Hi all, I've recently prepared a series of slides, some of which I've made into a mini-presentation that I thought might be interesting to the folks
here on ATS. The subject presented here is the nature of light, and how we perceive it; the broader topic is information loss in human systems of
I thought this might be interesting because people really don't seem to understand the difference between color and light, and how our human-specific
systems of perception work. This is an important thing to understand, especially when understanding spectroscopy and 'false color' images, for
example those which NASA releases.
Anyway, I've resized some slides and written some language around them; hope you enjoy and perhaps find it informative.
A Brief Overview Of Color
Our eyes react to light, which we perceive as having various 'colors'. It's important to realize that 'color' is not a property of the light
itself, it's a property of our perception
of the light.
Light, being electromagnetic radiation, can be at different wavelengths, and with different energy levels at each wavelength. The light we see is
almost always a blend of different wavelengths of photons, in various proportions, ranging from about 400 nanometers to 700 nanometers (the 'visible
The Cells Of The Retina
The retina is the portion of the eye that reacts to light. Cells on the retina are stimulated by photons, build up an electrical potential, and
transmit 'activation' signals to other nerve cells in the eye, eventually transmitting the information to the visual cortex.
There are two major categories of light-sensitive cells in the retina: rods and cones (named for their rough shapes). Rod cells are more sensitive to
light intensity, along a wider range of wavelengths, than cone cells. They're the primary cell responsible for night-vision and monochrome
perception. Cone cells are responsible for color vision.
The different types of cells are distributed in varying densities along the surface of the retina. The areas of the retina that generate peripheral
vision contain mostly rod cells, while the center of the retina (the fovea) is mostly cone cells. This explains why, when it's very dark, you can
sometimes see dimly-lit objects with better contrast by looking to one side or the other, instead of staring straight at them.
The cone cells are of three types: known as S-cones, M-cones, and L-cones. Each type responds differently to different wavelengths of light.
They're sometimes called blue, green, and red cones, but that labeling isn't really accurate -- remember, wavelengths aren't quite the same thing
The M-Cone And Monovariant Color Perception
Let's look first at M-cones (the 'green' receptors of the eye). When photons are absorbed by a cone, they cause that cone to build up an
electrical potential -- the 'signal'. Due to the type of photosensitive molecule in the cone, it gets more or less potential built-up from
different wavelengths of photons. Here's a graph of the M-cone activation response:
Let's look at how two different samples of pure-wavelength light affect the M-cone. The first sample, 'A', is at 485nm, the second, 'B', is at
It's important to notice that even though these samples each have the same number of photons per second impacting the retina, they result in
different activation energy levels from the cone. Clearly, a field of L-cones can distinguish between 'A' and 'B'. But what if we 'turn up the
brightness' of 'B', doubling the number of photons per second?
So a color perception system with a single type of 'sensor' can be quite easily fooled!
The L-Cone And Divariant Color Perception
The second type of cone in the eye is called the L-cone (or 'red' receptor). It has a different activation graph than the M-cone, as it uses a
slight variant of the same photosensitive chemical:
This different activation curve allows the L-cone to quite neatly detect differences that the M-cone misses, and vice-verse. For example, the
previous examples of 'A' versus 'B' and 'A' versus '2 * B', in terms of the L-Cone response:
But can a divariant color perception system still be fooled? Yep! Let's add another pure-wavelength of light, 'C', at 580nm:
While that's obviously distinguishable from either 'A' or 'B', lets look at how it is perceived versus the blend of 'A + B':
The S-Cone And Trivariant Color Perception
The normal human eye has three types of cone cells. The third type is known as the S-cone, it has a much lower range of wavelength activation than
the other two types of cones; it is more sensitive in the 'blue range':
Here's what the wavelengths 'A', 'B', and 'C' look like, with the S-cone activatation curve added:
This obviously clears up the situation that was indistinguishable with only a two-cone system, as only wavelength 'A' causes any activation energy
in the S-cone:
Trivariant Color Aliasing
But can a trivariant system, with all three different types of cones, be fooled? Of course! For example, considering the following, with new
locations for 'B' (at 412nm) and 'C' (at 520nm):
The trick is to remember that different wavelengths of light can be blended in different proportions. Here's a comparison of 'A' with 'B + 2/5 *
These two very different compositions of light are perceived by humans as virtually identical colors.
Enough for now; hope this was interesting and/or informative!