Building Design and Construction Handbook - Merritt - Ventech!

15.56 SECTION FIFTEEN
TABLE 15.5 Weighting Factors for Determining
Visual Conditions*
Task and worker characteristics Weight W
Workers’ ages
Under 40 �1
40 to 55 0
Over 55
Importance of speed or accuracy†
�1
Not important �1
Important (errors costly) 0
Critical (errors unsafe)
Reflectance of task background
�1
Greater than 70% �1
30 to 70% 0
Less than 30% �1
* Based on data in ‘‘Selection of Illuminance Values for
Interior Lighting Design,’’ RP-15A, Illuminating Engineering
Society of North America. Calculate �W by adding the
weighting factors W for the specific environment or required
performance and determine the corresponding visual
condition from Table 15.7.
† Use W � 0 for environment categories A to C.
TABLE 15.6 Visual Condition Number for Determining
Recommended Illuminances
Category
�W*
3 2 1 0 �1 �2 �3
A to C 4 4 3 2 2
D to I 4 4 3 3 3 2 2
* �W � sum of weighting factors given in Table 15.5 for a
specific environment or required performance.
building. Consequently, for task lighting, illumination should be kept to the minimum
necessary for maintenance of adequate quantity and quality of lighting.
For many years, single values for minimum illuminance, or level of illumination,
fc, developed by the Illuminating Engineering Society of North America (IES) and
listed in tables in the ‘‘IES Lighting Handbook,’’ have been widely used in the
United States. In 1979, however, the IES revised its criteria to a recommended
range of target illuminances. These recommended values take into account many
of the factors listed above. Following is an example and abbreviated tables to illustrate
the use of the tables in the IES handbook.
To determine the target illuminance, start by ascertaining the type of activity, or
illuminance category, for the space to be illuminated. (The ‘‘IES Lighting Handbook’’
contains a detailed table correlating specific areas or activities with categories
labeled A to I. For some of these categories, the effects of veiling reflections should
be evaluated, for which purpose equivalent-sphere-illumination, ESI, calculations
may be used, as indicated in Art. 15.10.5.) The first column of Table 15.7, which