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.

Different lighting schemes

 Different lighting schemes may be classfied as:

(i) direct lighting, (ii) indirect lighting, (iii) semi-direct lighting, (iv) semi-indirect lighting, and (v) general diffusing systems.

(i) Direct Lighting
As the name indicates, in the form of lighting, the light from the source falls directly on the object or the surface to be illuminated. 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) 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.

(iii) Semi-direct System

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.

(iv) Semi-indirect Lighting

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.

(v) General Diffusing System
In this system, luminaries are employed which have almost equal light distribution downwards and upwards.