I contemplated using one of those that had been torn. Patch it up and inflate it just enough to get a sphere. Probably would be far too big to be practical, and it got thrown away anyway.
No, for practical purposes the reflectivity does not matter much at all. White styrofoam is pretty darn reflective. Whether the surface reflects 95% of the light or 90% does not matter much once you calibrate your sphere… been there, tested that… Even with a rather tight thrower, the detector is seeing reflections from a LOT of angles… unless you are using a focused laser as the light source.
What you want to eliminate is a specular/non-difuse reflective surface that can direct a direct reflection of the light to the sensor. That is where the barium sulphate/sanding the styrofoam surface helps.
To test how well your sphere integrates, aim the light at different parts of the sphere and see how the readings change.
When I first got my sphere, I did not sand down the flat spots to make them spherical (I put the 4” diameter light port where one half of the sphere had the flat spot and the hole pretty much took up the whole flat area). The sphere still worked rather well with the flat spot on the other side of the sphere where the light hit it square on. There was maybe a 5% improvement in directional sensitivity after I sanded the flats down to a spherical radius.
My sphere sits on my kitchen floor and the room is lit by around 2500-10,000 lumens of room lighting. I have not opaqued the outside of the sphere. It works just fine and the room lighting does not appreciably affect the readings… unless you are trying to measure a small/dim light. I get around 1-2 lumens of room light leaking into the sphere. For dime lights I wait until night and turn off the room lights or just have my system subtract the background light level reading. (For color temperature testing of dim lights I use the darkened room).
For the US/UK users I see most people use Lux. 1 FC x 10.76 = 1 Lux. Other than way bigger numbers while using Lux, any reason not to use Foot Candles?
For whatever reason my 14.5” IS/Meter combo uses in LUX:10.40697674418605. The correction for 172/1790. After looking at the correction number for my (only) ANSI/NEMA FL-1 certified light of 172 lumens, I switched to FC on my meter. Basically same readings but now can use direct readout. I understand if I change meter or repaint sphere, everything changes.
I see that many others got correction numbers from 7.xxx to 12.xxx, so while fun, I would need to send IS, meter and light to BLF laboratories, INC? I like the suggestion to buy a few more calibration lights, so will continue to enjoy using the IS.
The one i built above took a bit of time to build, not sure if there is any profit in it for the time and parts used.
I have it calibrated very well now though.
Nice work DBSAR! It looks very handy with all the mounting options around it.
The accurateness of professional spheres has much less to do with the actual sphere, much more with the better measuring equipment and calibrated sources, a home made sphere like we make already has excellent integrating properties.