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uninteresting lighting scheme. Present thinking suggests that the careful

introduction of shadow and sparkle will enhance the appearance of a lighting

scheme greatly without affecting task illuminance unduly. Further information

on illuminance / luminance variation is contained in section 2.4.4 of CIBSE code

for interior lighting.

5.6 AdaptationIt is possible for the eye to adapt to widely varying levels of illumination. For

example, a bright day may provide an illuminance of 100,000 lux whilst at night,

the human eye can adapt well and provide a reasonable performance at 0.1 lux -

the approximate illuminance provided by moonlight.

The eye however, needs time to adjust to these changes. Providing a bright

reference point in a dark room can lead to glare. Most problems with glare in

interiors is caused by windows or luminaires either directly or indirectly.

5.7 Glare

The human eye can adjust from 100,000 lux in bright sunlight to 0.1 lux under

moonlight - a ratio of 1,000,000 to 1. However, it takes time to adapt fully from

from one lighting level to another. When moving from a bright exterior to a dark

interior, the eye can cope well with a ratio of 200:1. It takes about 90 seconds for

the first 70% of that adjustment to occur and a further 15 minutes for the

remainder. This accounts for the difficulty the human eye experiences when

simultaneously coping with different surface luminances - the phenomenon

known as glare.

With internal lighting design care must be taken to ensure there are no

excessively bright sources within the normal field of vision of people using the

installation. If a bright source is visible to the eye, the lens of the eye will close

in proportion to the brightness of the source. A bright source in an otherwise

bright interior may not present difficulty but a bright source in an otherwise

dimly lit interior will certainly present problems.

Care must be taken by lighting designers to ensure that bright light sources

are not placed in the field of vision. The source can be direct as in the case

of a luminaire or window or indirect, reflected from a working surface suchas a display screen. Glare can also be caused by excessive contrast such as a

dark background coupled with a bright foreground.

Glare from luminaires is reduced by decreasing the luminance of the source.

With opal and prismatic diffusers the source luminance is reduced by increasing

the area of the source. Special low glare luminaires utilise louvres to shield the

lamp from direct view. ( see special category luminaires, Chaper 6)

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Glare occurs whenever one part of an interior in the field of vision is much

brighter than the general interior. A task to immediate background ratio of 3:1

and task to general background ratio of 10:1 is recommended by the CIBSE

Code for Interior Lighting. Glare can be subdivided into disability and

discomfort glare.

5.7.1 Disability Glare occurs when vision is actually impaired. It can be the

cause of accidents and a serious reduction of visual performance. This is a more

extreme form of glare and is most likely to occur when there is an area close to

the line of sight which has a very high luminance. The most common causes of

glare indoors are windows and electric light sources which are seen either

directly or indirectly by reflection. Glossy magazines and visual display

terminals (VDTs) are common causes of indirect disability glare because of their

reflective surfaces.

5.7.2 Discomfort Glare often occurs, after continuous exposure to high

background contrast or high source luminance either directly or indirectly from

say VDTs. It can cause eye fatigue, headaches and other symptoms related to

"Sick Building Syndrome".

5.7.3 Glare Index of discomfort

The CIBSE code for interior lighting uses a glare index system. This is a

numerical index which may be calculated for lighting schemes. Section 2.6.4 of

the CIBSE guide provides limiting glare indices for most building interiors.