Electronics, Lighting, & Gadgets - Magicshine P-7 Headlight Measurement

I just bought a Magicshine MJ-808 LED headlight from Geomangear, as recommended on this forum. Thanks a lot guys!!! First thing I wanted to do with it is measure its output, as that's been the topic of much discussion, but I'll start with a few basics.

Ordered over the internet even though the notice was up that it was out of stock. Immediately was informed that the order shipped, though, and free two day delivery! Very nice!! Nice basic packaging, competent workmanship but I'd really like to see a beefier attachment to the mount, everything very acceptable for twice the price. The battery indicated a full charge!!!

Now a little on my background for context. I'm a bicyclist riding with toe clips since 1973, transcontinental with Bikecentennial in '76, club riding untill the early '80s, fair weather commuting all over the Silicon Valley for the past 25 years, now riding on an '82 Bruce Gordon framed 10-speed, and '69 Raleigh Superbes around town and with the San Jose Bike Party. I'm a holographer and photonics technician since 1980, working with all kinds of lasers, optics and light measurement systems.

So now that I want to ride much more at night, I decided to get a headlight. After looking around and reading here, I decided to go for the Magicshine. My measurement set up is shown below:

http://sites.google.com/site/bobspublicpics/Home/MagicshineTest.jpg

The headlight is held by a test tube clamp over a circular target with an area of 0.5 square meters. I drew small circles on the target indicating where I'd need to place the detector to adequately sample the whole field. Instead of measuring all of them, I measured the center, and one for each of the five rings of little circles. Then, I multiplied each ring's sample measurement by the number of little circles in that ring; 6 in the first, then 12, 20, 26, and 32 in the outermost ring. I figured this was OK because of the symmetrical pattern emitted by the housing.


I used a Lutron LX-101 Lux Meter. One Lux equals one Lumen per Square Meter. I added the lux measurements calculated as above, divided by 97 to get the average, then multiplied by 0.5, for the high and low settings for the headlight. For the high setting I got 764 Lumens, and for the low setting it was 267 Lumens. This compares with the specified 900 and 200 respectively.

There's a glass window (they call it a "lens") protecting the aluminum reflector, and despite it being described as coated, I can't see any evidence of any antireflection coating. So allowing for a 7% reduction because of the window, my measurement was about 92% of the specified amount. I can certainly see 8% loss in reflectivity from bare polished aluminum and my measurement inaccuracy (especially in the hot spot), so I think they're doing everything they say they are as far as light output is concearned. Better than spec at the low setting. The center setting is in the middle.

The spot of light output from this headlight covers 85 degrees, and the central hot spot is about 16 degrees of that. That comes out to a hot spot about 5.5 feet wide at 20 feet distance. I increased the distance between the headlight and target, so that the central hot spot covered it. I stopped when the lux measurement at the edge was 1/3 of the center measurement. Here I calculated 337 Lumens, or 44% of the total output.

Next thing I want to do is measure some other lights out there for comparison. This was my first go at it, though, and am really happy with the results. There's plenty of light for my purposes, and if the run time is also as they say, I'll be a very happy camper.

thanks so much for finally putting real numbers to this discussion. your work is very appreciated:thumb:

I second the thanks for doing this work. If you wanted to, I think you even get local cyclists to loan you some popular lights and help with the measurements.

Could this setup also measure tail light output? I think the tail light LED's emit almost monochromatic red light. Does the wavelength sensitivity of the Lutron LX-101 Lux Meter correspond to the wavelength sensitivity of the human eye either when light adapted or when dark adapted?

Again, I greatly appreciate your effort.

Thanks for doing this!

Dan

I second the thanks for doing this work. If you wanted to, I think you even get local cyclists to loan you some popular lights and help with the measurements.

Could this setup also measure tail light output? I think the tail light LED's emit almost monochromatic red light. Does the wavelength sensitivity of the Lutron LX-101 Lux Meter correspond to the wavelength sensitivity of the human eye either when light adapted or when dark adapted?

Again, I greatly appreciate your effort.

I thought I'd cut a few holes in the end of a box and take it to the San Jose Bike Party, which rides monthly between 8pm and midnight. Taillights are a bit trickier because you want to capture all the light, and it's going everywhere. Headlights with well defined edges to their pattern are easy. But for taillights, I have an 8" diameter integrating sphere with a 2" diameter input port, so I can measure anything that'll fit in there. May be easier to measure one of the LEDs and do a napkin calculation.

Monochromatic LEDs' spectral bandwidth is typically around 30nm, so a luxmeter is still probably the way to go. Radiometric units (Watts, Joules, ergs) is for when you want to know how much energy is in the light. Photometric units (lux, lumens, footcandles) are what you want when you're asking "How bright does it look to me?". A Watt of green light is waaaay brighter than a Watt of red light, day or night,
but it's the daylight adapted eye the photometric curve is based on.

Nice work.

The output is actually higher than I would have expected--I poked around the P7 datasheet for a bit and estimated the actual output would be somewhere in the ballpark of 600 lumens.

900 lumens is actually the high end of the 'typical' P7 luminosity spec. The P7 used in the Magicshine light is from the 'C' luminosity bin (rated at 700-800 lumens @ 2800 mA of current). Actual output is even lower, since the Magicshine drives the LED with 2400 mA of current, not 2800 mA, and the junction temperature is well above 25 C (elevated junction temperature also causes output to drop).

I was considering doing some measurements of my own (I'm a PhD student in an atomic physics/optics lab) but I don't have access to a luminosity meter, just a power meter--I would have had to do some sort of conversion from watts to lumens based on the LED color temperature or something.

Ok I'm confused, probably something simple that I missed, but you say it's 764 lumens on high at one point, but then say it's 337 lumens at another point.

Is the 764 lumens at the "hot spot", and the 337 the lumens of the "spill"? Or am I just still confused! LOL!

The 764 lumens is the total light output from the head, defined by making the entire patttern fit into an area of 0.5 square meters. The 337 lumens is the light output into the central hot spot only, defined by making just the hot spot fit into the same area.

Hey Jefferee, there's a Lutorn LX-101 luxmeter on ebay for about $50 now. They're not expensive. I'd love for someone to verify my measurement!

Well, I have good news and bad news. The bad news is that I found a huge mistake in my measurement technique above, thanks to the prodding of a member here off line. My new measurement now gives 375 lumens on high, 180 lumens on medium, and 98 lumens on low. These numbers were taken after a ten minute warm-up of the headlight, with a fan blowing on it for cooling.

I realize this is a huge error, and puts a question mark on my technique, but that's OK as long as I keep going here and refine it over time. It's why I really want to compare different headlights, to see the relationship between measured numbers and real world brightness relationships that everyone can see. I'll have my set-up at this month's San Jose Bike Party (on the 16th) for that purpose.

The good news is that this headlight is as bright as anybody needs (certainly for on the road), despite my low measured numbers. So don't be hung up on the numbers. And please bear with me while I get it right. Absolute light measurement is difficult to do on the cheap, as I'm doing it here, which is why I feel it's important to nail it down.

400-500 lumens is right in line with most p7 output.. Even though many say 700-900 lumens, the actual lumens out the front is much less.

Comparatively, my Trinewt which is rated at 490 lumens has slightly less output than all of my p7's.

So this light is right in line with many lights that advertise 500+ lumens..

LEDs are capable of much, much more output when really cooled properly and modulated way above the flicker rate. Considering the compromises obviously made to make this product cheap, and the poor job of getting the heat away from the emitter itself and into the air flow around the head, I certainly believe there's much more light to get out of this unit than it's giving now.

measurement now gives 375 lumens on high.


believe there's much more light to get out of this unit than it's giving now.

http://www.bikeforums.net/showpost.php?p=9756226&postcount=216

Those guys at CPF take that stuff seriously. I was surprised too when I learned that all these so-called 900 lumen P7 lights are really 400.

The MS has a crappy reflector, the pill is barely contacting the housing, and the driver is suboptimal. The emitters are probably the earliest C bin too. The wiring and driver are pretty cheap too, but this will affect longevity rather than output.

The biggest mistake is the reflector. A bunch of light is visible at the periphery of the beam, and it's wasted. It's not reflected properly, in part due to the hole in the reflector being about 2mm in diameter too large to match the diameter of the emitter.

My DX MTE P7 flashlight (now $34) is brighter than the MS for about the first 30 minutes, until the 18650 cell is burned down a bit. I use it as a "high beam" only, not for more throw, but better illumination of nearby objects.

I don't agree with your evaluation of the reflector. I'd have put a lens immediately in front of the emitter to send more of the light into the hot spot, as a reflector gets way too big very fast. That's something I'll probably do as a modification.

Also, the diameter of that hole isn't critical as no light is lost through it. The emitters don't sent light backward. If the emitter was mounted behind the hole, you'd lose some light then, but I don't see that happening.

I LIKE the extra spill. I think the ratio is just about right. If it had any less spill, I'd have to frost the lens or reflector to get more spill to evenly illuminate the road. I don't want just a spot.

So working backwards from 370 lm, 20% loss in optics, and 10% for the 2.4A drive current, -> 510 bulb lumens.

Assuming the LED is in the 800-900bin its losing 40% of the output to heat. How hot is it?

So working backwards from 370 lm, 20% loss in optics, and 10% for the 2.4A drive current, -> 510 bulb lumens.

Assuming the LED is in the 800-900bin its losing 40% of the output to heat. How hot is it?

I believe there are some other losses: some of these high-power lights have been known to cause interference with wireless cyclocomputers, so either magnetic or RF fields are created, causing some of the lost power. I'd also guess that the lumen/power input ratio is not a linear ratio.

I believe there are some other losses: some of these high-power lights have been known to cause interference with wireless cyclocomputers, so either magnetic or RF fields are created, causing some of the lost power. I'd also guess that the lumen/power input ratio is not a linear ratio.

All that RF is generated in the regulator, so it does draw the battery down some but it happens BEFORE the LED, so given that the current has been measured at the LED, this doesn't count.

Again, I say, regardless of the numbers, the answer is "plenty bright enough." Arguing about the numbers is just pissing into the wind, or trying to justify having spent hundreds more for some other light. I certainly have spent hundreds more in other lights before I bought this one.

Again, I say, regardless of the numbers, the answer is "plenty bright enough." Arguing about the numbers is just pissing into the wind, or trying to justify having spent hundreds more for some other light. I certainly have spent hundreds more in other lights before I bought this one.

My intent is really to find a simple and consistent method to measure these headlights with as much accuracy as possible, because I like doing that kind of thing. I've recently started riding a lot at night, and needed something better than the dynohub powered headlight on my '69 Raleigh Superbe. That's what got me going with this.

So much of the discussion about bike lights is about "how bright" they are, which is as subjective as it gets. Then there's lumen numbers thrown about, comparisons made by cameras, and marketing BS all over the internet but I couldn't find any real numerical comparison measurements. Now that I have a nice bright light, though, I want to know if it can be made brighter with some modification.

I also want to know how bright is too bright for a bike headlight. In the meantime, though, I'd like to measure other lights and maybe make a credible comparison chart. I hope it will reduce the argument about numbers, but you might want to stand upwind anyway because there are those on this forum who have much more belligerant confidence in their wordy drivel than real knowledge about light, much less the arcane activity of measuring it accurately.

Appreciate the honest disclaimers on the measurements.

I don't know if you have any other LED flashlights lying around, but if it's not too much trouble, it would be even more helpful to find out the numbers of the Magicshine relative to one of these other lights using your measurement system.

For example, if your measurements rate the Magicshine at 375lumens, but a Fenix L2D flashlight at 180 lumens, it would be helpful in getting a sense of where your measurements lie relative to the rest of the manufacturer claims.

http://www.bikeforums.net/showpost.php?p=9756226&postcount=216

Those guys at CPF take that stuff seriously. I was surprised too when I learned that all these so-called 900 lumen P7 lights are really 400.

The MS has a crappy reflector, the pill is barely contacting the housing, and the driver is suboptimal. The emitters are probably the earliest C bin too. The wiring and driver are pretty cheap too, but this will affect longevity rather than output.

The biggest mistake is the reflector. A bunch of light is visible at the periphery of the beam, and it's wasted. It's not reflected properly, in part due to the hole in the reflector being about 2mm in diameter too large to match the diameter of the emitter.

My DX MTE P7 flashlight (now $34) is brighter than the MS for about the first 30 minutes, until the 18650 cell is burned down a bit. I use it as a "high beam" only, not for more throw, but better illumination of nearby objects.

Great link with neat data. I love this stuff, and it's quite useful, especially for relative comparisons.

I will note that the FENIX L2D is listed on this link with 145 lumens measured, whereas the manufacturer and other home-measurers have generally pegged this light at 180-200 lumens (it's a popular light, was the "it" light 2 years ago.) While I personally don't care about 145 vs 200 lumens since the Fenix works great and is super bright, this measurement discrepancy becomes a bigger deal when talking about a light like the Magicshine, which has more lumens in question. I could easily see a 150-200 lumen differential due to varying light measurement technique, with this measurement coming in on the low end.

I mention this just to alert the possibility of a "lowball" estimate using the revised measuring technique. I don't own a Magicshine and am not defending it per se, but I was surprised to see how low something as studied as the Fenix L2D measured out on that link, given how effective and well-regarded it is. (I'd still love my Fenix even if it was 50 lumens - it works great!)

I do appreciate the measurements being made. Sorry if I gave the impression that I was blowing off the efforts here. I was just a little bugged by people (probably who just spent $300 on a 400 lumen light) saying in effect that the MS was obviously worthless crap since they said it was 900 lumens and it was actually 400. My response is, REAL numbers are good to have for comparison, but in the end the truth is that for $80 this is plenty of light for almost anyone's use.

Hey ItsJustMe, no problem! I agree with you, it's the real world performance that counts and not the numbers. I hate marketing BS with numbers. Like MS using the output rating for the raw emitter instead of measuring their headlight output and publishing THAT. Also, knowing the lumens in the "hot spot" as a percentage of the total, as well as the relative size of the "hot spot" is nearly as important as the total output.

Well I needed a new flashlight, so I just ordered a Fenix L2D as suggested above. Should be here in a week or so, and I'll post the results of my measurements with it then.

+1 good info, thanks.

My $.02, Lumens are for marketing and sales, Lux is for the ride :-)

Yesterday I received a new Fenix L2D flashlight, and immediately put it to the test. A picture of my set-up is below.

http://sites.google.com/site/bobspublicpics/_/rsrc/1255218162571/Home/LumensTest.jpg

The light was positioned to fill the target circle fully as with the Magicshine headlight, with the "hot spot" centered on the detector in the middle reading at the maximum. Ten measurements were taken, each being one sensor width away from the previous, starting in the center and going radially toward the edge. When finished, the center was measured again to verify the constant output of the flashlight. The measurements were averaged and the number of lumens calculated.

The output of the Fenix L2D flashlight in "Turbo" mode, after about five minutes of warmup and with the fan on it for some cooling, was 110 lumens by my calculation. This is 61% of the specified 180 lumens, and 76% of the measurement in the thread mentioned above. My calculation of the output of the Magicshine headlight was 36% of its specified output on high. I plan on measuring lots of other headlights in the next few weeks, and will start another thread with the results afterward.

Ooh, hologram on the wall. We still have some holograms around somewhere, I should put them back on the wall.

Ooh, hologram on the wall. We still have some holograms around somewhere, I should put them back on the wall.

Here's one I made in "84. It's a '72 Campy derailleur I'm using to this day. First one I sold was to one of the owners of Wheelsmiths bike Shop in Palo Alto, where my studio was at the time.

http://sites.google.com/site/bobspublicpics/_/rsrc/1248655628337/Home/Derailleur.jpg

I have both the Magicshine LED and a Fenix L2D. I've used the Fenix for 2+ years on my handlebar, and moved it to my helmet when I got the MS. The Magicshine puts out way more light than the Fenix, which served my needs just fine for the past two years.

The lumen numbers are useful for comparing lights, but the main point to keep in mind is that there are few options that can compare with the MS in its price range. If you can spend $300-500 on a light system, I am sure there are better options. For less than $100, not many. The P7 flashlights are main competitors for MS in that price range, but they have a number of drawbacks (in my view) -- including size, weight, non-standard batteries, and the size and shape of the lightbeam.

In another forum there was a thread titled "lumen schoomen", and I gotta say I feel the same way at this point.
All these numbers and all this talk and comparing, I'm over it.
All I know is this, when I turn my MS on on the trail, it's BRIGHT. How many lumens? Don't know, don't care anymore. I just know that it's brighter, lights up a bigger area and throws further then just about every light on my group rides, and costs less, much less.

"Lumen schoomen" is the appropriate response to all the wildly inaccurate and often maliciously misleading numbers thrown out by manufacturers too lazy to measure their products, and advocates getting all their information from the boxes those products come in. I'm "in to" measuring light, and that's the motivation for my little project here.

So what's your point? That I shouldn't post my results here? That now that YOU are happy with the light YOU have, that nobody should be concerned with the real output of these lights? Is that why you chose to come onto this thread and essentially say the topic of the thread is unnecessary?

"Lumen schoomen" is the appropriate response to all the wildly inaccurate and often maliciously misleading numbers thrown out by manufacturers too lazy to measure their products, and advocates getting all their information from the boxes those products come in. I'm "in to" measuring light, and that's the motivation for my little project here.

So what's your point? That I shouldn't post my results here? That now that YOU are happy with the light YOU have, that nobody should be concerned with the real output of these lights? Is that why you chose to come onto this thread and essentially say the topic of the thread is unnecessary?

RR - Don't get me wrong. I appreciate the testing you've done. It's always good to see real data and facts to make comparisons. My post was mainly directed at some of the negative comments I've seen about the MS light by people who obviously have not actually seen or used the light.

I think this is one of the more useful threads out there: at least some real numbers to compare lights with versus the 'my light works great' thing.

I was running a 10W halogen prior to getting my Airbike 900. The halogen cost me (2 years ago) almost what a MagicShine costs now, and puts out a mere fraction of the light of my Airbike.

I ride mostly on really bumpy country roads with no streetlights, and at my age I don't think I'd want anything less than what I have now.
I've ordered one of the '900 lumen' P7 Ultrafire flashlights from hkequipment, not because it is good value (the MS is a better deal with the batteries and bicycle mount) but I needed a light for hiking the trails around here at night, and will see how it compares to the Airbike as far as beam and brightness goes.

I also welcome some real numbers in the debate and discussion of lights. Many things might effect which light we like best for our own riding (beam shape, color, spill, etc) but some really comparable numbers on actual output with the optics in place is a very useful place to start. Thanks for your work and God bless those who love to collect useful data the rest of us have neither the tools nor time to collect! I loom forward to seeing some of your other data.

Thanks for all the testing. Hopefully you can get a bunch more to compare it to so people can make a really informed purchase. I'd be interested in seeing some taillight figures too.

I love my MS and it is one of the top 3 brightest lights in my group (20 or so) and WAY cheaper than most of them.

I also know it's brighter than a scooters headlight and cars dim their high beams now.

I'll be happy if the magicshine is at least 2x brighter than my 200L. I paid $125 for the 200L (batteries extra), so the magicshine will actually be less.

Good job on the testing. It's really helpful to see that your test also pegs the Fenix L2d at a much lower lumen rating than the manufacturer - which suggests that your Magicshine results (tested similarly) will likely come in significantly (much signficantly, judging from the Fenix numbers) than the manufacturer claims.

With your measurements, sounds like the Magicshine is at least 3x brighter than a Fenix L2D on turbo - that's hella bright, and I know you won't find any more inexpensive commercially non-DIY solutions with close to comparable brightness. Your helpful review further encourages that the Magicshine is the real deal with brightness as claimed by people and reviewers.

RapidRobert;

If possible try to test some Niterider lights. They CLAIM on their web site that the light outputs they list are based on the average of ten production units tested using an integrating sphere. If true this might allow you to calibrate your test procedure.

I do appreciate the measurements being made. Sorry if I gave the impression that I was blowing off the efforts here. I was just a little bugged by people (probably who just spent $300 on a 400 lumen light) saying in effect that the MS was obviously worthless crap since they said it was 900 lumens and it was actually 400. My response is, REAL numbers are good to have for comparison, but in the end the truth is that for $80 this is plenty of light for almost anyone's use.


This is the truth.

Well I finally got the time to build up a more portable system to measure bike headlights. It's an 8" diameter Labsphere integrating sphere with my Luttron LX-101 luxmeter attached to the exit port and mounted on a tripod as shown below.

http://sites.google.com/site/bobspublicpics/_/rsrc/1260257804815/Home/IntegratingSphere.jpg

I used a fan to cool the Magicshine headlight mounted on my handlebars, which was pointed into the entrance port of the sphere. The airflow (8 mph) and its temperature (51 degrees F) past the light are shown on the windmeter held next to it.

http://sites.google.com/site/bobspublicpics/_/rsrc/1260257838895/Home/IntegratingSphere%26LampCooling.jpg

I took the sphere over to the home of a local young laser enthusiast, LaserBen (who buys, repairs and sells surplus lasers), to calibrate it. He used a Spectra-Physics Millenium laser and a Coherent power meter, as seen below. According to the Photopic Spectral Luminous Efficiency Function, one Watt of laser light at 532nm equals 601 lumens. We put measured amounts of that wavelength into the sphere, between 0.1W and 1.9W, and noted the corresponding values displayed on the meter. I then converted Watts into lumens and graphed the results. The result was a straight line, as expected.

http://sites.google.com/site/bobspublicpics/_/rsrc/1260257872546/Home/IntegratingSphereCalibration.jpg

Using this method, I measured my headlight to be 330 lm on high, 185 lm on medium, and 95 lm on low. Not bad! Despite wishing I verified the calibration laser's output power with more than one meter, these results agree quite well with those reported above. Certainly NOT 900 lm, or even the 450 lm reported elsewhere. So now i'm pretty confident with my measurements, and ready to start gathering data about other headlights and flashlights (for comparison), and modification of my light to increase its output.

Well I finally got the time to build up a more portable system to measure bike headlights. It's an 8" diameter Labsphere integrating sphere with my Luttron LX-101 luxmeter attached to the exit port and mounted on a tripod as shown below.

http://sites.google.com/site/bobspublicpics/_/rsrc/1260257804815/Home/IntegratingSphere.jpg

I used a fan to cool the Magicshine headlight mounted on my handlebars, which was pointed into the entrance port of the sphere. The airflow (8 mph) and its temperature (51 degrees F) past the light are shown on the windmeter held next to it.

http://sites.google.com/site/bobspublicpics/_/rsrc/1260257838895/Home/IntegratingSphere%26LampCooling.jpg

I took the sphere over to the home of a local young laser enthusiast, LaserBen (who buys, repairs and sells surplus lasers), to calibrate it. He used a Spectra-Physics Millenium laser and a Coherent power meter, as seen below. According to the Photopic Spectral Luminous Efficiency Function, one Watt of laser light at 532nm equals 601 lumens. We put measured amounts of that wavelength into the sphere, between 0.1W and 1.9W, and noted the corresponding values displayed on the meter. I then converted Watts into lumens and graphed the results. The result was a straight line, as expected.

http://sites.google.com/site/bobspublicpics/_/rsrc/1260257872546/Home/IntegratingSphereCalibration.jpg

Using this method, I measured my headlight to be 330 lm on high, 185 lm on medium, and 95 lm on low. Not bad! Despite wishing I verified the calibration laser's output power with more than one meter, these results agree quite well with those reported above. Certainly NOT 900 lm, or even the 450 lm reported elsewhere. So now i'm pretty confident with my measurements, and ready to start gathering data about other headlights and flashlights (for comparison), and modification of my light to increase its output.

I'd like to see what a Dinotte 200L rates with the same test.

excellent thread. lumens are not very interesting without knowing what methods the manufacturers used to arrive at that number. unless every manufacturer measures their light output using the same, standardized method, it's not exactly an apples-apples comparison. not sure it is feasible for RapidRobert to do this, but it would be a great service to the community if more lights can be tested using the same setup (or at least some of the more popular/recommended setups). at least that way we can know the relative performance of the different lights.

Beam pattern is also quite important. I was doing my highway commute route the other day with my DiNotte 600L and my Dereelight DBS, and while the DBS is "only" a ~250-lumen light, it completely dominates the 600L for actually seeing forward down the highway, because the light goes where I need it. I'd compare it to driving with one's parking lights, then switching on the low-beams. Lumens are one factor to look at, beam pattern's another.

Beam pattern is also quite important. I was doing my highway commute route the other day with my DiNotte 600L and my Dereelight DBS, and while the DBS is "only" a ~250-lumen light, it completely dominates the 600L for actually seeing forward down the highway, because the light goes where I need it. I'd compare it to driving with one's parking lights, then switching on the low-beams. Lumens are one factor to look at, beam pattern's another.

Absolutely right! That's why I advocate describing headlight performance by the following four numbers:
1) Total Lumens
2) Hot Spot Lumens
3) Total Divergence
4) Hot Spot Divergence

All four numbers are easier to measure with my first system above, which is why I started with that one instead of the sphere. The sphere is easier to take to bike shops for comparisons, but the benchtop system gives a better picture more accurately. Perhaps with the addition to a camera looking down on the target to see beam shape as well.

I believe there are some other losses: some of these high-power lights have been known to cause interference with wireless cyclocomputers, so either magnetic or RF fields are created, causing some of the lost power. I'd also guess that the lumen/power input ratio is not a linear ratio.The RF noise power loss is real, but miniscule. As for the lumens/power ratio not being linear -- you're right, it's not.

RapidRobert;

Personally I believe that beam shots taken on a surface perpendicular to the light beam are also definitely worthwhile. The majority of LED lights seem to have what is basically a round beam like a flashlight while those that are road legal in Germany have a sharp cutoff, like a auto low beam headlight., to prevent blinding oncoming traffic and pedestrians. For most American LED bike headlights the differences in beam pattern are primarily divergence amount and spill divergence. Amount of light in the main beam and the spill beam also vary.

Lux figures are not much help as they are normally given for the brightest part of the beam and are totally distance related. Busch & Muller of Germany give their Lux figures for a distance of 10 meters while a British dynamo headlight maker lists 4000 Lux but this is at a distance of 1 meter. At 10 meters this should equate to 40 Lux if my calculations are correct.

Now I have more to talk about at the coffee shop.

Absolutely right! That's why I advocate describing headlight performance by the following four numbers:
1) Total Lumens
2) Hot Spot Lumens
3) Total Divergence
4) Hot Spot Divergence

All four numbers are easier to measure with my first system above, which is why I started with that one instead of the sphere. The sphere is easier to take to bike shops for comparisons, but the benchtop system gives a better picture more accurately. Perhaps with the addition to a camera looking down on the target to see beam shape as well.


Huh. I just slap 'em on the bike and ride. This is the DBS alone on a pitch-dark 600-foot straightaway in an area infested with deer, which is its showcase environment. Light metering was adjusted to match how it looks to my dark-adapted vision... might be a tad overcooked, but pretty close actually:

http://i240.photobucket.com/albums/ff237/mechBgon/036.jpg

Nice flashlight. I give it a 7 :)