it would be to substitute more cheap lights for one expensive one.

Oh, and to address one other issue mentioned - interference fringes (constructive/destructive interference) won't occur unless you can get the sources as close as a few hundred nanometers apart.

Okay -- who's got a light meter and too many headlights?

No such thing. :p

Lumens are additive. No you wont see twice as far with twice the lumens because the beam spreads. To See twice as far you need 4x the lumens...or you can halve the beam angle.

The other issue is perception. If your 250 lumen light illuminates well for 16m (how far you ride in 3s at 20kph) but you want to see for 32m for those 40kph tailwinds ... well if you turn up the power to 1000lumens you get the same illumination out at 30m. Only now you have a really bright patch out to 15m which blinds your eyes, so you aren't SEEING quite out to 30m even though the illumination there is fine. :rolleyes:
Solution...put a second light in with the same 250lumens but half the beam angle pointed out to 30m. Same illumination out at 30 but 500lumens instead of 1000.

Technically, in the audio world 3db is considered twice as loud...

.

Not really. A 3 dB boost indeed requires twice the electrical power but this is percieved as only a moderate increase in loudness. The industry standard Daven step attenuators used in audio mixing consoles through the 1960s had twenty steps of 2 dB each. Alexander Graham Bell, who basically invented the transmission of sound by electricity, figured that one bel (i.e. 10 deci-bels) represented a "doubling" (or halving) of loudness. Of course this is all subjective but who are you or I to argue with him?

I figure that light works the same way. The most obvious example to me is the fluorescent "trouble" light I use in the garage. It has two 13 watt fluorescent tubes which can be switched on individually. The brightness increase obtained by using both tubes is underwhelming to say the least. You may have noticed a similar effect in your car when one headlight is out...usually a noticeable but definitely not an earth shattering difference.

Bottom line: doubling the lumens is noticeable, but not that dramatic. Multiplying them by ten is truly significant. Lamp salesmen may beg to differ...

Higher intensity should give better distance, though, at least enough to be noticeable by the eye. I've also got a Dinotte headlight, and between its high & low settings, it's easier to see farther when it's set at its brightest.

Yes, higher intensity will give a higher light density at a distance but that's for a single light source. Two light sources will give you a higher light density at the same distance but it won't give you more distance. A single source that produces twice as many lumens will give you more lumens at a longer distance.

For example, I use lamps that push out 750 lumens. I just built a lamp that pushes out 1500 lumens. I can stand in my back yard and illuminate a tree a block away with both lamps. If I shine 2 of the 750 lumen lamps on the tree, the tree has about the same amount of illumination as if I had a single 750 lumen lamp on it. I can see that it's a tree. If I shine the 1500 lumen lamp on it I can pick out detail...individual leaves...on the tree. Because the 1500 lm lamp has more light at the focal point of the lamp, it puts more on the target at a longer distance.

One of your 750 lumen bulbs needs changing.

Can a lower Lum light have a longer throw than a higher lum light?

Flash light vs Bike lights...seem to be about throw...

bright where? in the distance / right in front?

If you have the same lights, will more of them give them more throw? From my experience with my 3 200Ls
I see more detail with all 3 on high but not really much farther...

I also have a 5 watt "flood" halogen light running on 8 AAs for my commuter light. I like it more than the 200ls for my single speed town-e bike that will never go over 12 mph...the light is ideal for that speed even if it has less lum than a 200L.

My point, dunno, I forgot...

Not really. A 3 dB boost indeed requires twice the electrical power but this is percieved as only a moderate increase in loudness. The industry standard Daven step attenuators used in audio mixing consoles through the 1960s had twenty steps of 2 dB each. Alexander Graham Bell, who basically invented the transmission of sound by electricity, figured that one bel (i.e. 10 deci-bels) represented a "doubling" (or halving) of loudness. Of course this is all subjective but who are you or I to argue with him?

I figure that light works the same way. The most obvious example to me is the fluorescent "trouble" light I use in the garage. It has two 13 watt fluorescent tubes which can be switched on individually. The brightness increase obtained by using both tubes is underwhelming to say the least. You may have noticed a similar effect in your car when one headlight is out...usually a noticeable but definitely not an earth shattering difference.

Bottom line: doubling the lumens is noticeable, but not that dramatic. Multiplying them by ten is truly significant. Lamp salesmen may beg to differ...



My understanding of it, which of course could be way off, is that 3db is a scientific doubling of sound pressure, ie twice as loud but for our ears to perceive a source being twice as loud requires about a 10db increase. So for every 3db spl goes up the meter calls it twice as loud.

Not that I expect this will help much....

Fundamentally, irradiance (W/m^2) is the basic measure of electromagnetic energy. All light meters measure irradiance. Watts are the radiometric units of irradiance, like radio and microwaves. Lumens are irradiance detectable to the human eye.

As such, having two 200 lumen lights gives 400 lumen's. The lumens from a point source (such as a few feet away from a bare bulb) obeys the inverse-square law. The lumens decrease proportional to 1/r^2 for a spherical distribution of light due to conservation of energy.

The reflector, as demonstrated by the MR16 vs MR11 halogen bulbs, makes a big difference in how this energy is distributed. So, a better reflector will provide more lumens in a given direction. However, the lumens decrease with distance (in accordance with energy conservation) due to beam spreading. Also, what we see from the light on a road is it's reflection from a surface which spreads and diffuses the light (i.e. creates many small point sources) approximating the inverse-square law.

Clear as mud now? Yes, lumens are additive.

Ref:
Hecht, E. "Optics", Addison-Wiley, 1987, pp 44.
http://www.sunriseinstruments.com/terminology.html

Can a lower Lum light have a longer throw than a higher lum light?


Absolutely yes. For example, my measily little laser pointer probably puts out a mere lumen or less (I'd guess significantly less than a lumen), yet it has enough "throw" to reach clear across my yard to the tree line, a distance of perhaps 50 yards, with ease. My flashlight probably makes (I'm guessing) roughly 50 lumens or so yet it can't manage to make a visible spot on the same tree line. A laser is the extreme example of optics versus lumens.

One of your 750 lumen bulbs needs changing.

No, it doesn't. That's the crux of the issue. 2 lamps putting out the same amount of light as a single lamp won't throw a beam the same distance. If you could direct both beams through a lens to combine the light, you could get them to go the same distance but from two different sources using 2 different sets of optics, you won't get the same intensity at the same distance as a single source.

You had it right in your first post. Two lamps don't let you see twice as far which is what BarracksSi was asking. It just provides more light at the same distance. If you want to see twice as far, you need more intensity at the focal point of the reflector...or less angle.

That's one of the advantages of the MR16 vs the MR11 in halogen. The lamp at the focal point of both might be putting out 20 W but the lamp of the MR16 is larger...as is the reflector...and puts out more light. That's why an overvolted MR11 puts out 750 lm and an overvolted MR16 at the same voltage puts out 1500 lm.

That's one of the advantages of the MR16 vs the MR11 in halogen. The lamp at the focal point of both might be putting out 20 W but the lamp of the MR16 is larger...as is the reflector...and puts out more light. That's why an overvolted MR11 puts out 750 lm and an overvolted MR16 at the same voltage puts out 1500 lm.

If the bulbs in both MR11 and MR16 lamps are exactly the same, then where's the extra light from the MR16 coming from? My guess is at the cost of sidespill, which is of course perfect if all you want to do is see ahead. But if you want to be seen by traffic coming at you from the side, MR11 is better as it's much more visible at close to perpendicular angles.

If the bulbs in both MR11 and MR16 lamps are exactly the same, then where's the extra light from the MR16 coming from? My guess is at the cost of sidespill, which is of course perfect if all you want to do is see ahead. But if you want to be seen by traffic coming at you from the side, MR11 is better as it's much more visible at close to perpendicular angles.

The bulbs aren't exactly the same, and the reflector on the MR16 is much better.

The bulbs aren't exactly the same, and the reflector on the MR16 is much better.

And the bulb itself is larger...about twice as big.

Additive or not, it seems the key to compare one system to another should include some sort of normalizing factor such as area. Lux, as I see stated above, is lumens per square meter. To compare one lamp to another wouldn't you need to create a spec that would give you something like the lux @ 30 meters. But then you'd need to factor in the pattern dimensions.

Pragmatically it seems if you wanted 400 lumen it would be better to have multiple lights so you could create a pattern that fits the need. A very well lit spot only 3 feet in diameter is less helpful than a more poorly lit spot that is 20 fit in diameter. Of course poorly and well are relative but.....I'd like the flexibility everything else being equal. Something must be said for appearance as well.

Additive or not, it seems the key to compare one system to another should include some sort of normalizing factor such as area. Lux, as I see stated above, is lumens per square meter. To compare one lamp to another wouldn't you need to create a spec that would give you something like the lux @ 30 meters. But then you'd need to factor in the pattern dimensions.

I think an intensity vs. angle off center for both vertical and horizontal would be a nice thing to see. Unfortunately properly spec'ing a light fixture for an application is actually a technical task and I don't think your typical cyclist desires the challenge. Here's where the LBS comes in handy. You wouldn't buy a bike from a spec sheet so why buy a light from a spec sheet? As we've seen here stats (lumens) don't tell the whole story, just like you can't tell all you need to know about a bike from its weight. The best way to know if a light is right for you is to try it out under the conditions where you'll actually use it.

You had it right in your first post. Two lamps don't let you see twice as far which is what BarracksSi was asking. It just provides more light at the same distance. If you want to see twice as far, you need more intensity at the focal point of the reflector...or less angle.

How about this analogy --

Take firehoses, as an example.

Turn on one with a given line pressure, and see how far it goes. Get a second, and maybe a third, each seeing the same line pressure, using the same spray pattern, and see that they all go just as far as the first.

Get another hose with a higher pressure and set it with the same spray pattern as the others. It's going to shoot farther, right?

The single hose with higher pressure ("more lumens") will spray farther than multiple hoses with lower pressure ("less lumens").

How about this analogy --

Take firehoses, as an example.

Turn on one with a given line pressure, and see how far it goes. Get a second, and maybe a third, each seeing the same line pressure, using the same spray pattern, and see that they all go just as far as the first.

Get another hose with a higher pressure and set it with the same spray pattern as the others. It's going to shoot farther, right?

The single hose with higher pressure ("more lumens") will spray farther than multiple hoses with lower pressure ("less lumens").

That's pretty close. But the other issue of more lumens go further fits this analogy also. Take two hose spraying water so that their combined pressure is equal to the high pressure hose. Spray them from the two nozzles and see how far they go. They won't go as far as the high pressure hose by itself. They put out as much water but they just don't go as far. If you were to connect them with a tee so that they combine before they leave the nozzle, then they would go as far.

Same thing with the lights.

Good analogy;)

Okay -- who's got a light meter and too many headlights?

http://img148.imageshack.us/img148/6075/youguysarebothnutstb8.jpg

As I stated above, something must be said for appearance as well. :) But don't get me wrong. It looks nice. It must dim the house light when you put all that stuff on the charger. :)

So can someone help understand how to compare lumen to lux? Busch and Muller sell what appears to be some pretty nice lights but they rate their stuff in lux - 10 lux, 17 lux, 150 lux. How many lux does it take to get the equivalent of 500 lumen?

BTW: B & M now have a 1 led headlight capable of 50 lux. On their web site they have some pretty impressive pictures. Peter White is one of only a handful of distributors of this brand in the US. I do like German engineering, however, so I'm giving this a hard look. My current headlight is rated at 1000 candle power. What we really need is a few more standards. :)

Lux:
http://en.wikipedia.org/wiki/Lux

Lumen:
http://en.wikipedia.org/wiki/Lumen_%28unit%29

Comparing the two is like comparing the position of your car and its speed. If someone says "I'm 39 miles North of LA on the PCH" you can't tell how fast they're going with that info. Likewise you can't take Lux and convert to lumens or visa versa without knowing the beam pattern intimately.

http://img148.imageshack.us/img148/6075/youguysarebothnutstb8.jpg

How about a 3 meter powr up on that setup in a dark backyard/park/road? And also a shot at say 3-5 meters on a garage door. Cursious mans want to know the beamshots. Curious minds also want to know how Baileys doing. Curious minds also are curious if Bailey spots shades when everything is turned on. :D