sill height of 3 ft to the ceiling; by means of simple conversion factors the internally reflected component can be calculated for rooms of 100 and 900 sq. ft floor area, with ceiling heights ranging from 8 to 12 ft.
Changes in the height of sill and of window head will not as a rule affect the estimate of the internally reflected component, neither will the shape of the room, so long as the ratio of length to width does not ex- ceed 2: 1.
In Table 3 the ceiling reflection factor is assumed to be 70%, but other values can be allowed for by means of conversion factors. A con- tinuous horizontal external obstruc- tion of 20° measured from the cen- tre of the window is assumed. Where there is more than one window in a room, the internally reflected compon- ents are calculated separately for each window and then added together, re- membering that the minimum in- ternally reflected component for each window occurs at points in the room farthest from the window. For very large rooms, and/or different
ternal obstructions, the inter-reflection formula or the nomograms should be used.
The values given in Table 3 are minimum values at points farthest from the window. Where the average value of the internally reflected com- ponent is required the following con- version factors are applied:
Wall reflection
factor %
20
40
60
80
Conversion
factor
1.8
1.4
1.3
1.2
For more detailed information on the use
of the tables see National Building Studies, Special Report 26 (list on p. 6).
The B.R.S. Nomograms
Three nomograms have been de- vised for determining the internally reflected component of the daylight factor:
TABLE 3
Nomogram I for the average in- ally reflected component in side- lit rooms
Nomogram II for the minimum internally reflected component in side-lit rooms
Nomogram III for the average internally reflected component in top-lit rooms.
Minimum internally reflected component of daylight factor (%)*
of Window
Radio
Window
area as
10%
percentage
of floor
area:
floor
area
area
2 5
Floor reflection factor
20%
40%
Wall reflection factor
20% 40% 60% 80% | 20% 40% 60% 80% 20% 40% 60% 80%
0.1 0.1 0.2 0.1 0.1 0.2 0.4 0.1 0.2 0.3 0.5
——
0.1 0.2
1:50
——
ex- 1:20
1:14
7
1:10
10
The information required to use Table 3 is:
1:6.7
15
1:5
20
(a) Ratio of window
area to floor
1:4
25
area
1: 3.3
30
1:2.9
35
1:2.5
40
1:2.2
45
1:2
50
0.1 0.2 0.3 0.5 0.1 0.2 0.4 0.6 0.1 0.2 0.4 0.7 0.2 0.3 0.6 0.9
0.2 0.4 0.6 1.0 0.2 0.5 0.8 1.3 0.4 0.7 1.1 1.7 0.2 0.5 0.8 1.4 0.3 0.6 1.1 1.7 0.5 0.9 1.5 2.3 0.3 0.6 1.0 1.7 0.4 0.8 1.3 2.0 0.6 1.1 1.8 2.8 0.3 0.7 1.2 2.0 0.5 0.9 1.5 2.4 0.8 1.3 2.1 3.3 0.4 0.8 1.4 2.3 0.5 1.0 1.8 2.8 0.9 1.5 2.4 3.8 0.5 0.9 1.6 2.6 0.6 1.2 2.0 3.1 1.0 1.7 2.7 4.2 0.5 1.0 1.8 2.9 0.7 1.3 2.2 3.4 1.2 1.9 3.0 4.6 0.6 1.1 1.9 3.1 0.8 1.4 2.3 3.7 1.3 2.1 3.2 4.9
0.1 0.2 0.2
0.1 0.2 0.4 0.6 0.2 0.3 0.6 0.8
0.3 0.5 0.8 1.2
(6) Average reflection factor of the floor (including furniture) (c) Average reflection factor of all the walls (excluding the win- dow)
For rooms of 100 sq. ft. and 900 sq. ft the following conversion factors can be applied to the results obtained by Table 3; conversion factors for intermediate sizes can be obtained by interpolation.
Floor area Wall reflection factor, %
20 40 60 80
100 sq. ft 900 sq. ft
0.6 0.7 0.8 0.9 1.4 1.2 1.0 0.9
For rooms with ceiling reflection factors other than the 70% assumed in Table 3 the following conversion factors can be applied:
Ceiling reflection
factor %
40
50
60
70
80
*Assuming ceiling reflection factor-70%, angle of external obstruction=20°
TABLE 4
Use of Nomogram III for horizontal rooflights
Variation of function K with angles of obstruction from centre of rooflight (degress above horizontal)
Obstruction on one side of rooflight
0°
Obstruction on second side of rooflight
0°
88
10°
87
87
20°
87
87
Conversion
30°
85
85
85
10° 20° 30° 40° 50° 60° 70° 80°
87 87 85 82 78 72 65 57 87 85 82 77 71 64 56
86 85 82 77 71 64 56
83 80 75 69 62 54
factor
40°
82
82
82
0.7
50°
78
77
77 75
72
0.8
60°
72
71
71 69
66
0.9
70°
65
64
1.0
64
62 59
1.1
80°
57
56
56
54
51
80 77 72 66 59 51
47
62 56 48 41
54 48 41 47 41 33
68 62 54
33
25
THE HONG KỌNG & FAR EAST BUILDER-VOLUME 19, NUMBER 4
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