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IGHTING
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Copyright © BEAM Society Limited. All rights reserved. Page 114
[3]. These methods are also summarised in Section 4.5.6 of the
CIBSE Code.
The calculated glare index shall be checked for compliance with the
recommendations given in Section 2.6.4 of the Code or Chapter 5 of
the Lighting Guide.
For assessment using the IESNA Lighting Criteria, the calculation
methods described in Chapter 9 of the IESNA Lighting Handbook can
be used for the calculation of the following parameters:
i.
horizontal and vertical illuminance;
ii.
glare: VCP or UGR;
iii.
luminance;
A validated computer programme such as Radiance, Lightscape etc.
can be used for the calculation. The calculated results will then be
checked for compliance.
B
ACKGROUND
Lighting quality is a complicated subject and is an integration of task
performance, visual comfort, social communication, mood, health,
safety and well-being and aesthetic judgement. It is also related to
economics and the environment in respect of the installation,
maintenance and operation of the lighting system.
The uniformity of illuminance distribution on the task area and its
surrounding area have a great impact on how quickly, safely and
comfortably a person perceives and carries out a visual task. A task
area is not usually the entire area of a workstation. On an office desk,
for example, the task area may only be about the size of a desk
blotter, but in interiors such as drawing offices the visual task may
cover the whole area of a drawing board. Where task areas may be
located anywhere over an area of a room, the recommendation
applies to all potential task areas within that area.
Glare is another important factor which affects lighting quality. It
describes the sensation produced by bright areas in the field of view,
and may be experienced either as discomfort glare or as disability
glare. In any proposed lighting installation, the likelihood of discomfort
glare being experienced can be estimated by calculating the unified
glare rating (UGR).
It is also important for visual performance and the feeling of comfort
when objects and human skin are rendered naturally and correctly. To
provide an objective indication of the colour rendering properties of a
light source, the general colour-rendering index, Ra, has been
introduced. The maximum value of Ra is 100, which stands for the
quality of natural light, and this figure decreases with decreasing
colour-rendering quality.
2
Commission Internationale de l'Eclairage (CIE). Maintenance of indoor electric lighting systems. CIE Technical
Report - Publication No. 97. Vienna. The Chartered Institution of Building Services Engineers. Code for interior
lighting. London.
3
The Chartered Institution of Building Services Engineers. Technical Memoranda TM10. Calculation of glare indices.
London, CIBSE, 1985.