as
(a) The B.R.S. daylight tables (b) The grid method known
the Waldram Diagram' (c) The B.R.S. Daylight Factor
Protractors
B.R.S. Simplified Daylight Tables
These tables, published in full in National Building Studies, Special Report 26, have been prepared to enable all the components of the daylight factor to be determined when scale drawings are not readily
general, the sky component at any other reference point can be obtained by addition or subtraction. Comprehensive details for the use of the Table are given in the Special Re- port referred to.
Waldram Diagram
Grid methods of calculating sky components can be applied with accuracy to a wide variety of circum- stances but are inclined to be tedious. They are particularly useful when obstructions and the window are
The area of the patch of sky plotted on the diagram is then proportional to the sky component at the reference point.
The diagram can be of any con- venient height and length but in practice it is most convenient to make it some simple multiple of 50 units in area. As the complete diagram represents half the hemisphere of un- obstructed skv. one unit represents 1% sky component. The outline of window and external obstruction is distorted in shape when transferred to the diagram but the area enclosed
J&
13
#
ני
+3
+
ני
H
Fig. 1. (a) and (b). Illumination, in lumens per square foot, received from the sky, on a horizontal surface out of doors.
available, and are thus most ap- propriate for use in the early stages of a design. Table 1, reproduced here, gives sky components from an overcast sky for rooms with vertical rectangular windows glazed with clear clean glass. Allowance can be made for simple external obstruc- tions, the use of other types of glass. the presence of dust on the glass etc.. as will be explained in the Digest.
next
The following information (Fig. 4) is needed to use the Table.
116
(i) The effective height H of the window head above the work- ing plane after allowing for any obstructions
The effective widths W1 and #2 of the window on each side of a line drawn from the reference point normal to the plane of the window. taken separately
(iii) The distance D from the re- ference point to the plane of the window. The ratios HD.
D and 2D can be worked out and the sky com. ponents can then be read directly from the table. In
plotted against time.
complex in outline. The most widely used grids are based on the Waldram diagram, which, in its latest form (Fig. 5), is based on the luminance distribution of an overcast sky and allows for glass losses.
The diagram comprises a grid representing 50% sky component (i.e. half the hemisphere of sky) and is so constructed that equal areas represent equal sky components. The area of sky visible through the win- dow from the reference point is plot- ted on the grid from information obtained from a scaled plan and section of the room being examined.
1.09-
1-17+ 60*
Y LUMINANCE FATIOS OR FACI
0.72
0 56
0.98.
40+
10
0-50 5.
22*
relative to the area of the whole grid gives an accurate direct measure of the sky component. The superim posed curved lines on the grid known as droop lines correspond to the horizontal edges of obstructions parallel to and at right angles to the plane of the window.
B.R.S. Daylight Factor Protractors
Of the methods for determining the sky components at the design stage, the Daylight Factor Protractors de- signed by the Building Research Station are the most widely used by
1-27+80°
1-24 70*
ZENITH
,59
42°
Fig. 2. Luminance distribution of densely overcast sky com. pared with the average luminance taken as unity.
THE HONG KONG & FAR EAST BUILDER-VOLUME 19. NUMBER 2