Light Sphere

Where could I get one of these so I could measure lumen output and candela? Not sure if this is the correct forum for this.

Have you considered making your own? It's not too difficult if you are patient. There is a thread on another flashlight forum that goes through the process from start to finish. I think CPF member Match was the one who published it.

- Matt

Could you post the thread? I'd like to check it out.

PM Sent. Just not sure if I can link to other forums in public - better safe than sorry

Thanks, Mattaus. What are the best lux meters? I don't want to start on this and end up with a crappy lux meter.

Bully,

Sorry I forgot to subscribe :/

No idea to be honest. Most guys seem to use the cheap ones you can get on eBay for cheap. Remember you just need a relative reading of the light intensity. You use a light that you know the lumen rating of, measure it using your light sphere and the lux meter, and apply a multiplication factor to what value the lux meter gives you to get to that lumen rating. Every other light you then test has it's lux measurement multiplied by that factor. It doesn't mater how accurate the lux meter is, as long as it's consistent.

No idea how to work out if it's consistent or not though. Maybe test it against your first light every now and then - just as if you were calibrating any other piece of test equipment.

Sorry that's all I've got.

- Matt

Matt,
Coming from you, even a little bit is held in high regard. I'd always been interested in building myself one as well. Thank you for the info!

Mattaus said:

Remember you just need a relative reading of the light intensity. You use a light that you know the lumen rating of, measure it using your light sphere and the lux meter, and apply a multiplication factor to what value the lux meter gives you to get to that lumen rating. Every other light you then test has it's lux measurement multiplied by that factor. It doesn't mater how accurate the lux meter is, as long as it's consistent.

Click to expand...

This works really well... you may not even need to build an integrated sphere for decent accuracy. I'm using my DSLR as a light meter, and bouncing the light between two 8.5x11" pieces of white cardboard about 4 ft apart (bounce beam off one, meter off the other). Of course, it's only a sample of the total illumination but, none the less, once you know X, then all the ratios of X (10x, 1/10th, 1/1000th, etc.) hold true and consistent. Course, 1/3 stop increments of a DSLR ain't going to be the finest instrument in the tool shed, but it's certainly way more accurate and objective than the naked eye.

....and yet I still​ forgot to subscribe.

Thanks for the kind words climber

reppans said:

This works really well... you may not even need to build an integrated sphere for decent accuracy. I'm using my DSLR as a light meter, and bouncing the light between two 8.5x11" pieces of white cardboard about 4 ft apart (bounce beam off one, meter off the other). Of course, it's only a sample of the total illumination but, none the less, once you know X, then all the ratios of X (10x, 1/10th, 1/1000th, etc.) hold true and consistent. Course, 1/3 stop increments of a DSLR ain't going to be the finest instrument in the tool shed, but it's certainly way more accurate and objective than the naked eye.

Click to expand...

Pretty expensive lux meter haha. But you're right - the idea behind a light sphere is to improve the accuracy and consistency of the measurements. Plenty of guys on these forums do simple ceiling bounce tests for an estimated measurement. Not fantastic but it's better than "I estimate the output at about such-n-such lumen."

Like I said early on - building a light sphere is not tricky and the results will be pretty good if you're patient about it.

- Matt

I found a few lux meters on eBay. I'm gonna try and build on this weekend. Where do I get the glue mentioned in the thread you pm'd to me?

Man I'm horrible at being prompt. Mix water soluble wood glue with water. Any hardware shop sells it. We called it aquadere here in Australia but I'm not sure what they call it in the US (assuming you live in the US!). The mix is a bout 30/70 water to glue. Basically you just want to thin the wood glue down enough that it completely soaks into the paper without leaving a thick residue on the paper. In practice, when each layer is dried, it should just look like normal newspaper that is rock hard to the touch. You'll know your glue mixture is too thick if the paper appears a shade of white or has glue residue left on it.

Remember: slow and steady, one paper layer at a time, smooth as possible and allow it to fully dry before adding a second layer. If you do too many at one time patches won't dry properly and the wall will be weak and expand. By expand I mean the paper layers won't be stuck together properly and little air gaps will form between them, further weakening your sphere. Strength is very important for two reasons:

1) When you cut the sphere in half to paint the inside, and weakness in the walls (or the walls being too thin) will result in the two halves of the sphere becoming irregular which will make putting it back together a pain in the a**.

2) When you're using the finished article, any weakness could result in cracks in the new join, which leak light. Also general wear and tear from attaching lights, lux meters and moving it around will quickly show weaknesses in the build.

- Matt

EDIT: Added this as a subscription so I'll be notified sooner

I found that using the search phrase "Home Made Integrating Sphere" rather than "Home Made Light Sphere" returns much more relevant results in Google. Now to see if some professor friends can get one of my lights measured for me. There has to be at least one IS on campus that they can get a quick flashlight test done on. If I can get a calibrated light to go on I think I'll build one of these.