Chimerical Colours – How to See the Impossible

Pick a colour, any you like.  Now picture it in your head.  Easy enough, right?  Now picture a colour that doesn’t exist.  Something brighter than white, or perhaps darker than black.  It seems impossible, but it can be done.

Chimerical colours are shades that cannot exist in the real-world, but can be perceived due to the way in which our eyes see colour.  They are part of a larger collection of hues called impossible colours.

To understand how chimerical colours work, we need to understand how we see colour in the first place.  (If you want to skip straight to the demo, scroll to the bottom!)

This is the visible light spectrum.  Those numbers along the bottom are the wavelengths of light, measured in nanometres.  Wavelengths outside this spectrum can’t be seen by the naked eye.  Longer wavelengths are the home of infrared light, microwaves, and radio waves.  Shorter wavelengths contain ultraviolet light, X-rays, and gamma radiation.

Visible light is detected by specialised photoreceptor cells in our eyes called rods and cones.  Rods deal with low-light conditions, but don’t play a part in colour vision (this is why your vision goes greyscale at night).  Cones, on the other hand, are very involved in colour vision.

We have three types of cone.  These are officially referred to as the short, medium, and long-wave cones.  However it is simpler to think of them as the blue, green, and red cones.  Each cone responds most strongly to light of that particular wavelength.

So, if there is a lightbulb that gives off a lot of energy around 500nm (green) and around 625nm (red), the green and red cones are excited.  Here, excited means that the cells start sending more signals to the brain.

The combination of green and red light is not perceived as red-green, but as yellow.  Yellow is not a primary colour when it comes to light. Your art teacher wasn’t wrong per se, but when it comes to light, yellow is a secondary colour.

Photoreceptor cells are a bit like sprinters.  They can run at a very fast pace for a short period of time, but then they get tired.  When they are tired, they slow down to a lower speed than before they started sprinting.  In the same way, the cones in the eye can fire a lot to a colour they are specialised for, but only for a short time.  Once that colour is gone, their firing rate (speed) drops to less than it was before they were exposed to that colour.

Due to the way the brain works out colour, a drop in the firing rate of a photoreceptor is interpreted as the opposite of that colour.  If your red cone falls below the original amount of signal, this is perceived as green.

This is the origin of the colour aftereffect – the phenomenon whereby looking at a colour for a long period of time makes the opposite colour appear in your vision for a while afterwards.

In order for us to create a chimerical colour, we need to use the colour aftereffect.

Creating a Chimerical Colour, Step by Step

1. Stare at the cross in the centre of the left circle below for around 30 seconds, trying to keep your eyes still, and blinking very little.
2. When 30 seconds are up, look at the cross in the centre of the right image.
3. Marvel at the impossible colour produced!

The demonstration above produces a colour called Self-luminous Cyan.  This turquoise-blue colour is brighter than the white background.  This is an impossible colour because colours cannot be brighter than white, as white represents the maximum possible amount of energy a colour can have.  Yet you should see Self-luminous Cyan as being brighter than white, and also blueish at the same time.

This is only one kind of chimerical colour, there are two others.  Self-luminous colours are ones that are brighter than white.  Stygian colours are darker than black, and are so named for the River Styx which was the boundary of the underworld in Greek mythology.  Finally there are hyperbolic colours, which are more orange than orange, more green than green, and so on.

What we consider to be impossible colours is dependant on our own colour vision system.  We have only three cones, making us fairly rare in the animal kingdom.  Most mammals have just two, and many nocturnal animals have only one.  However the reigning champion is the mantis shrimp, which has sixteen cones it can use for colour vision.

Using chimerical colours, we can at least see some colours that don’t exist.  Even if we are stuck with just three cones.