ViperZ

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.




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.




Reference

ovrrdrive

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...

BarracksSi

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

Mr. Underbridge

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.

znomit

No such thing.

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.
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.

bikingbrit

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...

cyccommute 

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.

znomit

One of your 750 lumen bulbs needs changing.

ggg300

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...

ovrrdrive

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.

Road_Biker

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.
https://www.sunriseinstruments.com/terminology.html

Chris_F

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.

cyccommute 

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.

Sir Bikesalot

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.

jeff-o

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

cyccommute 

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

dbs

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.

Chris_F

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.

BarracksSi

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").

cyccommute 

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

2manybikes

dbs

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.

dbs

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.

Chris_F

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

Lumen:
https://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.

Zero_Enigma

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.