6. Integrating Sphere EN
Language: Deutsch | English
The base is a white ping‑pong ball. With a utility knife I cut two 11 mm holes: the entrance pupil for the LED and the exit pupil for the gate. The holes should be roughly 90° to each other. Between the two holes goes a slot. Why?
To prevent the LED from shining directly into the exit pupil. For that you build a small light barrier. Fittingly, I cut a piece from a lab spool; it’s nicely light‑tight.
The cut barrier is slid into the slot and glued.
A control view from one hole through the other shows: we see only black. That’s how it should be.
For the ping‑pong ball to scatter and reflect perfectly and neutrally, it should be coated with barium sulfate. The light barrier should be coated as well. Here I mix a suitable liquid in a magnetic stirrer; it takes a while until the barium sulfate has no more clumps.
A mix that works quite well:
- 100 g barium sulfate
- 65 ml ethanol
- 35 ml distilled water
- 20 ml polyvinyl alcohol (PVA), clear
Use a pipette to put the emulsion into the ball and swirl it so the coating spreads everywhere without pooling. After about 60 minutes at room temperature, it’s dry enough for a second coat. Keep it moving during drying!
Note: This mixture developed slight cracks after a few weeks and seems a bit too glossy. So far that hasn’t been a practical issue, but I’ve since found another recipe that still needs testing.
- 20 g BaSO₄
- 10 ml 5% PVA solution (1 part diluted with 4 parts 90° distilled water)
- 10–15 ml distilled water (should be “a little thicker than cream”)
- 1 drop IPA
For space reasons I placed the integrating sphere inside the lamp‑house cover. That’s not ideal because you have to power the lamp there... see below. In any case, the black heat‑shield must be removed.
The top vent grille is removed and the housing widened to make room for the fan (needed with the YujiLED).
A lot of complicated measuring and calculating followed to place everything correctly... after three attempts I had it. :)
Mark the center of the LED COB on the heatsink...
LED attached with thermal adhesive...
...and an old PC fan screwed onto the heatsink in blow mode.
Fits pretty well! The original latch mechanism holds everything together.
Meanwhile I feed the LED via a spring‑contact strip...
...whose counterpart is mounted on the projector. Not pretty, but it works. This is just the first version of a 5‑year tinkering project. :)
The exit pupil of the integrating sphere should sit as close to the gate as possible—then the effect of completely unidirectional light is strongest.
Have fun experimenting!
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