Ballpark Lumens with light meter?

[Killforfood]

I purchased light meter A while back and would like to know if there is any way to calculate ballpark Lumen readings with it. It only reads in Candle Power, LUX and Luminous Intensity.

[LightRider]

I’m not sure but I’ve been wanting to learn how to do this as well so I will watch the thread to see what knowledge is shared.

[1dash1]

The preferred method would be some sort of integrating sphere, light tube, or light box:

Integrating sphere #4 (the fast and cheap one)

(Wait, I never mentioned sphere #3 ! [image]. Well, I did make it, but although sphere #3 was supposed to be the simple one, during the build I got carried away and added all kinds of useful things. I have not done a build thread on it yet, but I will, one of these days.) Another day, another integrating sphere. The aim of this one is to really be simple: see how much I can leave out without sacrificing too much on the performance. In this integrating sphere is much more thinking time involv…

It would have to be individually calibrated:

Help with my D.I.Y. Sphere Calibration

I’m confused on calibrating my 12” D.I.Y. Polystyrene (“Foam”) integrating sphere, modeled after this build thread by djozz with pics in post #64. Few things about my sphere: it’s sanded with 100 grit sandpaper inside, luxmeter is the “good” LX1330B and set into the foam a bit (no baffle, but shouldn’t get any direct light). Entrance hole is approximately 90mm at the top and 80mm at the bottom inside edge. I did NOT paint or cover the outside. Lux readings were taken in nearly complete darkness …

The less accurate (ball-park) approach would be a calibrated ceiling-bounce test or a wall-bounce test.

• Position your lightmeter in exactly the same location every time you run the test (preferably, you’d have the spot permanently marked).
• Position every flashlight in exactly the same location every time you run the test. (Make sure that flashlight and light meter are positioned such that the shadow of the flashlight does not interfere with the light meter sensor reading.)
• Shine the flashlight onto the wall or ceiling and record the reading of the foot candles or lux.
• Develop a database of all the flashlights in your inventory: flashlight, batteries used, recorded output levels in foot candles or lux, manufacturer’s or other reference (reviewers/users) output levels in lumens.
• Ideally, you would then send a few of your flashlights to someone with a reliable integrating sphere for them to do some baseline testing. In lieu of such testing, you’ll have to make some assumptions. You’ll need to assess which flashlights you are most confident in matching their respective reference output information. Develop an average multiplication factor correlating readings between those flashlights and the reference output information.
• You can then apply this multiplication factor to any new flashlight that you test.

Notes:

1. Try to take the readings under the same ambient conditions: freshly charged batteries, room temperature, total darkness.
2. Try to take the readings at the same time after turning on the flashlight: initial reading within 3-5 seconds of turn-on (optional), primary reading at 30 seconds, secondary reading and tertiary readings as desired (optional).
3. You should adjust the multiplication factor as you get more and more information.
4. You may also want to develop a similar database for beam intensity (shine the beam directly onto the light meter from a fixed distance, record the maximum intensity). Use one of the online calculators for converting lux to candela. Compare against manufacturer’s listing of beam intensity.

[chadvone]

1dash1 nailed it, great post.

Here is my light box I made about 7 years ago, Out of a Orange juice box. I stuck a piece of glass over the hole and made a hole for tube lights and a flap to open up a larger hole. I use 3 HDS lights to come up with a scale. It is very accurate between 10 and 200 lumens. I divide the light meter number by 50. On lights brighter than 500 lumens, It reads much lower compared to others specks.

I can set a light on the glass, write down the LBL ( light box lux) divide by 50 for ballpark lumens. Check it as battery drains to see how much time till 50 percent I can get on a cell. I have a lot of fun with mine, I am sure it isn’t the most accurate way, but to compare between light I have it is just right for me.

[Killforfood]

Really good post 1dash1,

I have a good area in my house that would work. Just have to wait for nightfall.

Ooops, I blew right past your light box links. I’ll go read that now.
That’s what happens when you’re doing 90!