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Color Vision

The human visual system can detect the range of light spectrum from about 400 nanometers (violet) to about 700 nanometers (red). Our visual system perceives this range of light wave frequencies as a smoothly varying rainbow of colors. We call this range of light frequencies the visual spectrum. The following illustration shows the visual spectrum approximately as a typical human eye experiences it.


The human eye has a lens and iris diaphragm which serve similar functions to the corresponding features of a camera. In other respects the eye is quite different from a camera. A camera has a flat image plane where the resolution and spectral response is reasonable constant across the entire plane. The eye is not at all like that, having evolved to serve several different purposes. It provides a motion sensor system with nearly 180 degrees horizontal coverage. The eye's peripheral vision system only supports low resolution imaging but offers an excellent ability to detect movement through a wide range of illumination levels. This motion detection has been useful to human kind for protection from aggressors and for spotting game while hunting. Peripheral vision provides very little color information. The retina is a thin layer of nerve cells which consists of partially of light sensor cells. The majority of the eye's inside chamber has this retina layer, accounting for the very wide angle of our peripheral vision. The following illustration shows a cross section of the human eye.




The eye's high resolution color vision system has a much narrower angle of coverage. This system can flexibly adapt to widely varying illumination colors and levels. It evolved primarily as a daylight system and ceases to work well at very low illumination levels. The sensors associated with this system are concentrated around the eye's fovea. The light sensor cells capable of working over a wide illumination levels and of providing quick response to changes are called rods. High resolution color imaging is provided by light sensor cells called cones. The following illustration shows a functional diagram for the rods which are spread throughout the retina, and the cones which are localized in the area of the fovea.




The cones in a typical human eye have the ability to separately sense three different portions of the spectrum. We identify these peak sensitivities as red, green and blue - the primary colors. All rod light sensors have the same broadband sensitivity and therefore only provide luminance information. Rods cannot create color images. The brain and nervous system perform extensive processing of the rod and cone outputs in order to generate an image.

Our eyes have three sets of sensors with peak sensitivities at light frequencies that we call red (580 nm), green (540 nm) and blue (450 nm). Light at any wavelength in the visual spectrum range from 400 to 700 nanometres will excite one or more of these three types of sensors. Our perception of which color we are seeing is determined by which combination of sensors are excited and by how much. The following illustration shows the spectral sensitivity of the typical human visual system. It is customary to denote the RGB sensors with the Greek letters Rho (red), Gamma (green) and Beta (blue).



The sensitivity curves of the Rho, Gamma and Beta sensors in our eyes determine the intensity of the colors we perceive for each of wavelengths in the visual spectrum. The following illustration is an approximation of the visual spectrum illustration adjusted for the sensitivity curves of our Rho, Gamma and Beta sensors.


Our visual system selectively senses the range of light wavelengths that we refer to as the visual spectrum. Selective sensing of different light wavelengths allows the visual system to create the perception of color. Some people have a visual anomaly referred to as color blindness and have trouble distinguishing between certain colors. Red-green color blindness could occur if the Rho and Gamma sensor curves exactly overlapped or if there were an insufficient number of either rho or gamma sensors. A person with this affliction might have trouble telling red from green, especially at lower illumination levels.

The human visual system has much greater sensitivity in low ambient illumination. The cones contribute little or no sensitivity in this condition. Imaging is primarily accomplished by the rods when illumination levels are very low. The plot that follows shows the spectral sensitivity of the rods.


Notice that sensitivity is peaked at yellow-green, not the day-for-night blue of motion picture moonlight. Since rods all seem to share the same spectral selectivity characteristics, they cannot create a color image so one color of moonlight is probably as good as the next. The most accurate representation of moonlight is a contrasty black and white image since we cannot see any colors at very low illumination levels.


Where is the Visible Spectrum located in relation to the rest of the electromagnetic spectrum? The following chart shows the relative locations of various sections of the spectrum.



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