Have you ever been a subject in a color-matching experiment? If so, you may have encountered…
by Michael H. Brill, Datacolor
The Maxwell spot is an entoptic image of the eye's macula, a yellow-pigmented retinal area extending 3 or so degrees about the center of fixation. Until this year I regarded the Maxwell spot as an arcane effect that I would never see. Reportedly the spot is inconspicuous because it is fixed to the retina and hence the retinal receptors adapt to it. But even with rapid fading of the spot, I still should have seen it transiently in moving my gaze, say, from a blue sky to a white sheet of paper. But that didn’t happen. The paper showed me yellow journalism, but never a yellow spot. Ethan Montag  gave a demo (alternating blue and yellow field) to show the Maxwell spot---but no guarantees. (Evidently Montag also found it hard to see.) Also, Montag's demo shows the spot as a dark smudge on the blue field or a light smudge on the yellow field. It's still not yellow.
Then, twice in the past year I saw the Maxwell spot, both times in the context of a white light created by three narrowband LEDs. In neither case was the spot yellow. It was rather like a pink lipstick smudge on a white collar---betraying color infidelity by interfering with my ability to match colors. What a nuisance!
I first saw it when looking at a broad white surface in a light box that simulated daylight by mixing LED illumination. Several light mixtures flashed on and off in sequence, and curiously the “three-band lamp” always revealed a pink smudge for a few seconds. Could it be spatial inhomogeneity of the three-band lamp? No, the smudge covered less area when I got closer, and it always was centered about the direction of my gaze.
I saw it again at the latest IS&T/SID Color Imaging Conference. Abhijit Sarkar (a PhD student at Technicolor Research in Rennes, France and University of Nantes) gave what was judged to be the best student paper at the conference, on devising observer categories to reduce observer metamerism. He performed abbreviated color-matching experiments on multiple observers, using two 3-primary displays powered by different primaries. The observer categories he found did not agree well with the age dependency found by earlier investigators. As an on-site demonstration, Sarkar brought a 10-degree matching setup powered by a pair of LED triads, with wavelength peaks (452, 508, 642) nm and (462, 522, 592) nm. I was amazed how difficult it was for me to make the match, because the left-hand semicircle always had a fuzzy pink spot that faded away when I attended to the right-hand semicircle. When I backed away from the apparatus, the left-hand side of the match appeared uniformly purplish-pink. This latter effect had been noted by Sarkar. I thought we were seeing the Maxwell spot, and Mark Fairchild agreed.
Why is the spot called yellow and yet looks pink? Because the macular pigment absorbs strongly in a broad band about 450 nm , it would appear yellow when transilluminated by a full-spectrum daylight. When there are gaps in the light spectrum (as with 3-band lamps), attenuation of the green band can enhance the relative weight of the red, hence we see pink.
Not all three-band lamps show the effect, but Sarkar’s left-side green wavelength (508 nm) is low enough to be highly absorbed by the macula, leaving the 642-nm red primary to predominate. Because the G primary carries a lot of luminance, lack of that luminance in the Maxwell spot makes the pink darker and enhances my perception of it (relative to the yellow I'd managed to escape all the rest of my life).
Jack Moreland  describes a related way to reveal the Maxwell spot: “A large bipartite field (14 deg square) is presented. The two half-fields are approximately matched in colour: the appearance being a near-white. The mixtures are cyan and reddish-orange (490 + 610 nm) on the left, and blue and yellowish-green (460 + 470 nm) on the right […] An observer sees [a] patch about 3 or 4 deg in diameter [that] changes from ‘pink on green’ (left) to ‘green on pink’ (right) on switching gaze between the two half-fields.” So the Maxwell spot has shown itself to be pink to other eyes before mine.
Together with the best-paper prize, Sarkar now has a new factor to consider in selecting LED primaries. Also, I begin to understand how color-matching subjects must feel when told to "ignore the Maxwell spot." When the spot is lipstick-pink, that task is hard enough to make one consider “cosmetic” surgery.
 Ethan Montag, JIMG 774: Vision & Psychophysics, Chapter 8, Part 3: Parts of the eye.
 Jack D. Morehead, Entoptic visualization of macular pigment, J. Physiol. 485, 4P-5P (1995).