Fun with Light Meters.....

[Chris M.]

Well I have finally joined the ranks of Craig, Brock, Doug and all you other Light meter owners- a nice new digital showed up today which I ordered through work. And just now I put it to work blasting it with a box full of LED flashlights, making note of the readings.

The following are peak foot-candle readings, taken from 1 foot away from the sensor window, after being lit for approximately 1 minute to let them settle, and moved around gently to find the highest reading. The meter was set to Flourescent light mode- for colder colour temperature light, I believe this was the best setting for white LEDs. The other settings are sodium, incandescent and mercury- too cold/warm for LEDs I thought. Ambient light in the room is zero aside from the dim glow of my monitors which did not register on the meter- and it has a zero offset adjustment to compensate for light ambient light anyway. Most of the flashlights had very little use on the original batteries, no more than about 10 minutes maximum. Sorry, no Photons yet since those have been used a lot and would give low readings. I`ll get new batteries sometime.

OK enough waffle....

White lights-
AAA Arc- 15.2
"Smart Planet Mile Light"- 9.3 (from ebay- coming very soon to my preliminary review listings)
Mini LED flashlight from Ebay- 13.2 (bottom one on my LED review main page)
ASP Aspen- 9.1
Stylus-3- 19.6
Turtlelite 1- 52.3 (very narrow beam)
Energizer DB18 conversion- 104.3
CMG 04- 13.5
Infinity- 5.5
6-LED Flashled- 26.9 (with 3 N cells)
......................5.3 (with 2 AA cells)
LEDcorp keychain- 8.5

Coloured lights- not sure if my meter is compatible with non-white lights but anyway...

ASP Sapphire- 1.4 (deep blue, wide beam- but reading possibly too low)
Green Infinity- 8.5
Turquoise Infinity- 7.0

and just for fun-
Surefire 9Z- 1700!

Of course there are inevitable variations from light to light due to Nichia having a hard time making all their LEDs the same (well they are very small- it`s not easy!)but I hope this is of some use to someone? Not sure how it compares to Craig and Brock`s tests a while ago- anywhere near?


I read somewhere that readings from a meter calibrated in foot-candles, taken from a light source 1 foot away from the sensor, are the same as Candelas- is this right? In which case those up there would be Candela readings proving that most of those 5.6 candela LEDs are being pushed a little harder than intended.

 

[The_LED_Museum]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Chris M.:

I read somewhere that readings from a meter calibrated in foot-candles, taken from a light source 1 foot away from the sensor, are the same as Candelas- is this right? In which case those up there would be Candela readings proving that most of those 5.6 candela LEDs are being pushed a little harder than intended.<HR></BLOCKQUOTE>

If your physical measurements are accurate, then yes, you can directly measure beam candelas from LEDs this way.
For mcd just add some zeros.

A newer, good-grade Nichia NSPW500BS will generally read 6000 to 9500mcd when driven at 20mA; substantially more when overdriven as they usually are in LED flashlights.

It is not at all uncommon to see 18,000mcd out of a garden variety white that's in a 3-cell light with no resistor.

Colored LEDs tend to give funny readings, because the photodetector in most light meters is calibrated to read white or mostly white light. If your meter has adjustable color temperature settings, setting it for "sunlight" or better yet, "cool white fluorescent" should give you reasonably accurate results with white LEDs.

Ambient temperature can also affect readings.
Try to keep it between 70°F and 75°F for the absolute most accurate results. It's probably still alright around 80°F, but if its much warmer than that, wait till after dark to do your tests. My recording thermometer reads 94°F right now, so I'd better leave the thing off for awhile.