Building Design and Construction Handbook - Merritt - Ventech!

15.52 SECTION FIFTEEN
space, whereas a large, bright luminaire in the field of vision would cause discomfort.
In contrast, glare decreases as the distance of the light source from the line of
sight increases. Also, glare decreases with increase in general luminance of the
visual environment, or level of eye adaptation.
The Illuminating Engineering Society has established standard conditions for
determining a criterion, called visual comfort probability (VCP), for rating discomfort
glare. VCP indicates the percentage of observers with normal vision who
will be visually comfortable in a specific environment. Tables of VCP values for
various luminaires are available from their manufacturers. VCP values should be
applied with caution, because they may not be applicable under conditions that
depart significantly from the IES standard.
In general, direct glare should not be troublesome if all of the following conditions
are satisfied for an overhead electric lighting system:
1. VCP is about 70 or more.
2. The ratio of maximum luminance of each luminaire to its average luminance is
5:1 (preferably 3:1) or less, at 45, 55, 65, 75, and 85� with respect to the vertical,
when viewed lengthwise and crosswise.
3. The maximum luminance of each luminaire, when viewed lengthwise and crosswise,
does not exceed the values given in Table 15.4 for the specified angles.
Reflected Glare. Also called veiling reflection because of the effect on visibility,
reflected glare results when incident light from a bright light source is reflected by
the task into the eyes of the observer and causes discomfort or loss of contrast.
Occurrence of glare depends on the brightness of the light source, overall luminance
of the task, reflectance of the task surface, and relative positions of light source,
task, and observer. When a bright light source is reflected into an observer’s eyes,
it casts an apparent veil over the image of the task. The result is a loss of contrast
that would otherwise be useful in perception of task size and silhouette details; for
example, print that would be readily legible without reflected glare would become
difficult to read in the presence of a veiling reflection.
Several techniques have been found useful in maintaining an adequate quantity
of light while limiting loss of contrast. These include the following:
Observers, tasks, and light sources should be positioned to reduce reflected glare.
If there is only a single light source, positional change should remove it from the
field of vision. When daylighting is used, occupants should be faced parallel to or
away from windows, rather than toward them. When overhead luminaires are used,
they should be positioned on either side of and behind the occupants, instead of in
the general area above and forward of them. When continuous rows of linear luminaires
are used, the occupants should be positioned between the rows with the
line of sight parallel to the longitudinal axes of the luminaires.
TABLE 15.4 Recommended Maximum
Luminances of Light Sources
Angle with vertical, degrees Luminance, fL
45 2250
55 1605
65 1125
75 750
85 495