This section discusses the various types of lamps that are available, how they work and their pros and cons, particularly with respect to their efficacy and, therefore, their contribution towards sustainability.
1. Categories of lamps
The diagram below summarises the main categories of lamps.
The diagram also highlights the types of lamps that are referred to by the lighting industry as high intensity discharge (HID) lamps. These lamps share the characteristic that light is generated from the excitation of atoms of certain metals in an electrical discharge between two electrodes, through inert gases.
HEADS UP: Lamp Choice and Energy Efficiency
Choosing the lamp type has an enormous impact on energy efficiency, although it is only one link in the design chain. It is possible to design a very inefficient lighting system, using very efficient lamps.
As can been seen in figure below, gas discharge lamps are more efficient than incandescent lamps. Incandescent lamps are discouraged for general purpose illumination, except for special effects such as highlighting.
Low voltage halogen reflector lamps (dichroic lamps) have become very popular in recent years and are marginally more efficient than tungsten incandescent lamps. However, from a design perspective, their highly directional light output makes them a poor choice for general purpose illumination, meaning that large quantities are required to light open spaces.
LEDs are an emerging technology and are often claimed to be very efficient. However recent experience shows that LEDs have a range of efficiencies, thus great care should be taken in selecting LEDs for any lighting purpose.
The name lamp is the generic term for a device that creates light either by thermal emission or by discharge radiation. Light can be produced from electricity in many ways, of which the following are the most important in lighting engineering.
Incandescence or thermo-luminescence is the production of light from heat. Light from a filament lamp is produced in this way; electricity is used to raise the temperature of the filament until it is incandescent.
Electrical Discharge is the production of light from the passage of electricity through a gas or vapour. In lamps using this principle the atoms of the gas are agitated or excited by the passage of the electric current and this atomic excitation produces visible radiation, ultra-violet and infra-red energy.
Phosphorescence & Fluorescence are the processes of converting the invisible ultra-violet energy emitted normally from an electrical discharge, into visible light. Material called phosphors cause ultra-violet energy to make the transition into visible light.
The efficiency of a lamp (also known as efficacy) is measured in lumens per watt. The chart below shows the typical efficacy of the standard lamps including standard control gear losses. This allow the relative efficiency comparison of lamps to be made. As an example, a 100 watt incandescent lamp produces approximately the same amount of lumens as a 20 watt fluorescent lamp. Similarly, a 250 watt metal halide lamp produces approximately the same amount of lumens as a 400 watt mercury vapour lamp.
In general, fluorescent lighting provides the most efficient lighting system mounted up to the height of 4 - 5 metres. Above this height the use of high bay luminaires incorporating high intensity discharge lamps are ideal for providing general background lighting.
Combined with localised fluorescent task lighting for finer detail tasks, the overall lighting system can be made far more efficient. In general, the task lights used need only be single tube fluorescent luminaires. Care should be taken not to “over-light” areas for specific tasks.