Mixing light from multiple leds

Question

I'm trying to design a device that produces visible light with a somewhat controllable spectrum. My goal is to measure the human eye cone cell responses by switching between two different spectra and finding spectra that are hard to distinguish by eye, so an RGB led won't be sufficient for this purpose.

My first draft PCB has 13 LEDs in different wavelengths set in a circle with a diameter of 20 mm, but I can change this arrangement in whatever way is desirable. The LEDs I plan to use come in 2 mm x 2 mm packages, so they could also be fit more tightly if that's useful for the mixing (see image below). Their "typical viewing angle" is between 145° and 170°. (LED datasheet)

What kind of optics could I use to produce a well mixed light out of these? I would not need more than a (say) 2 cm x 2 cm diffuse surface with well mixed light. The output power need not be large—ideally it would be comfortable to look at from a distance of tens of centimeters—and it is not a huge problem if a fair amount of the light is absorbed.

I have looked into light pipes in the Digikey catalog, and while I have a feeling they might be part of the solution, they generally seem to be designed for leading the light from a single led to a panel. I think a light pipe should mix light shot into it from a sufficient angle. I also have explored all kinds of weird ideas like installing LED reflectors upside down and having diffuse lenses in the other end. Another idea is to put the LEDs in a separate, smaller board that fits inside a reflector. In the end, it's clear to me that I don't know enough about optics to understand how to best approach this problem.

Here's a very early draft design of a Raspberry Pi hat (65 mm x 56 mm) with the 13 LEDs in a d=20 mm circle.

Answer 1

In photometry, the standard instrument for doing this is the Integrating Sphere.

The concept is to bounce light around inside a matt white enclosure before allowing it to escape, and arranging the light sources and exit hole so that you can't see any light source directly from the output.

How good or big does your enclosure need to be? Probably not as good as you would need in a photometer, just something better than you are getting by just putting the LEDs close together.

As a first attempt, try a thin translucent sheet of white paper, or a thick sheet of tracing paper, spaced off from the LEDs, to diffuse the light, it may be sufficient.

If you are fabricating the board the LEDs are on, then use white silk screen around the LEDs so that the board surface is white rather than dark green. Light scattered back from the paper will illuminate the white areas, which mixed light will then bounce forward again. Maybe make matt white walls around the LEDs to complete the enclosure. If you want to try with existing boards, use Tippex or white paint to paint the board around the LEDs.

A better approximation to the integrating sphere will be to have the LEDs facing away from the viewer, and bouncing off the back wall of the box.

Answer 2

Consider a construction like it's used in small TFT backlights. The LEDs inject the light in the side(s) of a transparent plastic panel. The back side of the panel has a white reflector and the light goes across the panel under different angles. Most of the light gets deflected several times by total reflection at the surface until finally it leaves the panel. Light distribution is equalized very good.