This is great, I tried to get the guys at CPF to do this years ago, when there was a member there who had access to a sphere at work (MrGMan - GMan?). This is the best way to go about this, have someone build a light source, have the lumens certified at different levels, and calibrate other spheres. What I suggested then was everyone chip in and build a light source, either a large bodied flashlight with CC regulation or, more preferably, a heat sinked emitter. The latter would have required users to have a lab power supply, or shipping one around to the different participants. The former would have required shipping the light, charger and batteries around. Not to mention everyone would have been responsible for building their own sphere. This way is much simpler for people who don’t have the time or equipment to build their own.
I don’t know about here, and I haven’t read past page one yet, but over there we got plenty of naysayers. Don’t listen, it’s a great idea, and while they may not be true IS’s, you can obtain readings which are repeatable, reliable and very close to an actual sphere or light output.
I built a light box, not even a sphere, when I first started in all of this and the readings were very surprising, right in line with manufacturers specs. And all that was before I built an actual sphere and bought an NIST light meter.
I have some PowerPoint presentations that might be interesting, they’re over the accuracy of off the shelf light meters and how modern LED’s affect the readings. They’re from a couple years back, and I’m not sure how I’d go about converting them and posting here, but I’m willing to give it a shot if you guys are interested?
Again, Great Idea!
-Michael
BTW: Here’s the old thread where I built my first light box, and the readings matched the manufacturer ratings closer than expected for a cheap meter and simple box.
Interesting Light Box Numbers…
ETA: Made it to page 2. Those saying the sphere needs to be bigger are correct, especially for the types of lights built around here. I believe mine is 16”, but all my stuff is packed away in boxes in storage. The balls can be expensive, but I’ll look for the link where I purchased mine, IIRC I paid around $36-38 shipped. Also, don’t forget to account for the thickness of the styrofoam in your calculations, for instance, a 16” ball may be 1” thick, thereby reducing the inside diameter.
Hole size on mine is around 2-1/4 or 2-1/2 inches IIRC, it would accomadate most lights but not some of the bigger multi-emitter lights or even some large reflector lights I have, one of the Lowes Taskforce lights that was posted as a deal here a couple of years ago, or some of my (comparatively) small handheld spotlights . Maybe I’ll start over and build another sphere, I’ve put new emitters/drivers in some of those and some are in que to be modded again with new LED’s, it’d be nice to be able measure those.
And to be inline with ANSI there is a small waiting period before taking the measurements. But unless it’s uncomfortable to hold your hand in front of the light for a short period I don’t think it’ll be an issue. With hotwires definitely so, not so much with LEDs. I’ve got some that you can feel heat with your hand in front of the reflector, but nothing hot enough to melt styrofoam. I would think the greater danger here would be in an extended runtime test, where the light itself became too hot to touch or hold, and if it was in contact with the edge of the opening might cause a problem. The meter I have is an Extech 407026 , it’s a data logging meter (if you have the cable and software) and when I’ve done extended runtime readings I’ve used a small PC type fan to keep the temperature under control, both for the light and sphere.