Understanding Colour

Colour by number

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By Colin Ramsden, January 2007.

What is colour?

Colour is an optical illusion, a perception of light created by your eyes and interpreted by your brain. Colour is a quality constructed by the visual brain and not necessarily a property of objects as such. In order for animals to respond accurately to their environments, their visual systems need to correctly interpret the form of objects around them. A major component of this is the perception of colours.

The history of colour perception

Through experimentation (most famously with sunlight split through a glass prism by Sir Isaac Newton), it was discovered that white light is composed of several colours which combine to create our perception of "white".

The colour prism

The colour prism.
See how as white light enters and exits the prism in air,
it is bent (refracted) and splits into separate colours.

Nature reveals the composition of light (during sun-showers) with beautiful rainbows.

Rainbow over Lake Mary

In this photo of a rainbow, look closely and see that
there is a second rainbow higher up in the picture.

400 years before Newton's optical experiments in 1665, Roger Bacon in the thirteenth century recognised the visible spectrum in a glass of water, and postulated, but could not demonstrate, that the colors of a rainbow were due to the reflection and refraction of sunlight through individual raindrops.

Biographical information About the first scientist

Roger Bacon was among the first to practise the "scientific" experimental method of acquiring knowledge about the world, and rejected the blind following of prior authorities, both in theological and scientific study, which was the accepted method of undertaking study of the day. Bacon studied extensively, experimented scientifically, and subsequently wrote treatises on the subjects of mathematics and optics, alchemy (and the manufacture of gunpowder), the positions and sizes of the celestial bodies, and anticipated later inventions such as microscopes, telescopes, spectacles, flying machines, hydraulics and steam engines amongst other things. He was imprisoned for over a decade, by the Roman Catholic Church, for being a possessor of forbidden knowledge and for practising witchcraft, however, now is widely regarded as the "first scientist".

Nearly a century and a half after Newton, another English scientist, Thomas Young, known for his work in physical optics, is said in the early years of the 19th century to have passed a beam of light through two parallel slits in an opaque barrier, which formed a pattern of alternating light and dark bands on a white screen beyond, instead of two bright areas as was otherwise expected. The resultant dark areas were considered to be caused by interference between the two light sources. This led Young to reason that light was composed of waves, with crests and troughs, and which did much to establish the wave theory of light (as first proposed by Christiaan Huygens in his 1678 "Treatise on Light").

Young has also been called the founder of physiological optics. Back in 1793, he explained how the eye accommodates itself to vision at different distances (focuses) by changing the curvature of its lens. He also raised the hypothesis that colour perception depends on the presence in the retina of three kinds of nerve fibres which respond respectively to red, green and violet light. (This trichromatic theory wasn't able to be experimentally proven until 1959 with the advent if microspectrophotometry.)

The visible light spectrum RGB

The visible light spectrum.
See that the colour ranges gradually from deep red on the left through to deep violet on the right,
yet three major colour groupings dominate: red, green, and blue.

During the mid 19th century, the Scottish mathematician and theoretical physicist, James Clerk Maxwell, explained light as the propagation of electromagnetic waves. Maxwell showed that waves of oscillating electric and magnetic fields travel through empty space (vacuum) at a speed that could be predicted from simple electrical experiments using the data available at the time. Maxwell obtained a velocity of 310,740,000 m/s, and wrote in 1865:

"This velocity is so nearly that of light, that it seems we have strong reason to conclude that light itself (including radiant heat, and other radiations if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws."

Maxwell was correct in this conjecture—as modern measurements have determined light to travel in a vacuum at a little under 300,000km/sec—though he did not live to see the first experimental confirmation by Heinrich Hertz in 1888 who generated and detected radio waves in a laboratory. Hertz demonstrated that these waves behaved exactly like visible light, exhibiting properties such as reflection, refraction, diffraction, and interference. Maxwell's theory and Hertz's experiments led directly to the development of modern radio, radar, television, electromagnetic imaging, and wireless communications. 

Maxwell is also known for creating the first permanent true-colour photograph in 1861 using red, green, and blue filters, to create 3 separate images (on photographic plates). When subsequently used to project onto a screen with three different projectors, each equipped with the same colour filter used to take its image, and brought into focus, the three images reformed a create a full colour image. (Modern digital projectors utilise these same principles.)

Biographical information About the forefather of modern physics

Maxwell's quantitative explanation of light as an electromagnetic wave is considered one of the great triumphs of 19th-century physics. He formulated a set of equations, eponymically named Maxwell's equations, which for the first time, expressed the basic laws of electricity and magnetism in a unified fashion. He also developed the Maxwell distribution, a statistical means to describe aspects of the kinetic theory of gases. These two discoveries helped usher in the era of modern physics, laying the foundation for future work in such fields as special relativity and quantum mechanics.

What is the electromagnetic spectrum?

The electromagnetic spectrum is the range of all possible electromagnetic radiation. Waves of electromagnetic fields vary in size from wavelengths of thousands of kilometres down to fractions of the size of an atom.

A wavelength is the distance between repeating units of a wave pattern, usually between two sequential crests (or troughs) in the same wave. The wavelengths of visible light are quite small, measured in nanometres (10ˆ’9) or billionths of a metre (0.000 000 001m = 1nm).

Electromagnetic Spectrum showing visible spectrum highlight

The full known electromagnetic spectrum showing wavelengths, frequency, and common names.
See that the visible spectrum is only a very small portion of the whole electromagnetic spectrum.

Red wavelengths at 650nm are slightly longer than blue wavelengths at 450nm.

Visible light covers a small portion of the whole electromagnetic spectrum. This is the range in which the sun (and stars similar to it) emit most of their radiation. It is likely no coincidence that the human eye is sensitive to the wavelengths that the sun emits most strongly, as that has been our primary source of light for all our existence.

Not surprisingly, human vision is perfectly adapted for vision on planet Earth, and can handle extremes from full glaring midday sunlight, to low level night-time moonlight or starlight, and everything in-between. Or so it would seem? Actually, studies have revealed that human vision is sensitive (more accurate) to some brightness levels and colours, and less sensitive to other brightness levels and colours, somewhat unevenly across the visible light spectrum.

Graph of visible spectral sensitivity

Representation of human visible spectrum sensitivity.
See that three peaks occur with one each in the blue, green, and red areas of the spectrum.

Perception of color is achieved in humans through color receptors (in the eyes) containing pigments with different spectral sensitivities which receive coloured light. The human visual system combines the information from each type of receptor to give rise to different perceptions of different wavelengths of light.

There are three types of colour receptors (commonly called cones—because of their shape) in the human eye which respond predominantly to red green and blue colours peaking at the frequencies of 590nm (red), 540nm (green), and 450nm (blue), however, this colour sensitivity overlaps as illustrated in the following graph.

Graph of human visible spectrum sensitivity

Graph of human colour sensitivity showing the sensitivity range of each colour receptor (cone) type Beta, Gamma, and Rho. The vertical index is a measure of sensitivity, and the horizontal index is a measure of light wavelength. See how each colour sensitivity overlaps each other.
For example, a predominantly green colour of say 520nm wavelength would stimulate each of the three cone types in the eye to different levels.

When a particular colour enters the eye and strikes the colour receptor cones, each will send a proportionate signal to the brain which interprets the signals and determines the perceived light in that portion of the eye as a particular colour.

For example, in the 'graph of human colour sensitivity' above, if light at the frequency of 450nm was received by the eye, you can see that the blue cones would send their strongest signals, whilst the green and red cones would send much weaker signals appropriate to their respective levels as shown in the graph. With a light of 550nm, see that the green cones would send a strong signal, the red cones would send a less strong signal, and the blue cones would send no signal at all, as the blue type cones do not respond to wavelengths above about 535nm. With a light of 650nm, the red cones would send a stronger signal then the green cones, and the blue would send none.

The different combinations of signal strengths from the three cone types are hard-coded into our very DNA, and cannot be learned or unlearned by any individual. Our perception of which colour we are seeing is predetermined by which combination of colour sensor cones are excited by the light and by how much. This combination process is termed 'additive colour' because the primary colours can produce secondary colours when the lights are added to a dark background.

Additive colour diagram

Additive colour primaries and secondaries.
See that the secondaries (CMY) are created where the primaries (RGB) overlap,
and that "white" is the composite of all these colours.

This is the principle employed by light generating display devices such as Cathode Ray Tube (CRT) monitors and TVs which use a combination of closely packed red, green and blue (RGB) light emitting pixels to produce their range of visible display colours. Have a close look at any 'picture tube' type TV screen, and you'll see that it consists of rows of tiny individually coloured pixels grouped in threes, red, green and blue (trichroic).

Compare the principles of additive colour of light (as illustrated above), with the following example of the subtractive colours of light. Subtractive colour uses pigments or dyes to selectively block white light:

Subtractive Colours

Subtractive colour primaries and secondaries.
See that the secondaries (RGB) are created where the primaries (CMY) overlap,
and that "black" is the composite of all these colours.

This is the method employed by most color printers which use at least cyan, magenta and yellow (CMY) inks to absorb all colours except those they wish to display. Many printers also include black ink in addition to CMY (known as CMYK) because CMY alone cannot produce deep enough shadows.

RYB Colour Wheel

Subtractive colour wheel.
See that the primary triad colours are Yellow, Red and Blue,
which are primarily used by paint artists during the mixing of paint colours.

 

RGBCMY Colour wheel

RGBCMY Subtractive colour wheel.

See that RGB are the major triad colours,
and that CMY are the secondary colours given equal? ranking in the circle.

Human colour vision is high resolution, but is restricted to the object currently being focussed upon and requires daylight lighting levels, as it ceases to work very well in low lighting conditions. Peripheral vision is monochrome (has no particular color—black and white) and is in low resolution, but offers an excellent ability to detect movement through a wide range of illumination levels.

It is our peripheral vision which allows us to see at night in moonlight or starlight. It uses the majority of light receptors in the eyes (commonly called rods—because of their shape). Unlike cones, rods do not detect color (chromaticity), just brightness (luminous intensity—luminance). Rods are used for contrasting levels of brightness or lack thereof, to provide the perception of texture and visual depth. The brain performs extensive processing of both the rod and cone output signals of the eyes in order to generate an image in your mind. 

Graph showing the spectral sensitivity of the rod-like light sensors in human eyes

Graph of human eyesight light sensitivity
showing the sensitivity range of rod-type light receptors.
See that the range spreads across the entire visible spectrum,
so no particular colour is perceived, which demonstrates why
night-vision is in shades of black and white (monochrome).

There are three variables in colour vision, hue, saturation and brightness, and all depend on wavelength.

 

 

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