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:
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.