Introduction to Color - Colors in Mind and Outside the Visible Light Spectrum

Inna Tishchenko on June 1, 2020

Photo by Jörg Bauer on Unsplash

Colors in mind

What a wonderful day to enjoy lying down on a blue meadow under the bright pink sun, watching birds in the green sky.. Or maybe not - unless the meadow is green, the sun is yellow and the sky is blue. Color perception is so natural to us that it appears almost impossible not to associate objects in our environment with particular colors. In the physical world (or the matrix, as Neo would refer to it), however, colors exist not quite in the form we usually anticipate them to be.

What is Color?

Visual perception is an essential ability to interpret the surrounding environment, and colors play a vital role in this process. By definition, color is the characteristic of visual perception from the stimulation of photoreceptor cells on the eye retina by electromagnetic radiation, usually coming from a light source. Depending on the surface and material, the electromagnetic waves get reflected and emitted by objects in a different manner. Our mind does an amazing job of interpreting such constellations and converting them into what we perceive as colors, commonly used to describe and recognize objects.

Color Perception

Having evolved on the planet Earth sunbathed by the electromagnetic waves of the star Sun, it is no surprise that our eyes have developed a particular sensitivity for the light wavelengths associated with the Sun’s maximal emission power. Furthermore - adopting the assumption that our eyes evolved more than 500 million years ago in the water environment - the visible light spectrum also corresponds to the light wavelengths propagating well underwater.

Our eyes have two types of photoreceptor cells on the retina: cones and rods. Cones are able to detect colors in the visible light spectrum and rods function in dim light without color perception. There are three types of cones comprising a total of approximately 6 million color-associated photoreceptors on the retina.

American Academy of Ophthalmology - Eye Health.

Each of the three cones is linked to the maximal sensitivity for a different light wavelength: one peak is associated with what we perceive as the color yellow-orange (called “red” cone), one “green” and one “blue”. Typically, our cones detect wavelengths between 380 nm and 760 nm, which is approximately the size of a virus or bacteria.

By OpenStax College - Anatomy & Physiology, Connexions Web site.

The blue light corresponds to shorter wavelengths and is considered cool. The red light is associated with longer waves and is warm. For instance, its' neighbor on the spectrum is the invisible infrared, which is known to generate the feeling of heat.

Relative Luminance

The distributions of light sensitivity of different cones raise an imminent question: if we only have three types of cones, how do we see more than three colors? The answer lies in our mind: it processes the information it receives from cones and converges it to one color corresponding to the anticipated original wavelength. For instance, if only blue cones are firing, we see the color blue. If there are blue and green cones firing, we see a cyan hue (note: in practice, the green cone mostly activates in conjunction with the red cone due to a significant overlap in the light sensitivity).

Each cone type is linked to different absolute light sensitivity. Although these variations differ from person to person, in general, the shorter wavelength (blue) cones are linked to a significantly smaller sensitivity than the long (red) ones. This is the reason the blue color in its full brightness appears much darker than green or red colors. The anticipated relative luminance is shown in the below figure. It demonstrates that yellow is the lightest color in its full brightness, followed by cyan and green.

ArtyClick - Reative Luminance and Hue.

In practical terms, the varying relative luminance demonstrates that some colors are naturally light or dark, and it is virtually impossible to obtain certain color tones in high saturation and brightness. For instance, dark yellow and light (pure, or ultramarine) blue are impossible to obtain without significantly compromising their brightness or saturation. Further, the yellow and blue color combination can be used to create a big impact since it is linked to the highest contrast between two fully saturated colors.

The Invisible Magenta

Comparing colors associated with electromagnetic wavelengths within the visible light spectrum and the hue range we perceive, it strikes that magenta (colors including fuchsia and hot pink) is not associated with any light wavelength and is virtually non-existent. In fact, there is no magenta in the rainbow or in the spectrum of the white light passing through a triangular prism.

Since our vision is limited to the blue-violet color on the side of the short wavelength and orange-red on the side of the long-wavelength, where does magenta come from? Perceived colors are derived from the difference in the cones activation rates and for most colors, it is located in the visible range. Magenta is an exception: it is perceived when the red (long wavelengths on one end of the spectrum) and blue (short wavelengths on the other end of the spectrum) cones are activated simultaneously while the green cones are silent (green cones activation would indicate that the original wavelength lies between the short and long wavelengths). Thus, instead of the green color, which would be inferred if the green cone was activated, our mind plays a trick and defines magenta as a new color outside the visible light spectrum. Isn’t our mind just mind-blowing?