Chasing bright flashlights can be kind of fun. First upgrading the LED drivers (with a Attiny13 MCU based driver) and then upgrading the MCU to ATTINY85 to load my own MCU controller program. Then, upgrade the emitters and again when a newer better one is available.

But, back to the point I want to make, 100,000 lumens is definitely horse crap. If you don't know how to get an estimate and your flashlight has a zoom mode, here is a quick way:

Measuring lumens is kind of hand-waving magic anyhow. Since lumens is the 360 degree 3d output (all directions spherical), you need to capture and measure the entire spherical shell surface. To accurately measure lumens, one has to build a sphere with sensors covering the entire inner surface to capture all emitted light, or a no-loss lens/reflector to focus all the light in one direction (and then hand-wave magic about spherical surface vs flat).

**Lux** on the other hand is 1 lumen projected onto 1 meter-square surface 1 meter away. So, if at 1 meter away and project to 1m

^{2} you measure 100 Lux, you know is 100 lumens. Lux can be easily

**measured using a Smart Phone**^{[1]}. Since this sensor is for back-light illumination adjustment, it is not so accurate but at least it is a consistent gauge of brightness. The variation I see between my Smart Phones suggest at +-25% (and also as compared to my actual lumens meter which is probably not that accurate).

You can project your light 1 meter away onto a 1 meter-square surface and do the math. One meter away is easy. To project the light to 1m

^{2} you need a circular area of 1m

^{2}, so radius is

~~r=(1m/3.1416=0.3183m)~~ r=(1m/sqrt(3.1416)=0.564m). You need to adjust your

**flashlight's zoom** accordingly to get it to project to a circle of r=

~~0.3183~~ 0.564 meter. The flashlight probably has a brighter center circle and a dimmer outer circular ring, so the Lux in the two areas should be separately measured and prorating the two numbers by the fraction of the two areas. If the bright center is 20% of the total area and at 200 lux, and dimmer outer is 40 lux, you have 20% of 200 plus 80% of 40. That would be your estimated total lumens. It would still be an approximation as even within the brighter center circle, the lux will be higher when nearer to the center. You can of course use average within a ring, or may be dividing it into more rings, but that difference will be smaller than the difference due to the Smart Phone's ambient light sensor accuracy. Not really good, but better than 100,000 vs may between 1200 to 3000.

Note:

**[1]** If your Smart Phone have a diagnostics mode (my Asus Zenfone 2/2e, 3 do), use that. If not you can use an app, download the CPU-Z app. Under "Sensors" tab, you should find your Ambient Light Senor, it will show you the lux received by the ambient light sensor.

Edit: Minor wording to correct grammar.

Edit 2:

Just to make sure subsequent readers get the right numbers in case the reader read only this but missed Kjpye's later reply, I corrected this post. Kjpye is right, I made an error by forgetting

**the square root**.

...

Your calculation of the required radius is wrong. The area of a circle is πr², so for an area of 1m² you need a radius of 1/√π or 0.5642m.