I notice there are a few people here that use light meters to test the lumen outputs of their lights. Anyone find a good formula for converting the meter Exposure Values (EV) to lumens? I’m toying with one but wanted to see if anyone could give me a jump start before recreating the wheel. I have a good calibration light which I believe to be accurate, and also have a good ambient bounce sampling method which I trust.
Sorry if this has been discussed, I searched but didn’t find anything.
If your talking about a regular lux meter and just turning a light on in a room and taking readings, it’s VERY difficult to convert these numbers to lumens. If you start with a known baseline light and are very consistant in where and how you test for those readings you can get a ballpark lumen number. But these are way too many variables in a large area like that to get reliable numbers. It is better than nothing though. The smaller the room and the more consistant you are the more accurate that method becomes. Its best if you can make a lightbox to take the readings. Then you have a very controlled and exact volume that the lux meter reads each and very time. You can get pretty decent numbers using that type of method. Of course a lightbox will not do as good as a integrated sphere, but very few hobbiests ever have the chance to access one.
If you don’t want to deal with a lightbox, try taking readings in a closet or similar small area. But you do have to have a light (which you state you have) with known lumen values too create a baseline multiplier with first. If you can do that, then you can do some rough lumen numbers just fine. Big rooms start getting really questionable for testing like that though.
Thanks, but my ambient bounce sampling method does even use a small room (it can be outdoors), which I still find to be inaccurate and inconsistent, particularly with low- and sub- lumen levels which I care most about. I’m not new to this and have been measuring for a year or so, and can hit every mode on my trusted calibration lights from sub-lumen on up… everytime. I also reconcile with Selfbuilt’s findings for added verification, although I find his lumen scale to be too liberal, and his sub-lumen estimates to be, well, …. off. Happy to discuss “lightbox” methodology further, if anyone is interested, but for now, if we could stay on the topic of EV > lumens conversion formula.
Guess my question revolves around this baseline multiplier - EV figures are not linear, as lumens are… can you use a constant (?) multiplier to convert them. I’m curious what measurement people are using off their light meters and how they are converting them to lumens.
Thanks
If the light meter provides for an incident reading.(integrating sphere over the sensor) then EV to Lux is simple conversion. It’s the Lux/EV to Lumen conversion that gets complicated.
Thanks that sequence helps me, I googled EV > Lux conversion and found the formula:
Lux = 2.5 x 2^EV which covers of the exponential part of the equation. Then I recalled a post from a photographer that carried the Lux to Lumens conversion by simply using a constant as a multiplier (I guess what rdrfronty also referred too) so that his formula ended up effectively being:
LUMENS = C x (2.5 x 2^EV), where “C” is simply a plug multiplier derived from a known calibration light.
The formula I’ve played around with is:
LUMENSt = LUMENSc x [2^(EVt - EVc)], where “c” is calibration light and “t” is the tested light.
This formula works with the link above, and also the first Wikipedia chart on the same subject. Algebra is Algebra I guess….
Thanks for the help folks!
(edit… if anyone is still interested, be happy to discuss the ambient sampling method…)