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

Our perception deceives us. Many times what we think we see is not what it seems, and one of the examples of this we have in the curious case of the Abney effect.

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

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

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What is the Abney effect?

The Abney effect is the perceived change in hue that occurs when white light is added to a monochromatic light source. In other words, it consists of seeing a color from another color tone, with specific hue and saturation, when more lighting is applied to it. Adding white light produces, on a psychological level, a desaturation of the monochromatic source, giving the sensation of that the color has changed in hue and saturation, even though all that has happened is that it now has a greater luminance.

The nature of this phenomenon is purely physiological, not physical. That the human eye perceives a shade of another color when light is added to it is somewhat counterintuitive., since the logical thing would be to see that same color only brighter. For example, the color brown is really no more than a dull orange-red which, when white light is applied to it, becomes that color. It gives the feeling that we have achieved a new color, or that brown has turned into orange, when in fact it has always been orange.

This phenomenon It was 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 colors of light, that is, red, blue, and green, you could induce changes in the perception of certain colors, even though they remained essentially the same tones.

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.

In this type of diagrams, two aspects of colors are represented: hue and saturation.. Hue is the color itself or chromaticity, represented by how close the color is to pure green, red, or blue when we talk about light colors. Saturation corresponds to the degree of color intensity, 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, teal, turquoise, green...) while columns can represent saturation, from lighter to more saturated tones. The Abney effect occurs when white light is applied to these colors and changes are perceived as if their hue or saturation had changed.

Going back 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. On a chromaticity diagram both colors would be the same, reddish-orange. It would be when the lighting is changed, whether more or less intense, that the perceived color would look different, the brown being the result of a reddish orange with low lighting.

This is why chromaticity diagrams are so useful for detecting which colors are that, by changing only the lighting, we perceive them as new colors on a psychological level. It is through 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.

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Physiology of the phenomenon

According to the opponent process model of the visual system, Three neurological channels are involved in color perception: two chromatic channels and one achromatic channel.. The color 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 sayings. The achromatic channel is responsible for the luminance, seeing how close the color is to white or black.

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

There is a specific situation in which the achromatic channel emits a slower response than the chromatic channels, and this is when white light is added to a color that was already being observed. The achromatic channel shows a slightly slower response time than it would in bright dark conditions. However, its response magnitude will be stronger than the chromatic, giving the false perception.

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

This would explain why color judgments vary depending on differences in the color environment or exposure to a given color. It could also be due to the length of time the retinal cones have been stimulated, causing them, for a short period of time, do not emit an adequate signal when impinged on them by different types of wavelengths. vibe.

Bibliographic references:

• Pridmore, R. (2007) Effect of purity on hue (Abney effect) in various conditions.” Color Research and Application. 32.1: 25–39.
• W. from W. Abney. (1909) On the Change in Hue of Spectrum Colors 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.