Noob Question

LED Flashlights – General Info
1

Hi all you FlashlightGurus out there :slight_smile:
I just got myself a (cheapo) lux meter and quickly took some measurements at 1 metre.

What I’d like to know is: how do I translate my lux readings into KCd values?
For example, at 1 metre, my StarryLight SA-22 gives me a reading of 12,940 lux.

Also, on a couple of my de-domed throwers, if I were to take the measurements at 3 metres,
(since I figure the beam doesn’t really come into focus at only one metre) then how do I do the calculations?
Sorry for the dumb question, but I’m not particularly technical-minded.

2

Enter illuminance in lux: 12,940 lx

Enter distance and select feet or meters: 1 M

Luminous intensity result in candela: *12 cd*

Just a quick Google ;)

3

Did you mean 12 kcd? 8)

4

Just square the distance and multiply.

At 2m, you multiply result by 2^2 = 4.
At 10m, you multiply result by 10^2 = 100.

The easiest is at about 3.16m, just multiply by 10. Close enough for measuring indoors, and enough collimation for many throwers.

5

Thanks a bunch, guys :slight_smile:

6

No, that's what the calculator says. 12 cd.


Calculator doesn't understand the thousand separator comma => input is considered to be 12 lx.

7

For low output measuring at 1ft can be useful. The rapidtables link posted earlier can convert this to candela.

I prefer taking measurements at different distances due to the way lux measuremants can vary with PWM, etc. This way you can better observe an overall improvement from a mod.

8

Yeah… after doing some preliminary testing I’ve found that the readings are more accurate
when taken at a distance of 3 metres or more.

However, even with brand-name flashlights the results are always somewhat lower than specc’ed :wink:

9

Hey I found it on the interwebz…it must be true :stuck_out_tongue:

10

Longer the better 5 to 10 meters. :slight_smile:

There is no guarantee that your cheapo meter is that accurate against quality calibrated meters. Best as a comparison between your own lights and as a guide only with other figures published.

11

Thanks xmint… Yes, I certainly don’t expect my super-duper ( :smiley: ) meter to be that accurate, but
like you say, it gives me a chance to compare all my different lights.
(and pretend that I’m ‘doing’ a selfbuilt type analysis :smiley: )

12

Someone who has done a lot of kcd testing, rdrfronty, seems to get the best accuracy at about 15m. Now he tests mostly high throw lights, but I know for sure he's right (within tolerances of the meters, setup, etc. of course). On one light (or more than one) I tested throw at 5m - sent him the identical light, he reproduced the #'s I got at 5m, then re-tested at 15m, and would get a higher kcd measurement. He found going more than 15m did not make a difference. I still use 5m but with the caveat the real # may be higher. It will vary from light to light... For me, 15m is not convenient - would require a helper and doing it outdoors, at night of course.

I happen to trust the measurement with $35 meters to be in the ballpark - identical meter several others here use, and match up well with production light measurements and expected results, etc. You can talk about quality of the meter, background effect, temperature effects, etc., but the bottom line of the environment is negligible compared to variations from unit to unit, batteries, method of performing the test, etc...

13

Using my lux meter has also shed some light on the batteries that I use.
Some batts are certainly better than others, and it shows clearly in the lux readings.