Light & Vision
Last Update: February 20, 2012
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The Visual System

The human visual system involves both the eye and the brain working together. The eye forms an optical image of the outside world on its retina. This image is then converted into electrical signals which are broken into different elements of information that are then transmitted up to the visual cortex of the brain where a model of the outside world is constructed. This model is based on the information from the retinal image and past experience.  The eye and the brain are in constant communication back and forth, so human vision truly depends on a visual system.

Because the visual system can adjust its sensitivity rapidly, it is able to operate over a vast range of lighting conditions, from starlight to strong sunlight. Of course, it cannot operate equally well over the whole range. As a general rule, the more light there is, the better the visual system will be able to perform. For example, by increasing the amount of light the ability to resolve detail and to discriminate colors are enhanced and the speed of response is improved (see the Visibility section). The challenge for lighting professionals is to understand how well the visual system can function under a given set of conditions, and to decide if that is good enough for the activities to be done. If it is not, the lighting professional has to design a lighting system that will allow people to do what is required. It is important to note that this is not always a simple matter of increasing the amount of light.  Often, a task can be made more visible by modifying or moving the task itself, by adjusting the type or direction of incident light or by removing competing light sources, rather than by simply adding additional illumination.

This section provides a brief overview of the mechanics of human vision.

The Eye
The eye is physically shaped to regulate and focus light. First, light reaches the transparent protective layer of the cornea where it is refracted by the convex shape of the cornea, much like a contact lens. The pupil, the aperture surrounded by the muscular ring (the iris) that determines our eye color, contracts or expands to control the amount of light entering the eye.
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The Retina
The retina is a complex structure. Light reaching the retina first passes through the ganglion and collector cell layers before being absorbed by the photoreceptors. The retina contains four types of photoreceptor, three cone types and one rod type, the names of the two forms, rods and cones, being derived from their shape.
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Photopic, Scotopic & Mesopic Vision
The human visual system has three operating states, depending on which photoreceptors are active:  photopic vision, scotopic vision and mesopic vision. Each operating state has different capabilities.
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The eye is a dynamic system that is constantly adjusting its sensitivity to its environment. The pupil obviously opens or closes to let in more or less ambient light but there are also subtle chemical and neural changes occurring.
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Color Vision
Color vision is one of the great joys of being human. Most of us can distinguish an enormous range of subtle colors, and find information and pleasure in doing so (Neitz et al, 2001). Colors add meaning to our environment.
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Perception is the ultimate output of the visual system. It is the outcome of a process of turning the flood of raw visual information from the eye into a meaningful, stable and coherent model of the outside world that holds steady in the brain. 
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As the eye ages, its capabilities are diminished. Eye movements are slower and adjustments to different light levels occur more slowly. The lens becomes increasingly rigid and loses some ability to adjust focus, especially close up.
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Light & Vision

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