Color perception principles

Essay by chickenUniversity, Bachelor's March 2002

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Our contemporary understanding of the concept of colour is rooted in Newton's work, Optics (1730), in which he demonstrated that white light was made up of coloured light. Soon after the publication of Optics, physicists were able to show that visible light resulted from electromagnetic waves of a cetrain frequency. The area of the electro-magnetic spectrum that we see as light lies between frequencies of 360 nanometres and 760 nanometres. However an objects hue is rarely of a single frequency, or monochromatic, rather it is comprised of a number of frequencies, with a dominant one determining the perceived colour. We are able to diffrentiate between approximately 200 monochromatic colours, and in the yellows, where our colour sensitivity is greatest, we are able to detect differences of as little as 0.1 nanometres.

Our colour perception mechanism begins with light sensitive cells in the retina, called cones. The light senitivity comes from a photosensitive pigment called iodopsin or visual purple.

When a photon of light triggers the iodopsin an enzyme is released in the cone that alters the membrane of the cell, such that the normal flow of sodium ions into the cell is halted. This causes the negative charge that normally exists across the cell membrane to increase from -40mv to -70mv. In its resting state a cone will trigger a constant stream of neurotransmitters, but when the negative charge across the membrane is increased, this stream slows or ceases, which in turn causes the bipolar cells to fire. The bipolar cells then trigger the ganglion cells via a complex network of horizontal and amacrine cells, the ganglion cells then send the resulting neural signals to the primary visual cortex.

It has long been recognised that colour perception would require a number of different types of sensor, sensitive to different wavelengths.