Anomaloscope — a citizen-science color-vision test

What this measures

This is a Rayleigh match. With normal color vision you can mix red and green light until it looks identical to a yellow — two different spectra that produce the same signal in your cones (a metamer).

The interesting number is not the mixture you settle on, but how wide a range of mixtures you accept as "the same yellow." Most people accept a fairly wide band. A tetrachromat — someone with a fourth cone type — breaks that metamer and accepts a narrower band. The width of your matching range is the signal.

Why a screen can't do this. A real match needs near-pure single-wavelength lights. Your monitor has only three broad primaries and can't make them — a "screen anomaloscope" would measure your display, not your eyes. That's why this test runs on a small LED device you build, with an orange filter and a diffuser. hardware required

Build the device

A fork of the BrainardLab Penn Anomaloscope: an Arduino Leonardo driving a yellow LED and an RGB LED, behind an orange long-pass filter and a diffuser.

Board: Arduino Leonardo. LEDs: yellow → D9 (100 Ω), RGB red → D6 (150 Ω), green → D5 (100 Ω), blue → D3 (unused).
Firmware: flash anomaloscope_leonardo.ino (dumb actuator: R/G/Y 0–255 over serial, 9600 baud).
Filter (required): an orange long-pass with cut-on between 546 and 589 nm — Rosco #23, Kodak/Tiffen Wratten 23A, or Lee 780 Golden Amber. Plus a diffuser (Scotch tape over the LEDs).
Then: Connect above, Calibrate, and Run the test.

↓ Lid cutting template (SVG — print at 100%) — two 1.5" windows, centerlines, septum mark.

Full BOM, wiring, and CAD are in the project repo. (Build-guide page expands in a later phase.)

Calibrate (do this first)

Two steps. 1) Balance red against green. 2) Bring the mix's brightness down to the yellow. Calibrate with the filter and diffuser in place.

yellow ref

R/G mix

Step 1 — red vs green. Flip between red-only and green-only and set the green gain until they look equally bright on the device.

green gain (G_GAIN): 0.45

Step 2 — mix vs yellow. Show the mix beside the yellow and lower the RGB level until they match in brightness.

RGB level (RGB_SCALE): 130 yellow reference: 200

Optional — check the balance with a DVD scrap. A DVD is a fine diffraction grating (~1350 lines/mm). Hold a piece up to your eye and look at the lit device through it: the RGB LED splits into separate red and green bands; the yellow stays a single band.

Balance: set G_GAIN until the red and green bands look equally bright.
Filter check: look through the orange filter too — the green band should drop well below the red.

And the giveaway: a matched "yellow" mix shows two bands (red + green) while the real yellow shows one — the metamer your eye is fooled by, pulled apart by the grating. For actual peak wavelengths, photograph the spectrum and measure it (the spectral-calibration step).

Run the test

yellow

your mix

Adjust (warm-up). Make the mix look like the yellow, then lock it.

ratio — · yellow —

Limits (the protocol). From your center, step toward each edge and judge. The range width is the result.