When it comes to colors, physics is easy. Physiology is hard. This is why the Abney effect is still debated. We know that it can turn blue into purple and orange into red, but we're not sure why.

William de Wiveleslie Abney was a chemist and physicist who got a lot of scientific work done in his lifetime. He advanced the field of photography. He studied how sunlight moved through the air. And in 1909, towards the end of his life, he left his most enduring legacy. Knowledge might be forgotten, but mysteries endure.


Given his interest in light and photography, it's not surprising that Abney noticed an interesting phenomenon dealing with how we perceive light. Desaturating light changes the perceived hue of it. Show someone a deep blue shade, and add a little white to it. The person won't only see the shade fade, they'll see it turn purple. This isn't the result of adding white paint to blue paint, which might result in a chemical shift of colors. The person is still seeing the same deep blue shade of light, with a little white interspersed throughout. The Abney effect doesn't even work the same way every time. Add white to blue and we see it as shifting the color up the spectrum to the smaller wavelengths of light and turning purplish. Add white to orange and it appears to shift down towards the longer wavelengths and turns reddish.

There are competing theories as to how this works. One involves the different color reception channels in the eye. One handles red to green wavelengths, one handles yellow to blue wavelengths, and one is monochromatic and sees general a scale of black to white. The monochromatic one is faster than the others, although its speed varies depending on how the luminance changes. The change in perceived hue is the result of the sum of the information from the competing channels, and it changes as the speed and dominance of the different channels change.

A study in 2011 introduced a different theory regarding the Abney effect. Instead of adding a set amount of white to a monochromatic color, researchers subtly changed the bandwidth of light. The range of wavelengths of light shown in the experiment was increased in various ways. The eye has different spectral sensitivities and will filter some wavelengths, so the hue should have changed as the bandwidth changed. Instead, to the subjects, the hue stayed the same.


The researchers concluded that the visual system grabbed onto the peak wavelength of the original light, and compensated for its own filtering of spectral light. Although the cones in the eye responded, the person didn't see the change. The visual system will, apparently, fool itself to keep things consistent. So why does it change when white light is added to the color? The researchers concluded that the Abney effect happens because the eye is trying to compensate for a filtering process that isn't happening. Adding white changes the quality of the color. The eye knows that a change happened, but it's fooled about what the cause of the change is. It compensates for a filtering process that hasn't occurred - and so we see a change in hue when we shouldn't.

Top Image: NOAA

[Via Tests of a Functional Account of the Abney Effect, PNAS]