Objects in our environment are only visible because the eye receives some of the light that is reflected off them. If an object reflected no light, it would appear to us as a featureless black silhouette, masking whatever was behind it but having no discernible features of its own. The light that reflects off a real material depends on both the wavelength content of the light incident upon it as well as how different wavelengths interact with the atomic structure of the material itself. Figure 1 shows a range of spheres with different colours ordered by wavelength and covering the entire visible light spectrum.
Visible Light Spectrum
The range of visible colours are spread across the band of wavelengths from red at 700nm up to violet at 400nm. White light contains all of these wavelengths in roughly equal measure. Black, by definition, contains none of these wavelengths. Thus, for a material to be perceived as having a distinct colour, it must contain some wavelengths with a greater intensity than others.
When light is incident on the surface of a material, some component of it will be reflected and some transmitted or absorbed. Light actually interacts with the material at a sub-atomic level. All solids, liquids and gasses are made up of molecules that contain atoms of one or more elements. These atoms contain varying numbers of protons, neutrons and electrons, with the electrons orbiting a nucleus made up of the protons and neutrons.
When a photon of light collides with an atom, it imparts its energy by changing the orbit of one or more electrons. Each atom can only support a certain range of electron orbit states. If the new state of the electron is supported, then that photon of light is effectively absorbed, increasing the kinetic energy of the atom and being dispersed as heat within the material.
If the new electron state is not supported, the electron quickly decays back to a supported state, releasing the excess energy as a photon of light. That photon either goes on to collide with another atom or, if it is close to the surface, escapes back out of the material as reflected light. Different atoms support different ranges of electron states and, as light with different wavelengths contains different levels of energy, the confluence of the two means that some wavelengths will be absorbed more than others depending on the type and distribution of atoms within the material.
Our vision depends solely on light that has been reflected off objects in the environment entering our eyes. The eye is a complex organ whose role is essentially to convert light into sensory signals that can be interpreted by the visual cortex within the brain.
The retina at the back of the eye is the beginning of the nervous system which transports visual information to the brain. It contains around 100,000,000 nerve endings that, because of their shape, are called rods and cones. Rods outnumber cones by a ratio of approximately 10:1 and are spread out relatively evenly over the entire retina except for a small area centred on the optical axis called the macula. This area contains a large concentration of closely packed cone cells, with almost no rods at its centre.
Rods are very sensitive to light levels but are unable to distinguish between different frequencies. Cones, on the other hand, are not as sensitive to light levels but can clearly distinguish different frequencies. This is possible because there are three types of cone cells in the retina, each sensitive to a different range of wavelengths. The first type, called L cones, are most sensitive to longer wavelengths in the red, orange and yellow band. The second type, called M cones, are most sensitive to mid-range wavelengths in the yellow and green band. Finally the third type, called S cones are most sensitive to shorter wavelengths in the blue and indigo band.
Thus, no matter how complex its actual wavelength content, all light is reduced to three color components by our eyes. This makes us trichromatic and explains why televisions and computer graphics displays use the RGB color model that can simulate a wide range of colours by only varying the relative levels of red, green and blue components.
The number of each S, M, and L cone cells in the eye vary significantly. Around 63% of all cone cells are L cones, with 31% being M cones (31%) and the remaining 6% being S cones. This explains our relatively poor ability to distinguish the color blue, especially at low light levels, as there are relatively few S cones. Given the number of L and M cones and the fact that their wavelength responses overlap so much, we are obviously most sensitive to orange, yellow and green light.
The eye is a very forgiving and adaptable light detector that, when combined with sensory processing in the visual cortex of the brain, is able to automatically compensate for all sorts of environmental factors that affect our perception of colour. For example, we automatically compensate for the fact that one half of a wall might be in shade by perceiving it as a single homogenous coloured surface. We make this perception even though the actual colour and brightness of the shaded and non-shaded areas may be very different. We can also automatically perceive the colour of two surfaces as being the same even if they are lit by lights with different colours.
In normal lighting levels, the rod cells are inactive with the cones cells being receptive to colour in quite fine detail - commonly called photopic vision. In very low light levels, however, the rods are the only receptive cells, hence we see only in shades of grey - a process called scotopic vision. As light levels change, there is a small band where both rods and cones are active - called mesopic vision.
The persistence of vision of cones is much longer than that of rods, around 1/20th of a second. This is why flicker is much easier to detect out of the corners of the eye rather than directly in front.
Another interesting phenomenon resulting from the difference between rods and cones can be observed on a dark night when viewing stars. It is generally not possible to see very faint stars if you look directly at them. However, if you focus on a nearby brighter star, you will be able to make out many more fainter stars which will disappear as soon as you look directly at them. This is because the macular, the focus point of the eye, contains very few rods, almost none at its very centre.
Units and Measures
The colour of light - whether emitted, reflected or transmitted - is characterized by its wavelength or frequency content and its intensity. Wavelength and frequency are simply reciprocal values, one measures the distance between peaks whilst the other measures the number of peaks that pass a fixed point in one second. Generally the unit nanometer (nm) is used to measure the wavelength and terahertz (THz) is used to measure the frequency of light.
Subjectively, colours can be described by properties such as hue, saturation and lightness/brightness. Objectively, the corresponding properties are dominant wavelength, purity and luminance.
The hue effectively refers to the dominant wavelength of a color, locating it somewhere within the red, orange, yellow, green, blue, indigo and violet range.
The saturation or purity, also sometimes called chroma, refers to the richness of the hue as compared to a gray of the same level of brightness. It can be thought of as the bandwidth of the color content. A higher bandwidth contains a wider range of wavelengths so the colour appears diluted. A lower bandwidth means that only a small range of wavelengths are present so the dominant wavelength is clearer.
The brightness of a light source or the lightness of an opaque object, sometimes together called value, is measured on a scale ranging from dim to bright for a source or from black to white for an opaque object. This corresponds to the amount of light emitted or reflected or its luminance.
These three values effectively describe the HSL/HSV system for describing colours, but there are several other color space systems.
What Color is this Spectrum?
A great interactive tool that allows you to hand-draw a spectrum and see what colour it is perceived as.
Wikipedia: The Color of Objects
Photometry, Radiometry and Measurement
Human Vision and Color Perception
Your Color Red Really Could Be My Blue
Wikipedia: Visual Cortex
Wikipedia: Color Vision
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