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Visibility and glare
 
VISIBILITY

Because the human eye is able to adapt to varying illuminance conditions, it is possible to see in both brilliant sunshine and dim moonlight. Adaptation is not instantaneous, however.

VISIBILITY
 
Cross-section of the human eye.
 
Light enters the eye through the pupil and then passes through the lens, which focuses the light on the retina, where an image forms. Contraction and dilation of the pupil controls the amount of luminous flux entering the eye.  In darkness, the pupil is fully dilated (~ 7 mm) while in bright illumination, the pupil contracts to a maximum (< 1 mm), to restrict the amount of light entering the eye.  Although the pupil adjusts to variation in luminous intensity in less than a second, it takes several minutes for the eye to fully adapt.  This process is even slower in older persons.  Under variable and rapidly changing light conditions, the eye can not adapt.  Visual perception is thus optimal when lighting is uniform.

Light falling on the retina interacts with the rods and cones, which generates nerve impulses.  The cones react to different colours while the rods detect different intensities.  Since the cones perform poorly in weak levels of illuminance, the rods are responsible for night vision, but only in black and white.
 
 
The full moon illustrates the characteristics of an ideal lighting system.  Illumination is weak but uniform, which allows one to see a great distance yet with a good depth of field.  Most artificial luminaires are unable to provide these important qualities.


GLARE

Glare is one of the annoying consequences of intense, non-uniform or poorly directed lighting.  It occurs whenever the eyes are exposed to a relatively bright source of light that forces the pupils to contract, and creates a strong contrast between lighted and background lighting levels.  Glare thus considerably limits one's ability to distinguish hazards, which can increase accident risk.  The following are the three causes of glare:



1. Bulbs of excessive strength

Even if precautions are taken to lessen the impact, strong bulbs always create glare when part of the light beam directly enters or reflects into the eye.
 
 
 
The two photos above illustrate the difference between good and glaring lighting in a small village.  In the photo on the left, a business is illuminated by strong bulbs that shine light everywhere.  This chaotic lighting blinds passersby and drivers.  The photo on the right shows how to correct the situation by installing new luminaires that control the direction of the light and also utilize weaker bulbs.
Source : Guillaume Poulin
 
2. Lack of uniform illumination

Light reflected off highly illuminated surfaces is very intense and produces glare when adjacent to darker zones.
 
In the consecutive photos above, a country road is lighted at 20 lux on average, which is three times what is generally recommended for this type of application.  The unilluminated section in the distance seems very dark in contrast with the foreground.  It is difficult to make out obstacles ahead and one can scarcely distinguish the pedestrian approaching the lighted area.  A more moderate illumination with gradual fading into the dark zone would greatly improve visibility and security.
 
3. Inappropriate direction of light beams

No matter what type of luminaire or amount of input power used, light emitted in the 10° angle below the horizon shines directly in the eyes and provokes glare.