Woah, what is an integrating sphere!? Don't worry, it's not as fancy as it sounds. It's basically a doohickey that lets me take direct measurements of the lumen output of my lights. A lot of companies list the "laboratory rated" lumens of a light which is typically 25% higher than the lumens you actually get OTF (Out The Front). Only reputable companies actually measure the output of their lights. Less reputable companies just "make up whatever sounds good at the time." Really, I'n not kidding. If you want to hear a rant about that let me know. For now, back on topic.
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| In this shot the light is actually "on" |
Typically an integrating sphere is a multi-thousand dollar piece of lab equipment. The other alternative is home brew. Giant styrofoam ball, a lolly pop looking thing, a couple holes, and a high quality light mete...viola. It's not perfect, but it's a whole lot better than guessing. This sphere should be accurate to within +/- 5% of the actual output. The design is a tried-and-true method
developed by several CPF members. It was nice not to re-invent the wheel (this time). More info after the jump!
This sphere is hollow and about 24" in diameter. The walls are 1" thick. I'm pretty sure it arrived in the largest/lightest box I've ever received...and according to my UPS guy...I receive a lot!
So, my fancy shmancy integrating sphere stand is...a smaller cardboard box. What did you expect? The two halves separate right at the center.
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| A view of the interior |
This isn't much to look at but it's where the magic happens. The hole of the left is cut for the size of my flashlight heads. I may need to cut it bigger some day if I make bigger lights. The hole on the right is for the sensor on the light meter. It's set to a very precise depth and even with the inside edge of the sphere. There is a tiny white post with a tiny white disk placed exactly half way between the light port and the sensor port. This prevents the sensor from "looking directly" at the light source and giving readings that are excessively high.
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| AEMC Model CA813 light meter...if you care |
So the sensor just gets plugged into a recessed opening on the side. I spent about half and hour cutting the foam with a dremel to get it EXACTLY the right size and depth.
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| Photo with the light off |
The sensor port and the light port are located EXACTLY 90 degrees apart...or as close as I could get it. I used a little high school geometry to calculate the distance in between and marked a string with the correct intervals Then I just taped the string to the outside of the sphere (wrapping it around) to get the exact hole locations.
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| Photo with the light on |
I met up with a local CPF member (Bigchelis) who also has an integrating sphere. I took some lights over to his place and tested them to get calibration data for my own sphere. This makes my current calculations slightly less accurate because I essentially have a copy of a copy. What I really need are reference lights that have been calibrated in a NIST certified lab-quality sphere. Bigchelis has a couple but they were out on loan to another CPF member. I'll post some official lumen readings when I get all of this figured out!
Good work, did you ever get that NIST calibration in?
ReplyDeleteYou need a better baffle. The light should reflect off of at least two surfaces before reaching the photometer. The setup you have will give exaggerated results, although this will be offset by the low reflectivity of the inner surface.
ReplyDeleteHi. This might be a late, but I'm looking into ISs for a project. You need to paint the inside of the sphere. Try a brilliant white matt paint (that doesn't dissolve foam). It will improve the reflectance and therefore light distribution within the sphere, and stop light escaping outwards. If the sphere glows, you're loosing light and hence getting a less accurate measurement.
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