Human stereo color vision is a very complex process that is not completely understood, despite hundreds of years of intense study and modeling. Vision involves the nearly simultaneous interaction of the two eyes and the brain through a network of neurons, receptors, and other specialized cells. The first steps in this sensory process are the stimulation of light receptors in the eyes, conversion of the light stimuli or images into signals, and transmission of electrical signals containing the vision information from each eye to the brain through the optic nerves. This information is processed in several stages, ultimately reaching the visual cortices of the cerebrum.

Introduction to Human Vision - The human eye is equipped with a variety of optical components including the cornea, iris, pupil, aqueous and vitreous humors, a variable-focus lens, and the retina. Together, these elements work to form images of the objects that fall into the field of view for each eye. When an object is observed, it is first focused through the convex cornea and lens elements, forming an inverted image on the surface of the retina, a multi-layered membrane that contains millions of light-sensitive cells. In order to reach the retina, light rays focused by the cornea must successively traverse the aqueous humor (in the anterior chamber), the crystalline lens, the gelatinous vitreous body, and the vascular and neuronal layers of the retina before they reach the photosensitive outer segments of the cone and rod cells. These photosensory cells detect the image and translate it into a series of electrical signals for transmission to the brain.

Sir George Biddell Airy (1801-1892) - Sir George Airy was a distinguished nineteenth century English Astronomer Royal who carried out optical research and first drew attention to the visual defect of astigmatism. Airy manufactured the first correcting eyeglasses (1825) using a cylindrical lens design that is still in use. The diffraction disks that bear his name (Airy Disks) were discovered in the spherical center of a wavefront traveling through a circular aperture. These diffraction patterns form the smallest unit that comprises an image, thus determining the limits of optical resolution.

Alhazen (965-1040) - Born in Iraq as Abu Ali Hasan Ibn al-Haitham, the great Arab physicist is more often known by the Latinized version of his first name, Alhazen. The efforts of Alhazen resulted in over one hundred works, the most famous of which was "Kitab-al-Manadhirn", rendered into Latin in the Middle Ages. The translation of the book on optics exerted a great influence upon the science of the western world, most notably on the work of Roger Bacon and Johannes Kepler. A significant observation in the work contradicted the beliefs of many great scientists, such as Ptolemy and Euclid. Alhazen correctly proposed that the eyes passively receive light reflected from objects, rather than emanating light rays themselves.

Euclid (325-265 BC) - Though often overshadowed by his mathematical reputation, Euclid is a central figure in the history of optics. He wrote an in-depth study of the phenomenon of visible light in Optica, the earliest surviving treatise concerning optics and light in the western world. Within the work, Euclid maintains the Platonic tradition that vision is caused by rays that emanate from the eye, but also offers an analysis of the eye's perception of distant objects and defines the laws of reflection of light from smooth surfaces. Optica was considered to be of particular importance to astronomy and was often included as part of a compendium of early Greek works in the field. Translated into Latin by a number of writers during the medieval period, the work gained renewed relevance in the fifteenth century when it underpinned the principles of linear perspective.

Thomas Young (1773-1829) - Thomas Young was an English physician and a physicist who was responsible for many important theories and discoveries in optics and in human anatomy. His best known work is the wave theory of interference. Young was also responsible for postulating how the receptors in the eye perceive colors. He is credited, along with Hermann Ludwig Ferdinand von Helmholtz, for developing the Young-Helmholtz trichromatic theory.

Human Eye Accommodation - Accommodation of the eye refers to the act of physiologically adjusting crystalline lens elements to alter the refractive power and bring objects that are closer to the eye into sharp focus. This tutorial explores changes in the lens structure as objects are relocated with respect to the eye.

Ishihara Color Blindness Test - Color blindness, a disruption in the normal functioning of human photopic vision, can be caused by host of conditions, including those derived from genetics, biochemistry, physical damage, and diseases. Partial color blindness, a condition where the individual has difficulty discriminating between specific colors, is far more common than total color blindness where only shades of gray are recognized. This interactive tutorial explores and simulates how full-color images appear to colorblind individuals, and compares these images to the Ishihara diagnostic colorblind test.

Reference Listing - References are presented in this section that explore human stereovision and the perception of colors. Included are pertinent review articles and books that discuss theoretical and applied aspects of color perception and the complexities of the human visual system. Several of the reference materials also address the topics of color blindness, astigmatism, and other physiological defects of the human visual system.

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