Are lumens cumulative? For example if I use two 900 lumen lights on my bike do I get a 1800 lumen output. I know that sounds really not very bright (me, not the lamps) and I understand my eyes might not be able to 'use' the full 1800 lumens etc but what is the basic formula?

Cheers!

im not sure about the formula but why do you need 1800 lumens!

I want to see the future…..

seriously, it's just a thought about using two DX p7 lamps for around £70 and whether it would cause planes to veer off course and tides to fluctuate wildly due to their massive output or if it would just be a bit rubbish

i can guarantee they will not be 900 lumens.

i can guarantee they will not be 900 lumens.

How can you?? serious question, if there labelled as 900, the box says so, surely you cant just make a light and put any old output on them?

welcome to the world of lights!

no-one bar Nite Rider and Light An Motion test their actual light output.

Its all theoretical. Scandalous really. I have been saying it for years – its misleading and tantamount to fibbing to consumers.

Regardless of whether the live up to their claims, what happens if I use 2? Does the useable light output double? Plus a quarter? Sorry, we can't provide an answer at the moment?

Sorry – dont know the answer.

Never mind, I could just shut up bangin' on about it on here and actually try it….no wait , why would I do that?

how dare you. dont actually try it!!!

You've got a point, the whole of STW would collapse if the hypothetical ever became the actual – on with the question…

Well what's the answer then light boffins?

How can you?? serious question, if there labelled as 900, the box says so, surely you cant just make a light and put any old output on them?

Ah, but that is the theoretical maximum and does not take into account any losses 🙄

I think the answer is yes, it does double.

Joe

Huzzah! I'm gonna Quadraphenia up my bike and blind a badger!

I use two MTE SSC P7 torches and they are 900 lumen total for the two.
One on the bars and one on the helmet and they are great. Have the bar mount on full and helmet on medium most of the time unless I am unleashing it downhill.

I've been looking for a way to explain, this is stolen from someone else as it makes good sense! There are too many variables to get exact results from two different sources, beam patterns wont meet etc etc.

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.

Now that's a quality answer. cheers!

How can you?? serious question, if there labelled as 900, the box says so, surely you cant just make a light and put any old output on them?

I think the Leds themselves are "up to 900 lumen" (at 2.8A I think). If you look at the Dx web, even the single 18650 torches are quoted as "up to 900 lumen"

My driver, for example though, puts out about 2.2A so i guess I wont be getting the magic 900 figure. I tried a simple driver tonight actually (a 1.8 ohm power resistor) Got 2.5A but chickened out when resistor quickly climbed to 130 deg! To hot an inefficiant to be practical.

The amount of light falling per second on a unit area placed at unit distance from such a source.

If all the emitted unit light (your claimed 900L torch) reaches without refraction, deflection or absorbtion the unit distance (the point at which you're measuring), then you would have your claimed Lumens.

But, of course, your reflector, lens, moisture in air, contamination, beam angle, means that your lamp will never reach it's max potential.

In theory, if all the light from 2 900L sources reaches a point uninterrupted and without any deflection you would have 1800L

So, that should sort it out for you, eh? 😀

Obviously, don't believe any of the tosh I come out with.

I have been saying it for years – its misleading and tantamount to fibbing to consumers.

I heard that Hope gold hubs weren't made of real gold either.

My Hope 4 (quoted 960lm) kicks out somewhere around 700lm I read in the L&M comparative test, however I'm more than happy with it.

Doesn't stop me wanting a DX for my lid though.

Sigh… the Q isn't really about the number of Lumens claimed by anything but how they act together i.e. is there a law of diminishing returns at work here when adding two light sources. I don't care if the torch is 900 lumens or not just what happens when you use two!

ta for the answers though!

OK, the real world answer is that if you put one on your head and one on the bars, it'll be much more effective than just one on the head or bars, simply because of the different applications. Effectiveness is down to a lot more than lumens. (I sound like an Ayup apologist when I say that, but it's true) I run 2 P7 torches as do a lot of people, it's a very effective setup (though a P7 flood as a helmet light is a bit of a liability in low cloud/light fog!)

To confuse you further, your eye doesn't respond to light linearly. So if lumens do add linearly your eye will percieve the change differently anyway!