Measuring lumens without an integrating sphere or calibration source

Some people were asking how to measure lumens without much equipment. I’d recommend making a cheap integrating sphere (or tube) but you can get a rough idea of lumens with just a phone and desk lamp.

Get the ceiling bounce app if you have an Android. Or search the app store for something like lux meter.

Try to find a desk or room light/lamp with a known brightness. In the UK they come with lumen ratings but I’m not sure about other countries. Search for something like “60W bulb lumens” for a rough estimate.

Measure the lux from the desk light, without any sunlight. Measure the lux with the phone, moving it around a bit so it’s mostly detecting light bouncing off the ceiling.

This gives you a rough lux-to-lumens ratio. Eg 1000lm shows as 2000lux means 2 lux per lumens.

Now point your flashlight at the ceiling and do the same. Divide the lux by the ratio you calculated to get lumens. Eg if the flashlight measured 200lux then it would be around 100lm.

This should give a good enough ball park. Inegrating spheres ensure you measure all the light equally.

If you have a multi meter and know the LED then another thing to do is you can look up other people’s LED specs (on BLF) to see how many lumens you get at different current levels. Unscrew the tail cap and measure the current from the body tube to the cell and that should allow you to work out how many lumens the LED could produce. Actual lumens will be a bit lower (10-20%?) due to resistance and output loss at the lens.

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