Estimating Daylight in Buildings-2
THIS article, which is a companion to the one the last issue, describes methods for
calculating the third component of the day lightfactor: that reaching a reference point indoors after reflection and inter-reflection from internal surfaces. It deals also with the corrections that may need to be made to the daylight factor to allow for alternative types of glass, and dirt on the glazing, and includes worked examples.
THE INTERNALLY
REFLECTED COMPONENT
Having calculated the amount of light reaching a reference point, first from the sky directly, and secondly, after reflection from surfaces outside the window by one of the methods described in the previous Digest, the next stage is to calculate the light reaching the reference point after re- flection and inter-reflection from sur- faces inside the room. This depends on the reflection factors of the walls. ceiling and floor, and on the amount of light received on them from the sky and the obstructions and ground outside. This process of reflection and inter-reflection is complex. but methods of calculation have been de- vised which are sufficiently accurate and yet simple enough for general application. The Building Research Station has developed the following three methods:
1. The inter-reflection formula 2. Tables based on the formula 3. Nomograms based on a modified
form of the formula.
TABLE 1
Effect of external obstruction on internally reflected component
Angle of obstruc-
tion measured
from centre of window (degrees above horizontal)
no obstruction
Coefficient C
(see equation 1)
39
10°
35
20°
31
30°
25
40°
20
50°
14
60°
10
70°
80°
7 5
In practice, the amount of inter- reflected light varies according to the
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distance of the reference point from the window. For most purposes, however, it is sufficient to assume an average internally reflected com- ponent over the greater part of the room with a lower minimum value at points far from the window. To comply with statutory regulations the minimum internally reflected com- ponent should be determined. In al! the methods, with the exception of Nomogram III. values for both average and minimum internally re- flected components may be calculated.
The B.R.S. inter-reflection formula
The formula is applicable where relatively high accuraiy is required in estimating the internally reflected component of daylight factor for side-lit rooms. It is given in the form:
Average internally reflected com ponent
0.85W X (CR +5R )%...(1) A(1 R) fw CW
where
W=Area of window
A =Total area of ceiling, floor and walls including area of window R=Average reflectance of ceiling, floor and all walls, including window, expressed as a fraction RAverage reflectance of the floor fw and those parts of the walls be- low the plane of the mid-height of the window (excluding the window wall)
R=Average reflectance of the ceil
cw ing and those parts of the walls
above the plane of the mid- height of the window (exclud. ing the window wall). CA coefficient having values de- pendent on the obstruction out- side the window.
Table 1 gives values of C for various obstructions. The values of C in the table depend on the follow- ing assumptions:
(a) The sky luminance distribution is that of the C.I.E. Overcast Sky.
(b) The ground and any obstruc. tions have a luminance one-tenth of the average sky luminance. (c) The obstructions are continuous and are horizontal and parallel to the window wall. (If the obstructions do not fulfil these conditions. an estimate should be made of the equivalent obstruction).
(d) The glass losses are those ap- propriate to clean clear window glass.
Where it is required to find the minimum internally reflected com- ponent of the daylight factor, the conversion factors in Table 2 can be applied to the average internally re- flected component (R is expressed here as a percentage instead of a fraction).
TABLE 2
Conversion of average to minimum internally reflected component
Average
Conversion
reflectance
factor
R(%)
30
0.54
40
0.67
50
0.78
60
0.85
The B.R.S. Tables
Table 3 was formulated by the Building Research Station to allow rapid assessment of the minimum internally reflected component of the daylight factor where certain limita- tions, about the size of room and scheme of decoration can be accepted. The table was designed primarily for rooms 400 sq. ft in floor area and 10 ft ceiling height, with a win- dow on one side extending from a
THE HONG KONG & FAR EAST BUILDER-VOLUME 19, NUMBER 4
Page 110Page 111
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