What is the role of day lighting in building science
If the building envelope is thought of as a barrier between the internal controlled environment and the external, undesirable conditions.
It must be realized that it should be a selective barrier, or rather a filter, which excludes the unwanted influences whilst admits those which are desirable. One such desirable effect is daylight.
Perhaps the most important communication channel of a man with his environment is vision. The eye is stimulated by light reflected from objects, thus light is a prerequisite of seeing.
Light can be produced artificially (e.g. electric light), but if it is available free of charge – it should be utilized.
In artificial lighting, the light source itself is under the designer’s (user’s) control. In day lighting the source (sun and sky) is given, thus if control is necessary, it must be in transmission and distribution.
Artificial lighting is practically independent of location, climate or even of the building fabric; daylighting, however, strongly depends on the externally given conditions and its control is only possible by the building itself.
The ultimate source of daylight is the sun, from which we receive a large amount of thermal radiation together with light.
In bright sunshine the illumination is around 100 klux (100000 lux), the intensity of thermal radiation is likely to be about 1 kW/m². In climates where the heat balance is negative, i.e. overheating is not likely, thermal considerations will rarely restrict the amount to be admitted in the tropics.
The admission of an abundant quantity of daylight will be accompanied by radiant heat which is probably excessive. Thus the filter function of the envelope will be even more important.
We must attempt to exclude radiant heat while admitting daylight. Where this cannot be done, thermal considerations will restrict the amount of light which can be admitted.
This means that there must be a fundamental difference in approach to daylighting design,
Discuss about the nature of light in detail?
What we perceive as light, is a narrow wavelength band of electromagnetic radiation from about 380 to 780 nm (1 nanometer = 10 9m).
This energy radiation shows dual characteristics: it consists of energy particle photons – but also shows transverse wave motion properties. The wavelength determines its color.
7 DAYLIGHTING TECHNIQUES FOR BUILDING
Light containing all visible waves is perceived as white. The human eye’s sensitivity varies with the wavelength; it is greatest around 550 nm (yellow) as shown by Figure 4.1.
What are the various photometric quantities of light?
The intensity of a light source is measured in units of candela (cd). This is the basic assumed and agreed unit in the international system (defined as the intensity of a 1/60 cm² uniformly emitting black body radiator at the melting point temperature of platinum) all other units are derived from this.
The flux (or flow) of light (symbol: F) is measured in lumens (Im). One lumen is the flow of light emitted by a unit intensity (1 cd) point source, within a unit solid angle. As the surface of a sphere subtends at its center 47 (= 12.56) unit of solid angle, a 1 cd point source will emit a total of 12.56 Im in all directions.
Illumination (symbol, E) is measured as the amount of flux falling on unit area, i.e. im/m² which is the lux, the unit of illumination in the international system.
Luminance (symbol: L) is the measure of the brightness of a surface. Units for its measurement can be derived in two ways:
1. If a light source of 1 cd intensity has a surface area of 1 m² (1 cd is distributed over 1 m²) its luminance is 1 cd/m². this is the official Sl unit
2. If a completely reflecting and diffusing surface (r =1.00) has an illumination of 1 lux, its luminance is 1 asb(apostilb)
3. The two units measure the same quantity and are directly convertible- 1 cd/m² =3.14asb.
What is the purpose of lighting? How will you measure visual efficiency to support lighting?
The purpose of lighting is twofold:
a. practical to facilitate the performance of a visual task and ensure visual comfort
b. artistic – to create certain emotional effects.
For practical purposes, we need to measure visual efficiency, as this strongly depends on the lighting. It can be measured on its three facets:
1. visual acuity, or sharpness of vision, measured as the reciprocal of the visual angle p (expressed in minutes) subtended at the eye by the least perceptible detail. For example, if the least perceptible detail subtends an angle of 2′ the acuity will be
1/p = 1/2 = 0.5
2. contrast sensitivity (cs), measured as the ratio of the least perceptible luminance difference (L2-L1) to the lower of the two luminance;
3. visual performance, i.e. the time required for seeing, expressed as the number of characters perceived per second or on any comparative scale
CS= l2-l1/l1 x 100 %
What is the source of day light? How light can reach in a building?
The ultimate source of daylight is, the sun, but the light arriving at the earth from the sun may be partly diffused by the atmosphere and the locally prevailing atmospheric conditions will determine how this light will reach a building.
If we consider a point inside a building, light may reach it from the sun by the following ways (Figure 4.2)
a. diffused or skylight through a window or opening
b. externally reflected light (by the ground or other buildings), through the same windows
C. internally reflected light from walls, ceiling or other internal surfaces
d. direct sunlight, along a straight path from the sun, through a window to the given point
What is the role of climate in light? Discuss in detail
In high latitude moderate climates, where the sky is typically overcast, the whole of the sky hemisphere acts as a light source.
Direct sunlight may occur, but cannot be relied on. The sky itself has a luminance sufficiently high to provide lighting in normal rooms.
On the basis of many observations, the commission International de l’ Eclair age (CIE) has established the luminance distribution of a typical overcast sky, as varying according to the function:
L = L x (1+2xsiny)
Where Ly = luminance at y altitude angle
Lh= luminance at the horizon
Thus the zenith luminance (L₂) =3x L, .
Hot-dry desert climates are characterized by strong direct sunlight from cloudless skies. Direct sunlight is usually excluded from buildings for thermal reasons.
The sky is of a deep blue color and its luminance may be as low as 1700 cd/m² – not enough to ensure adequate daylighting.
This clear sky usually has the highest luminance near the horizon and the lowest luminance at right angles to the sun.
The bare, dry sunlit ground and light-colored walls of other buildings will reflect much light which will be the main source of indoor day lighting. It may, however, also be the source of glare, when these strongly lit light surfaces are
within the visual field.
Light dust suspended in the air may create a haze and increase the apparent sky brightness up to 10 000 cd/m², but the frequent heavy dust and sand-storms can reduce it to below 850cd/m².
In warm-humid climates, the sky is typically overcast, with a luminance often exceeding 7 000 cd/m².
The proportion of diffused or skylight is predominant and the very bright sky viewed from a moderately lit room can cause discomfort glare.
In composite climates, wide variations occur in natural lighting, between overcast and clear sky conditions.
What do you mean by daylight factor concept? What are the various factor which contribute to the day light factor?
Due to the variability of outdoor lighting levels, it is difficult (and perhaps meaning- less) to calculate interior lighting in photometric illumination terms.
However, in a given building, at a certain point, the ratio of the illumination to the simultaneous outdoor illumination can be taken as constant. This constant ratio is expressed as a percentage is the daylight factor (DF):
DF = Ei /Eo x 100 (%)
Where Ei= illumination indoors, at the point taken
Eo illumination out-doors from an unobstructed sky hemisphere
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