Light Concepts

Light is so common that it is taken for granted. It is a basic requirement for the visual perception. For the human eye light is a very narrow band of electromagnetic radiation, from 0.38 microns to 0.78 microns. In this narrow range of 0.4 microns the human eye perceives all the colours, ranging from violet around 0.4 microns (or 400 nanometers) to deep red above 0.7 microns (or 700 nanometers) with the colours such as blue, green, yellow, orange and red in between. Sensitivity of the human eye peaks at 0.555 microns and drops to zero below 0.38 microns (termed as the ultraviolet range) and above 0.78 microns (known as the infrared range).

The amount of light produced by a light source is measured in lumens and when one lumen of light uniformly lights up a square metre area, the illumination level is one lux. The quality of light is determined by the distribution of energy over the visible range, i.e. 0.38 to 0.78 microns. One of the important features of light is the way colours are perceived under it. The ability of any light source to bring out the natural colour of objects is rated on scale of 0 to 100, and is referred to as Colour Rendering Index (CRI). Light from incandescent lamps and sun light are able to bring out true colours of objects and their CRI is rated as 100. Many other light sources, particularly discharge lamps, mayor may not bring out true colours of objects.

When a black body is heated to increasingly higher temperature, the light emitted by it changes slow ly from cherry red to white and then to bluish white. Colour temperature of a light source represents the temperature of black body which gives the same overall colour impression as that of the light source. Colour temperature of commonly used light sources range from 2700″K for a GLS lamp to 6500″ K for the cool day light Fluorescent Lamp.

All electric lamps could be broadly classified into two categories : Incandescent and Discharge. In an incandescent lamp a tungsten filament is sealed in vacuum or inert atmosphere and heated to high temperature by passage of electric current and the hot filament emits light. In a discharge lamp, current is passed through a gas sealed in a tube and the gas atoms emit their characteristic radiation in the visible range and also outside the visible range. In a fluorescent lamp, the emitted radiation is predominantly ultra violet which is converted into visible light by a phosphor coating inside the tube.

Incandescent lamps can be grouped into vacuum lamps and gas filled lamps, and the latter is further classified into halogen and non-halogen lamps. Discharge lamps can be grouped into low pressure discharge lamps (such as fluorescent lamps – FTL) and high pressure discharge (HID) lamps (such as high pressure mercury, high pressure sodium, metal hillide etc). HID Lamps are commonly used for out door applications, such as street lighting, while FTL and incandescent (GLS – General Lighting Service) lamps are more commonly used for indoor lighting. Efficacy of an electric lamp is expressed in terms of the amount of light (lumens) it produces for each unit of power (watt) and is expressed as lumen/watt. In general, discharge lamps are significantly more energy efficient as compared to incandescent lamps.

All lamps are generally operated inside a fitting or a luminaire which directs the light in the desired fashion. The overall efficiency of a lighting system is dependent on the efficacy of the lamps and luminaries combination.

Comparison of Various Light Sources

Type of Lamp Luminous Efficacy (lm/W) Colour Rendition Properties Remarks
Incandescent 10-15 Im/W Excellent Due to poor luminous efficiency, it has restricted use
Halogen 17-33 Im/W Excellent Used in flood lighting installations and in projectors & motorcar headlamps.
Fluorescent 65 Im/W Good-depending on the fluorescent coating Popular for indoor lighting.
Compact Fluorescent 55-65 Im/W Very good Tremendous potential for energy savings.
High Pressure Mercury Vapour 58 lm/W Fair Streetlighting, highway lighting
Metal Halide 90 Im/W Excellent High luminous efficiency.
High Pressure Sodium Vapour 145 lm/W Fair Suitable when colour rendering is not important.
Low Sodium Vapour Pressure 200 Im/W Poor Energy saving but poor colour rendition
Induction Lamp 65-70 lm/W Excellent Due to extremely long life, ideal for use in installation where relamping and maintenance are difficult or expensive.