In this article we will discuss about the types of interior lighting schemes and the factors that are required to be considered while designing the lighting scheme.
Types of Lighting Schemes:
i. Direct Lighting:
It is most commonly used type of lighting scheme. In this lighting scheme more than 90 percent of total light flux is made to fall directly on the working plane with the help of deep reflectors. Though it is most efficient but causes hard shadows and glare. It is mainly used for industrial and general out-door lighting.
ii. Semi-Direct Lighting:
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In this lighting scheme 60 to 90 percent of the total light flux is made to fall downwards directly with the help of semi-direct reflectors, remaining light is used to illuminate the ceiling and walls. Such a lighting system is best suited to rooms with high ceilings where a high level of uniformally distributed illumination is desirable. Glare in such units is avoided by employing diffusing globed which not only improve the brightness towards the eye but improve the efficiency of the systems with reference to working place.
iii. Semi-Indirect Lighting:
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In this lighting scheme 60 to 90 percent of total light flux is thrown upwards to the ceiling for diffuse reflection and the rest reaches the working plane directly except for some absorption by the bowl. This lighting scheme is with soft shadows and glare free. It is mainly used for indoor light decoration purposes.
iv. Indirect Lighting:
In this light scheme more than 90 percent of total light flux is thrown upwards to the ceiling for diffuse reflection by using inverted or bowl reflectors. In such a system the ceiling acts as the light source, and the glare is reduced to minimum. The resulting illumination is softer and more diffused, the shadows are less prominent and the appearance of the room is much improved over that which results from direct lighting. It is used for decoration purposes in cinemas theatres and hotels etc. and in workshops where large machines and other obstructions would cause trouble some shadows of direct lighting is employed.
v. General Lighting:
In this scheme lamps made of diffusing glass are used which give nearly equal illumination in all directions.
Outdoor Lighting Scheme:
(i) Street lighting
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(ii) Flood lighting
General Ideas about Street Lighting:
The main purpose of street lighting is to make the traffic and obstructions on the road clearly visible for safety point of view and convenience. The principle employed for street lighting is different from that of interior lighting. As the area is open i.e. there is no wall, the illumination level required is low. The question of colour rendering is also of minor importance.
In fact in case of interior lighting the objects are seen by light reflected by them but in case of street lighting the objects are seen in bright back ground.
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The reflectors used throw the light on the road at a very large angle of incidence. It is observed that a motorist needs to see the objects about 30 m away. Fig. 2.39 shows the principle of street lighting. The observer is able to see the object through reflected light as shown.
Generally an average number of 8 to 15 lumens per square meter is considered sufficient illumination on the street. Mercury vapour lamps and sodium discharge lamps has been found most economical due to lower power consumption for a given amount of light. Colour consideration does not matter much in street lighting.
Flood Lighting:
The meaning of flood lighting is flooding of large surfaces with light from powerful projectors.
Purpose of Flood Lighting:
Followings are the main purpose of employing flood lighting:
(i) Beautification:
For enhancing beauty of building at night such as public place, ancient building and monuments, religious building on important festive occasions etc.
(ii) Industrial and Commercial Flood-Lighting:
For illumination railway yards, sports stadiums, car parks, construction site, quarries etc.
For flood lighting it is necessary to concentrate the light from light source info a narrow beam. The type of reflector and its housing used for concentrating the light into narrow beam is known as flood light projector. The reflecting surface is made of silvered glass or stainless steel. Metal reflectors being more robust are usually preferred.
The casing and its mounting are arranged in such a manner that the beam can be in horizontal and a vertical direction on site. When higher wattage of projector lamp i.e. 500 W or 1000 W is used in projectors, then ventilation may be provided for cooling properly.
Projectors are classified according to the beam spread:
1. Narrow Beam Projectors:
In this type of projector beam is spread between 12 – 25°. These are used for distance more than 70 metres.
2. Medium Angle Projectors:
Projectors with beam spread between 25 – 40°. These are used for distance between 30 to 70 metres.
3. Wide Angle Projectors:
These are the projectors with beam spread between 40 – 90° and are used for distance below 30 metres.
Location and Mounting of Projectors:
One of the most important factor which affects the selection of projector is the location of the projector.
There are two possible location of projectors in practice locations of projectors in practice: Fig. 2.40,
(a) shows symmetric projector help 20 to 35 meters away from the surface to be flood lighted and providing approximately parallel beam spread of 25° to 30°. Fig. 2.40,
(b) shows the case when the projector cannot be located away from the building. In such a case, an unsymmetric reflector mounted in a basement area or on a bracket attached to the building is used which directs more intense light towards the top of the building.
Flood-Lighting Calculations:
The following points are taken into consideration while estimate the number and size of projectors:
1. Illumination Level Required:
The illumination level required depends upon the type of building, the purpose of flood-lighting.
2. Type of Projector:
The type of projector depends upon the area covered by the beam and illumination required. Beam angle of the projector is decided keeping in view the distance of projector from the surface.
3. Number of Projectors:
Number of projectors required for any desired intensity on particular surface is obtained from the following relation.
where,
N = Number of projectors
A = Area of surface in m2 to be illuminated
E = Illumination level required in lumens/m2
The other factors required for flood lighting calculations are:
(i) Depreciation factor
(ii) Utilization factor and
(iii) Waste light factor.
Monuments Lighting:
In monumental lighting aesthetics is of more important than level of illumination. Monumental lighting requires more of art than engineering. To have a clear perception of any object its boundaries and contours should have contrast with its general shape so that it shape can be judged.
Building surface having light marble, portland stone is having illumination level of 150 Lux, whereas light grey limestone is having illumination level of 250 Lux.
Design of Lighting Schemes:
i. Illumination Level:
This is the most vital factor because a sufficient illumination is the basic means whereby we are able to see our surroundings.
For each type of work there is a range of brightness most favourable to output i.e. which causes minimum fatigue and gives maximum output in terms of quality depends upon:
(i) The size of the objects to be seen and its distance from the observer. Greater the distance of the object from observer and smaller the size of the object, greater will be the illumination required for its proper perception and
(ii) Contrast between the object and back-ground-greater the contrast between the colour of the object and its background, greater will be the illumination required to distinguish the object properly. Objects which are seen for longer duration of time required more illumination than those for casual work. Similarly moving objects required more illumination than those for stationary objects.
ii. Uniformity of Illumination:
The human eye adjusts itself automatically to the brightness within the field of vision. If there is a lack of uniformity, pupil or iris of the eye has to adjust more frequently and thus fatigue is caused to the eye and productivity is reduced. It has been found that visual performance is best if the range of brightness within the field of vision is not greater than 3:1, which can be achieved by employing general lighting.
iii. Shadows:
In lighting installations, formation of long and hard shadows causes fatigue of eyes and therefore is considered to be a shortcoming. Complete absence of shadows altogether again does not necessarily mean an ideal condition of lighting instillations. Contrary, perhaps to popular opinion, a certain amount of shadow is desirable in artificial lighting as it helps to give shape to the solid objects and makes them easily recognised.
iv. Glare:
It may be direct or reflected i.e. it may come direct from the light source or it may be reflected brightness such as from a desk top, nickeled machine parts, or calendared paper.
Direct glare from a source of light is more common, and is more often a hindrance to vision. A glance at the sun proves that an extremely bright light source causes acute eye discomfort. Reflected glare is glare which comes to the eyes as glint or reflection of the light source in some polished surface.
v. Mounting Height:
In case of direct lighting it depends upon the type of building and type of lighting scheme employed. For rooms of large floor area, the luminaries should be mounted close to ceiling as possible. In case of indirect and semi-indirect lighting, it would be desirable to suspend luminaries enough down from ceiling to give uniform illumination.
vi. Spacing of Luminaries:
The distance of light source from the wall should be equal to one half the distances between two adjacent light sources. The distance between light fittings should not exceed 1.5 times the mounting height.