Abney effect: what it is and how it influences our perception of color

The Abney effect is a curious phenomenon of color perception. Let's see how it occurs.

Our perception deceives us. Often what we think we see is not what it appears to be, and one of the examples of this is the curious case of the Abney effect, which was discovered at the beginning of the last century..

Discovered at the beginning of the last century, this effect occurs when, when white light is applied to the same color, it is perceived with a different tone, as if the hue or saturation had changed.

Next we will go into more detail about the Abney effect, who discovered it and the physiological explanation behind this curious phenomenon.

What is the Abney effect?

The Abney effect is the change in perceived hue that occurs when white light is added to a monochromatic light source.. That is to say, it consists of seeing a color with a different color tone, with a specific hue and saturation, when more lighting is applied to it. The addition of white light produces, on a psychological level, a desaturation of the monochromatic source, giving the sensation that the color has changed in hue and saturation, although the only thing that has happened is that it now has a higher luminance.

The nature of this phenomenon is purely physiological, not physical. That the human eye perceives a hue of a different color when light is added is counter-intuitive, since it would be logical to see a different color when light is added to it.It is counterintuitive, since it would be logical to see the same color only brighter. For example, the brown color is actually no more than a dull orange-red that, when white light is applied to it, becomes that color. It gives the sensation that we have obtained a new color, or that the brown has been transformed into orange, when in reality it has always been orange.

This phenomenon first described in 1909 by the English chemist and physicist Sir William de Wiveleslie Abney. He discovered that by applying a white light source made from the three primary light colors, i.e. red, blue and green, it was possible to induce changes in the perception of certain colors, even though they remained essentially the same shades.

Chromaticity diagrams

To further understand this phenomenon, it is necessary to talk a little about a tool used in color theory. Chromaticity diagrams are two-dimensional diagrams in which colors are represented in XYZ coordinates. The X, Y and Z values, or tristimulus values, are simply used as values to create new colors from primary colors in the same way that the RGB model is used.

Two aspects of colors are represented in this type of diagram: hue and saturation.. Hue is the color itself or chromaticity, represented in how close the color is to pure green, red or blue when we speak of light colors. Saturation corresponds to the degree of intensity of the color, going from lighter to more intense. What is not represented in these diagrams is the illumination or luminance of the color.

Colors in chromaticity diagrams are represented in rows and columns. For example, rows can represent hue (blue, blue-green, turquoise, green...) while columns can represent saturation, from lighter to more saturated tones. The Abney effect occurs when, when white light is applied to these colors, changes are perceived as if the hues or saturations of the colors have changed.

Returning to the previous case, brown and reddish orange are the same color, with the same degree of hue and the same saturation, but with different degrees of illumination. In a chromaticity diagram both colors would be the same, reddish orange. It would be when the illumination is changed, either higher or lower intensity, that the perceived color would look different, being brown the result of a reddish orange with low illumination.

This is why chromaticity diagrams are so useful to detect which are the colors that, by changing only the illumination, we perceive as new colors at a psychological level. It is by means of these instruments and simply by shining white light on them that we can detect which colors our brain interprets as if they were different tones.

Physiology of the phenomenon

According to the opponent process model of the visual system, three neurological channels are involved in the perception of color: two chromatic channels and one achromatic channel.. The chromatic channels consist of a channel that perceives red and green (red-green channel) and a channel that perceives blue and yellow (yellow-blue channel), these being responsible for perceiving the tones themselves. The achromatic channel is responsible for luminance, seeing how close the color is to black or white.

Hue, saturation and illumination are perceived thanks to the joint and varied activity of these three neurological channels, which consist of axonal pathways coming from retinal ganglion cells. The activity of these three channels is intimately linked to the reaction time in the response to colors. Some activities depend on one or the other channel, or both types are also involved. The achromatic channel has a faster response speed than the chromatic channels, under most conditions.

There is one particular situation in which the achromatic channel emits a slower response than the chromatic channels, and that is when white light is added to a color that was already being observed. The achromatic channel exhibits a slightly slower response time than it would under conditions without intense brightness. However, its magnitude of response will be stronger than the chromatic, giving the false perception.

It is not well understood why we can see the same color as if it were a different color depending on the luminance.. The spectral sensitivity of the observer, the relative number of each type of cone or the age of the individual do not seem to be factors influencing how intense the perception of different hues is. What is clear is that the light of the environment in which one is has a significant influence, making the same image appear in a different color, as has been seen in illusions such as the blue or white dress.

This would explain why color judgments vary depending on differences in the color environment or exposure to a particular color. It could also be due to the amount of time the retinal cones have been stimulated, causing them, for a brief period of time, to not emit adequate signal when different types of wavelengths impinge on them.

Bibliographic references:

• Pridmore, R. (2007) Effect of purity on hue (Abney effect) in various conditions." Color Research and Application. 32.1: 25-39.
• W. de W. Abney. (1909) On the Change in Hue of Spectrum Colours by Dilution with White Light." Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character. 83.560: 120–127.