Maglite Reflector / Bezel / Lens Testing

[LilKevin715]

A lot of us guestimate how much light is lost from the emitter to OTF for LED flashlights. I frequently see members posting a inconsistent percentage number of emitter vs OTF lumens (anywhere from 66 to 85 percent, depending on the type of lens & setup used). This is one of the very few topics here on CPF that hasn't been fully tested (to my knowledge). Recently I built a lightbox (details here) to measure realative output that can be used to calculate estimated lumens, which I call Lightbox Lumens (LBL for short). Since I now have the ability to measure relative output its time to do some definitive testing and put these claims to the test (and to satisfy my curiosity). This testing will determine light loss from components such as reflectors, the bezel, and lenses.

Test equipment:

Lightbox (duh)
LX1330B Light Meter (+/- 3% claimed accuracy)
Maglite XM-L T6 1D w/ 3-mode driver, 3.5A on high (details of the build can be found in the link of my sig)
Reflectors: Stock Plastic SMO, Kaidomain (KD) Aluminum SMO, KD Aluminum OP
Lenses: Stock Maglite lens, Hardcoat Acrylic lens, Borofloat lens, UCL lens

Testing Procedure/Overview:

Baseline:
First a baseline test needs to be performed so we can use it as a reference point later on when doing any comparisons and calculations. This first test will be to unscrew the Maglite head and measure the light output in "candle" mode. I chose to use the low mode on the driver (150ma) to minimize thermal related output sag. This ensures test results are consistent and repeatable due to the emitter not heating up too much. Measurements throughout the entire experiment are taken at 30s to match the ANSI FL-1 standard.

Reflectors:
After the baseline test is done reflector efficiency will then be measured. The bezel and lens will be removed from the head and the reflector will be inserted. The head will then be screwed onto the body tube without the bezel and lens attached. Each reflector will then be measured for light loss. Reflectors will be tested in the same position height relative to the emitter.

Bezel:
Same testing method as the reflectors except screwing on the bezel with no lens attached.

Lenses:
Each lens will be tested with the the three different reflectors, generating twelve test data results. The bezel will be attached for each test. Each lens was thoroughly cleaned before testing so there wasn't a spec of dust, smudges, etc. As a side note stock Maglite lenses are notorious for scratching/smudging easily and this can lead to reduced output. I used a brand new out of the package Maglite for the testing with a lens that had no marks at all. A scratched/dirty/used lens would obviously reduce output and wouldn't be a fair comparison.

Testing Results:

The units in the table are given in both Lux values measured by my light meter as well as LBL values. There is a small difference in the Lux vs LBL percentages so I decided to include both for completeness.

Calculations:
Reflector Efficiency: Reflector without bezel and lens / baseline

Bezel Loss: Reflector Efficiency - (Reflector with bezel and no lens / candle mode)

Lens Efficiency: Reflector with bezel and lens / Reflector with bezel and no lens

OTF: Reflector with lens and bezel / baseline

Here is a slightly larger image for easier viewing (click to enlarge)

 

[LilKevin715]

Data Analysis & Thoughts:

After looking over the data for a bit I guess I'll go over the data the way it is presented in the table.

Baseline:
What is typically called emitter lumens (aka bulb lumens) is what was measured in this step of the experiment. This result is the basis for all the calculations.

Reflector Efficiency:
There are no big suprises here as far as the numbers go. Some of the light doesn't get reflected off the reflector and simply hits the inside walls of the lightbox immediately. The lower result of the KD OP reflector (barely) makes sense a little but. Light that hits the orange peel surface can be trapped and lost more compared to a smooth surface. However later on the results of the KD OP reflector are quite suprising.

Bezel Loss:
I was expecting this number to be constant but the spread is within reason. This could be attributed to the slightly different shape of the reflectors and/or surface coating/finish.

Stock Lens:
The stock lens doesn't do too bad in terms of performance, but this is assuming a pristine brand new lens. Performance is at the back of the pack by a decent margin. I might consider running a retest later on with a used lens that isn't in as good of a condition; results would be obviously worse. While not as good as the other lenses in terms of performance it is probably the cheapeast to manufacture.

Hardcoat Acrylic Lens:
While still a plastic lens it does perform very well with numbers very close to glass lenses. Besides the obvious benefit of offering higher performance vs the stock plastic lens, scratch resistance is improved dramatically. The lens can still be scratched, but nowhere near as easily compared to the stock lens. Cleaning the lens with a paper towel wont result in fine surface scratches compared to the same thing being done to a stock plastic lens. The only main drawback IMO is the required use of specialty cleaners for plastics like Novus when cleaning the lens.

Borofloat Lens:
The borofloat lens was a decent performer out of the bunch. While it doesn't have top-notch performance it makes up for the lack of optical performance with unmatched heat performance. It is the defacto go-to lens for high power incan setups.

UCL Lens:
There is a good reason why it is called a UCL (ultra clear lens), it is simply the best as far as optical performance goes compared to the others. The anti-reflective coating on both sides of the lens helps reduce the amount of light that is reflected back into the light. The AR coating combined with the substrate material makes it a top performer.

General Lens Comparison:
My results are roughly 3-4% higher for the stock/Hardcoat Acrylic/Borofloat lenses compared to the results tested by Foy (and published results on Chris's website). The numbers however are consistent in terms of ranking performance. Even though I tested three different reflectors for each lens the efficiency percentage is consistent.

General Reflector Comparison:
The stock SMO lens and the KD SMO lens are very close in performance when the two are compared directly, with the edge going to the KD SMO lens. The most suprising result is the higher performance of the KD OP reflector; it beats out both SMO reflectors by a healthy margin. The only reasonable explanation I can think of is that it as a better surface coating/finish compared to the SMO reflectors.

Emitter vs OTF Percentage:
There wasn't a huge spread in terms of OTF percentage with the various test results. While reflector choice can make a difference, it pales in comparison compared to the lens used. The stock lens was at the bottom of the results, followed by the borofloat and hardcoat acrylic in the middle, and lastly the UCL leading the way.

Conclusion:

The numbers speak for themselves. My test equipment and its accuracy aren't the best, but the results are consistent which can be used to determine a general trend. Please also remember there will always be sample variance. My results should not be interpreted as a basis for all flashlights (and LED flashlights in particular) as I only used a Maglite for testing. A different flashlight setup would most likely result in different results. So there you have it. If you have ever wondered how various components affect output for a Maglite you now have your answer.

 

[LilKevin715]

Reserved for something..