No_2_August_1964 — Page 121

Estimating Daylight in Buildings-1

IN view of impending revisions of B.S. Code of Practice on Daylighting the opportunity has been taken to revise and bring up to date the previous digest. The subject is dealt with in two parts. The first describes methods for estimating daylight received directly from the sky and by reflection from surfaces outdoors. In the next article will be described methods for estimating daylight received at a given reference point after reflection from surfaces indoors. Worked examples illustrate the application of methods most likely to prove useful in the majority of cases.

As was explained in Digest 70 (first series) the amount of daylight received in buildings is most con- veniently expressed in terms of the percentage ratio of indoor to outdoor illumination, the ratio being called the 'Daylight Factor. For any given situation, the value of this factor de- pends on the sky conditions, the size, shape and position of the windows and the reflectivity of the external and internal surfaces. The daylight factor can be predicted at the design stage, either by actually measuring the daylight received in a model of the building, or, more frequently, by calculation, using drawings and other necessary data. Some of the more commonly used aids to calculation which simplify what would otherwise. be a tedious operation are described here.

SOURCE OF DAYLIGHT

The quality and intensity of day. light varies with latitude, season, time of day and local weather conditions. In contrast with the tropics. direct sunlight in temperate zones cannot be relied on for lighting the interiors of buildings and therefore reliance has to be placed on the light received from the sky. In so doing, this Digest is concerned solely with daylighting this country and in those others where the sky conditions are strictly comparable.

a

The seasonal and daily variations in daylight can be seen from Figs. la and b, which show the illumina- tion received on a horizontal surface out of doors from the sky as whole, averaged for each month. These values are based on measure. ments of total sky illumination made by the National Physical Labora- tory for a wide range of sky con- ditions but excluding direct sunlight.

varies

The amount of light available is least when the sky is heavily over- cast, and it is this type of sky which is accepted in this country as the basis for daylighting calculations and the framing of minimum stand- ards. The overcast sky is amenable to mathematical analysis because its luminance (i.e. brightness) regularly from the horizon to the zenith and does not vary appreciably in azimuth. Figure 2 shows that the range of luminance from horizon to zenith of a densely overcast sky is about 1:3. Because of the uni formity of luminance horizontally no account need be taken of the ori entation of a particular window, and simple methods can be used to allow for variations of sky luminance, ac- cording to the altitude of the patch of sky visible from the room.

The illumination provided by the sky varies throughout the day, but for a heavily overcast sky it is most likely to be in the region of 500 lumens/ft2. This value is generally assumed as the basis for daylight- ing calculations in this country, but it is in fact exceeded for about 85% of the normal working time through- out the year.

THE DAYLIGHT FACTOR

The amount of light received in- side a building is usually only a small fraction of that received out of doors from the whole sky, and it varies with the outdoor illumina- tion. It is therefore impracticable to express interior daylighting in terms of the illumination actually attain- for able indoors at any one time, within a few minutes. perhaps, that figure will change with changes in the luminance of the sky. For prac tical purposes, therefore. use is made

THE HONG KONG & FAR EAST BUILDER-VOLUME 19, NUMBER 2

of the Daylight Factor, which can be expressed as:

Instantaneous illumination

indoors Simultaneously occuring

illumination outdoors

a ratio

× 100%

(direct sunlight is excluded from both indoor and outdoor illumina- tions).

Figure 3 shows that daylight reaching રી reference point in a room can be made up of three com. ponents:

(1) The sky component the light

received directly from the sky; (2) The externally reflected com. ponent light received after reflection from the ground, buildings or other external sur- faces;

(3) The internally reflected com- ponent the light received after being reflected from sur- faces inside the room.

In calculating the daylight factor the three components of daylight are calculated separately and simply added together. Corrections for glazing materials other than clear glass, dirt on glass, and reductions caused by the window framing can be applied to the total daylight fac- tor as described in the next Digest.

Much of the work in calculating (1) and (2) can be avoided by the use of the simple aids described be- low. Calculation of (3) is usually rather more

be complex but can made easier by the methods describ ed in the next Digest.

SKY COMPONENT

There are three aids in common use in this country for calculating the sky component:

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