Building Services Engineering 5th Edition Handbook

264 Lighting
Light sources
9
9
9
Window
3
3 3
6
7 8 8
5
2
Window
4
7
7
4 8 4
9
5
9
2
4
Shadow and brightness on a
scale of dark 1 to bright 10
11.3 Observed illumination pattern.
Definition of terms
Some of the terms that are used in lighting system design are as follows.
BZ classification: British Zonal classification of 1–10 for the downward light emitted from a
luminaire. The BZ class number relates to the flux that is directly incident upon the working
plane in relation to the total flux emitted. BZ1 classification is for a downward directional
luminaire. A BZ10 describes a luminaire that directs all its illumination upwards so that the
room is illuminated by reflection from the ceiling.
Contrast: the difference in the light and dark appearance of two parts of the visual field seen
simultaneously.
Daylight factor: the ratio of the natural illumination on a horizontal plane within the building to
that present simultaneously from an unobstructed sky, discounting direct sunlight. A standard
figure of 5000 lx is adopted for the external illuminance in the UK.
Efficacy: the luminous efficacy is the lamp light output in lumens per watt of electrical power
consumption.
Glare: the discomfort or impairment of vision due to excessive brightness.
Glare index: a calculated numerical scale for discomfort glare.