Light & Vision
Last Update: October 31, 2010
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Visibility

To make sense of the world around us, we need to be able to see detail and colors. To perceive the shape, curvature, color and texture of objects and people, we need to see differences that are often subtle. This section summarizes the main factors that affect the visibility of objects for people with normal vision. The limit of visibility for a specific variable is usually expressed as its threshold. A threshold of vision is conventionally taken as the level of the variable at which the visual task can be done correctly on 50 percent of the occasions it is attempted. A common example is visual acuity which is the threshold for the resolution of detail. This is often measured using a chart of random letters arranged in a series of rows of decreasing size. The observer is asked to stand at a set distance and read out the letters. The visual acuity is the angular size of the smallest letters that can be accurately read. There are three distinct groups of variables that influence visual thresholds: visual system state, target characteristics and the background against which the target appears.

Important visual system variables are the luminance to which the visual system is adapted, the position in the visual field where the target appears, and the extent to which the eye is correctly focused. As a general rule, the lower the luminance to which the visual system is adapted, the further the target is from the fovea, and the more mismatched the focal length of the eye is to the viewing distance, the worse will be the threshold values.

Important target characteristics are the size of detail, the luminance contrast and any color difference between the target and the immediate background. As a general rule, the larger the size of detail and the higher the luminance contrast, the better will be the threshold values. A color difference between the target and its immediate background only becomes important for visibility when luminance contrast is low (Eklund, 1999).

The important variables for the effect of the background against which the target appears are its area, luminance and color. The luminance and color of the background against which the target is seen determines the state of adaptation of the visual system. As a general rule, the larger the area around the target that is of a similar luminance to the target and neutral in color, the better will be the threshold measure.

This section will discuss key elements of visibility and their interactions with one another.

Threshold Contrast
Contrast is one of the most important factors in determining what can be seen. Objects with no contrast cannot be seen. There are several kinds of contrast that our visual system processes, but the two simplest are luminance contrast and color contrast.
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Visual Acuity
Size is another very important factor in determining visibility. For visibility, size is quantified as the angle or solid angle subtended by the object at the eye. If the size is too small, then the object cannot be seen no matter how high the contrast is. We can never see atoms with the naked eye.
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Color Discrimination
In the photopic state and to some extent in the mesopic state, the human visual system has an ability to discriminate colors.  The thresholds for discriminating different colors are given by the MacAdam ellipses plotted in the CIE 1931 chromaticity diagram.
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Flicker
All light sources operating from an AC electricity supply produce some fluctuation in light output, the waveform of the fluctuation depending on the physical properties of the light source and the characteristics of the electricity supply.
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Interactions
Visual thresholds are subject to interactions between variables. As an example, consider the effect of visual size on threshold contrast evident in Visibility Figure 2 – Threshold Contrast . As the size of the target is decreased, the threshold contrast is increased, though the magnitude of the increase depends on the background luminance.
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Light & Vision

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