How Does Colour Vision Works

Colour vision is the ability of any organism to distinguish different light based on their wavelength of light they reflect .The visual system derives colour by comparing the responses to light from several types of cone receptor in the eyes. Colour enable use to separate object form each other and from their background as different objects tend to have different colour. On the other hand, object of the same colour are grouped together.

One of the most important functions of the human eye is to pick up and detect light. Light is a form of electromagnetic energy. Electromagnetic energy also includes cosmic rays, X rays, ultra violet rays, infrared rays, and radio. The distance from one crest of waves to the next varies greatly from the shortest, which are cosmic rays to the longest, which are radio waves. The visual system is only sensitive to a tiny portion of electromagnetic energy wavelengths of approximately 380 to 760 nanometres. Colour vision is only present in daylight (photopic vision) or under high intensity light from other sources and is absent at night (scotopic vision) or under low intensity lighting.(optic lec no) Cones function in bright conditions and rods function in dim lighting conditions. There are approximately 7 million cones and 120 million rods in the human retina; hence, the two types of receptors are not distributed evenly. The fovea, located at the back of the retina contains most of the cones and none of the rods (it allows for high acuity colour vision). The rods however, reach their maximum density slightly peripheral to the fovea and both cones and rods start to diminish towards the retinal periphery. (Physic Lab., 2002)

To understand how colour vision works we need first to understand what is colour and where does it exist? Does it exist within an object or within the light? Colour is a sensory perception, which is caused by the interaction of light with our visual perception equipment (eyes and brain). We need to understand the study of the physical nature of light, its interaction with matter and the physiological as well as psychological understanding.

Light is a forms of ever-present energy, Its behaviour can be explained in different ways, one is by assuming that it is a wave and two by assuming that it is made up of particles having no mass (photons). When light interacts with matter it can be reflected back, absorbed or even transmitted through. It travels in a straight line, but it can change its direction when travelling through a medium. (dr hillabrand lecture note) The light waves themselves are not coloured. Colour is experience by a receptor when the light is absorbed or reflected. Sekular& Blake (2002)

The colour sensation is actually determined by three physical attributes of light (wavelength, Intensity and spectral purity) and three corresponding psychological qualities of the human colour sensation (Hue, brightness and Saturation). Hue refers to the wavelength of light that produces the - 2 -psychological sensation of colour, for example a short wavelength of light may produce the colour sensation of blue

( Millodot M.,2004) The brightness of a stimulus is directly related to the intensity of the wavelength. The more intense a light the brighter a stimulus will appear. However some stimulus can appear to be brighter then others even though they share the same intensity. Saturation is connected to the spectral purity of light. A pure light consists of a single

wavelength and is called monochromatic light. If other colours such as grey or white are added to a monochromatic light then it will appear to be 'washed out'. A pure

monochromatic light would very rarely occur outside of a laboratory setting rather the light that reaches the eye is generally a mixture of different wavelengths. One may wonder how the cones are mediating high acuity colour vision when most of them are located in the fovea. Luckily when we look around we do not just see two or three coloured details surrounded by a greyish seen instead we see an expansive handsomely coloured visual world. (Pritchard 1961).

Our visual system enables us to detect a different colour for every 2 nm of wavelength. That is an estimated number of 7 million wavelengths that we can discriminate amongst. The wavelengths that we see are not actually coloured. In fact there is no such thing as coloured light only the visible radiation of different wavelengths. As Isaac Newton so gracefully put it "for the rays, to speak properly, are not coloured. In them is nothing else then a certain power and disposition to stir up a sensation of this or that colour"( Boroditsky. L., 1999). Seeing colour is a subjective experience as it depends on the brain's ability to construct colour by analysing light wavelengths. However, it can also be thought of as objective given that any two viewers with the same kind of colour cones appear to construct colour in the same way. Not everybody sees colour the same way, there are certain genetically transmitted colour deficiencies that prevent us from doing so. In the worst case individuals would have no functioning cones and since all of their vision would come from the rods they would find photopic conditions almost unbearable. The next step up would be individuals with one functioning type of cone. Monochromatic individuals cannot distinguish between colours but are able to go out in daylight. Individuals in the first two categories are truly colour blind (very rare, but effects both genders equally). Dichromatic Individuals who have two types of functioning cones can see colour but have trouble with the wavelengths that correspond to their missing cone type (effects 9% of males and under 0.5% of females. (Nathans and at el., 1986).

There are two major theories of colour perception the Young & Helmholtz, Trichomatic theory and Ewald Hering's opponent processing theory. The Trichomatic theory asserts that the quality of colour is coded by the pattern of activity of three receptors rather then by specific receptors for each colour. Long before scientist knew about cones, Young 1801 documented that only a few receptors were needed to produce all of the colours of the visible spectrum. Fifty years later Herman Von Helmholtz further developed the theory subsequent to his discoveries involving colour mixture. Additive colour mixtures result when lights of various wavelengths are combined. Hermann Von Helmholtz 1894 demonstrated that by combining a set of three monochromatic light sources (known as primaries red, green and blue) in appropriate amounts they could match any other hue in the visible spectrum. This phenomenon can only occur as long as no combination of any two of the primaries can match the third. Some wavelengths can be added together to produce the same effect on the visual system as a pure colour. The law of three