The purpose of Daylight in design is to provide visual variety to the interiors by using it as a source of brightness contrasts.
The principal characteristics of Daylight is that it
changes with the time of day and year; even surrounding objects, pollution and
position of the house will affect how daylight can be utilized as a part of
design.
Daylight has two components: Sunlight and Skylight.
Sunlight is the direct beam of light from the Sun. Skylight refers to the
diffuse reflection of the light in the atmosphere (the light of the sky during
the day). Direct Sunlight in most cases is impractical as an element of design,
due to its glare. Just as uncovered light sources hit us with glare and need to
be shielded; similarly the intensity of sunlight needs to be carefully
controlled. Direct sunlight if used, will only light up some areas of the house
and not all, creating a high-contrast environment. This will result in
discomfort and visual fatigue.
Skylight on the other hand can be used without any
shielding mechanism. It can be used easily to light up non-critical areas for
seating, corridors, cafes, etc. Though using skylight to light up horizontal
spaces uniformly or to display art requires careful considerations, such as altering
building configurations accordingly.
People require change and stimuli to remain sensitive and
alert. Like gazing at distant objects relaxes our eye muscles; the changing
light of the day satisfies our psychological need for change; the changing
nature of natural light as clouds passes by breaks the monotony. Thus, using
daylight properly as a component of design satisfies our need for change and
stimuli, thereby removing visual fatigue and dullness. The goal of daylight
design should be to provide visual variety through the manipulation of
brightness contrasts. At the same time too much visual variety can cause
overstimulation and may also impair our visual performance.
Daylight and view do not necessarily go hand in hand, and
often are achieved through different types of opening. The criteria for
producing great exterior view and great interior daylight are different. The
more complex the view and the more often it changes the greater is our
satisfaction. Though large windows are desirable for great views, smaller
openings also serve this purpose very well.
Daylight Design
While using Daylight for design two factors need to be
taken into consideration: The size of the window (opening) and the height of
the window above the (task/work) area. The first is pretty simple - bigger the
size of the window, greater will be the amount of daylight. However, it is the
second feature which is more important – the placement of the opening. The
higher the window opening is placed deeper is the penetration of daylight into
the room. If placed very high it automatically prevents glare, which we
experience through lower windows. This is so because higher openings allow the
daylight to diffuse/spread out all over the room, which in turn also ensures
uniform inter-reflection among the objects present in the room.
Windows and opening which are set flush (at eye-level)
along walls, produce excessive contrasts. As the immediate space near the
window gets brightly lit and adjacent spaces remain comparatively dark. This
kind of contrasts is often harsh and uncomfortable. A softer
contrast/transition can be achieved with the use of splayed and rounded jambs
(vertical sides/arcs etc of window frames) and window wells. The jambs work by
reflecting light over a larger interior area; this can be also achieved by
painting the sides of the windows white (remember white as a colour reflects
out all light rays). Another method is to use drapes, curtains and blinds.
For comfortable viewing the luminance ratio between
fenestration (architectural term for arrangement of windows in a building) and
adjacent surfaces needs to be below 20:1. A similar ratio also applies for
electric luminance sources: The ratio between the surface luminance of electric
light sources and adjacent spaces ought not to be more than 40:1.
Fenestration Sections
The most common window arrangement (fenestration) is to
place windows on a single side of the room.
For such rooms to be completely and
uniformly lit, the depth of the room should not exceed twice the height of the
window head (upper part of the window) from the ground. (Of course full room
width windows will always work.) For example if the window head is at 10 feet,
the depth of the room should be 20feet. Properly placed narrow windows will
provide slightly less luminance than broader windows, but the difference is minor.
This 1:2 ratio between window head height and room depth can be extended to
2:5, by proper use of high reflectance surfaces in the room. If this ratio is
exceeded people seated in the deepest part of the room will experience
insufficient light, even with adequate electric lighting.
When windows are placed on opposite sides of the room, it
doubles the depth luminance of the room. The opposite windows only needs to
occupy the upper part of the wall to accomplish this, providing the same amount
of diffuse light as full height windows. This arrangement also reduces glare.
Skylights provide light to great depths especially in
storied buildings, with light reaching unto the ground floor. And is also a
means to provide light to top floors in high rise building. Using Light wells
and reflective surfaces on skylights will increase the depth and reach of
indoor light.
Skylights come in a variety of shapes, sizes and materials.
They are generally made of clear, patterned or translucent glass and a variety
of plastics. However, clear, grey-tinted or milk-white acrylics make the best
skylights, as they act very much like glass and are easier to maintain.
Regarding the shape, flat skylights are harder to maintain
as dirt accumulates on them easily; a problem that the exterior of dome-shaped
and slanted skylights do not face by virtue of their shape (however their
interior needs to be cleaned from time to time.) The angle of the skylight is
also important and needs to be placed in such a way that it avoids lighting up
the interiors with direct glares. Glare can be prevented by using Skylights
with deep wells, splayed wells (sloping sides) and louvers (blinds with
horizontal slants).
 |
| Clerestories |
Milk-white
(diffuse) plastic and glass skylight tend to block the exterior view -
obscuring the outside view means blocks the element of change and stimuli. On
the other hand, clear glass skylights over-heat the interiors and allow direct
sunlight and glares. An exterior shield, which allows daylight to pass through
but not direct sunlight, solves both these problems. In colder climates double
glazing is recommended. Double glazing involves merging of two thick glasses or
plastic with air in-between. This tends to protect the interiors from heat
loss.
 |
| Parallel Clerestories |
Clerestories (a raised structure on the roof or a series of
upper windows) can also act as skylights. Since they are vertical in nature and
not horizontal they allow daylight to pass through, while blocking direct
sunlight. When a light shell is added to
Clerestories, they act as reflecting surfaces, spreading uniform light in the
interiors. A series of parallel Clerestories on low roof greatly enhances the
daylight and are generally used in factories. When a clerestory is built
opposite to a window it doubles the light penetration depth in the room.
Clerestories can also be built on opposite walls and are to be found in many old
churches.
Tubular skylights are used in rooms where traditional
skylights are hard to build due to the design of the building. They are made up
of three parts: 1. A dome shaped structure on the roof which allows light to
pass. 2. A cylindrical aluminum shaft joining the dome to the room. The inner wall
of this structure is made of reflective surfaces to reflect light right down to
the lower reaches of the building. 3. And a translucent diffuser lens located on
the interior ceiling to evenly disperse the light in the room.
Heat Gain
Glazing and Double Glazing the skylights help in winters,
but pose a problem during summers with excessive heat. To avoid heat loss or
heat gain the building needs to be oriented (built) in such a way that it takes
into consideration not only the daily movement of the sun but also the change
in season. A good example is a house built to take in the rays of the sun in
the winter and avoid them during the summer. If the principal orientation of a
house is due south (exactly south of a location) or 30 degrees of due south,
the south facing walls may be built to take in the winter sun, while the roof
is designed to avoid the direct light of the sun. One should remember that North
light is softer, cooler and more uniform, than South light, which is harsher
and more variable. Skylights can be built in the south to tackle this natural
phenomenon.
Shading Devices
There are two types of shading device – interior and
exterior. The interior devices block and reflect back 60% to 70% of direct
sunlight. Exterior devices are more efficient and are capable of reflecting
back (blocking) 90% to 95% (if not more) of the sun’s heat and light. The
latter are more expensive to build and maintain, but they payoff in the long
run as they can be used in lieu of air conditioners and high electricity bills.
Moveable Controls
Draperies, shades and screens are available in a number of
materials and weaves, to pick and choose from, depending on the amount of light
we want (from soft transmission of light to complete blackout.) Generally draperies
consist of a pair, one to allow the required degree of transmission and the
other to block out light completely, when needed.
Venetian Blinds can be used to block light and also to
reflect light up to the ceiling (by keeping the slits in a slanted position)
and thereby disperse light throughout the room. This method also allows
exterior viewing. However both these methods need human intervention and
maintenance, where cleaning Venetian blinds can be a very time consuming
process.
Motorized controls that do not require human intervention
are another option. Though very efficient this method is expensive. Besides
controlling the amount of light according to the time of day they are also
capable of taking into consideration the amount of illumination received
through internal light sources and adjust the reach and degree of daylight
accordingly.
Exterior motorized shades not only controls heat and light
but also block out harmful UV Rays. These types of shades are expensive and
difficult to maintain.
Stationary Controls
The most common exterior stationary controls are simple shades
built to block glares and direct sunlight. Fixed exterior controls like
Overhangs and Awnings (exterior covering over the windows) provide shade from
direct light and blocks glare from penetrating through the upper parts of the
windows. But these lose out due to low aesthetic appeal. An overhang in the
south of the house is very useful, as during summers it blocks the direct
sunlight from entering through the upper parts of the window but allows in
enough skylight; and during winters the low angle of the sun ensures that warm
rays do not get blocked by the overhang.
Exterior horizontal louvers tend to allow direct sunlight,
which vertical louvers (both exterior and interior) avoid. Such vertical
louvers are generally located at the east or west side of the house, as they
allow light to pass in when the sun is at a low angle (morning and evening).
The primary difficulty with exterior overhangs and awnings
is that they are built to follow the solar seasons rather than the seasonal
climate. The middle of summer occurs in 21 June, but the hot days last into
July -August. Thus by following the yearly movement of the sun (in terms of
angles and positions) the house forsakes the climate immediately surrounding it
and thereby suffers.
The best counter-measure is to use plants, which follow the
climatic seasons: Vegetation that sheds their leaves and allows sunlight to
come in during the winters and blocks the harsh rays of the summer sun with new
foliage. Vines and deciduous trees work
best for such purposes.
Glazing Materials
There are a variety of Glazing materials to pick and choose
from in the market, on the basis of their light and heat transmittance (the
degree of light it allows to pass), colour and prismatic control. The best and
the most common glazing material is glass, but where breakage is a concern
acrylic or polycarbonate materials come in handy. Both glass and plastic can be
variously tinted (lightly) to control the amount of light and heat transmission
and at the same time allow an outside view.
Painted glass is to be avoided unless it is a stained piece
of art fulfilling an aesthetic need. Other translucent materials that transmit
light but block view are patterned, sandblasted, etched and opal glass and
plastic. Many of these materials also tend to get very glary, and hamper the
psychological need for change by blocking our view. As diffusion increases in
these materials so does the risk of glare.
Prismatic glass can be used to control the direction of
light entering the building. When positioned in a vertical angle they reflect
light onto the ceiling which further disperses light uniformly in the room.
Horizontal placement will result in glare-free, low luminance.
Quantity
Calculations overtime allows us to build systems based on
average daylight readings. But due to the many moods of nature, change in
seasons and climate, a precise amount of controlled interior daylight is hard
to ensure. Interior daylight illuminance is often expressed as a Daylight
Factor. This factor accounts for daylight received from the sky and external
surfaces, and inter-reflections in the room.
There are many methods, graphs and programmes to calculate
such details, some of which even take into account geographical features,
fenestration, time of the year, interior surfaces and room shape. The simplest
and the most accurate method is to build a scaled down model of the building. Light
will interact with this model exactly the same way as it will with the real scaled-up
building (when studied under similar sky conditions.) This need has given rise
to the invention of sky stimulators!
Energy Control
Photosensors (light sensing devices) have also been invented which switch off and on electric sources of light. This hi-tech devices switch on lights when external daylight decreases and vice-versa. Though efficient, sudden switching on and off of interior lights is often jarring. Hence systems with dimming controls are recommended which gradually dims electric lights off as daylight increases and increases the intensity of the electric lights with a fall in external luminance. More sophisticated systems are designed with built-in delay, to account for sudden changes in external light – i.e. a cloud swiftly passing over the sun, won’t trigger a response from such systems.
No comments:
Post a Comment