Principles of Lighting Design

Posted by Super User

Light for our living and working environments is produced a number of ways today.

Filament lamps use “incandescence” to glow as an electric current heates the filament inside a gas filled vacuum. Only about 8% of the electrical energy is converted into light.

Quartz filament halogen lamps use halogen gases such as iodine and quartz filaments to produce whiter and brighter lighting than incandescent lamps but are not so energy efficient.

Fluorescent lights came from filling a tube with low pressure gas and passing a current through it making the gas give off ultraviolet light that would strike the phosphor coated tube lining making it fluoresce. ie the phosphor atoms absorb light of one wavelength and release the energy as light of another wavelength.

Discharge lamps use gases such as mercury and sodium under both high and low pressures to emit different spectra.

Light emitting diodes (LEDs) differ from other light sources in the way they generate light. The first white LEDs were created by coating blue LEDs with phosphors that gave off yellow light which combined with the blue to create white light. Technology advances are making these LEDs brighter and more economic to produce. LEDs can now have higher brightness , no infra-red or UV radiation, no heat effect, no flicker, no noise, no pollution, no radio interference, no silver and other contamination. There is no danger of hurting our eyes and skin. Also artware and delicate objects are protected from the effects of long time lighting exposure.

Power consumption is half comparing with current fluorescent energy saving lamps. Life span is over 50,000 hours so costs of maintenance and replacement are drastically reduced.

Lumen (lm) is the amount of light produced from a source. Also referred to as luminous flux.

Candela (cd) is the unit of measure for luminous intensity and is used to show how bright a lamp is at a given angle of direction. Graphs can be used as shown opposite to illustrate the performance of reflector lamps.

Lux (lx) is the ratio of the amount of light (lumen) measured on a surface proportional to the area of that surface. Known as the illuminance. It quantifies the amount of light falling on the surface. Based on the measure that one lumen over one square metre produces one lux. 

Luminous efficacy is measured in lumens/watt (lm/w) and is the amount of lumens being emitted from a lamp proportional to the power in watts used by the lamp.. A lamps efficacy is determined by its efficiency to turn electrical energy into light.

Colour Temperature is the colour shown at a particular temperature measured in Kelvin units (ºK) which are a measure of thermodynamic temperature. ºC = ºK—273.15 ºC and absolute zero is minus 273.15 ºC and is 0 ºK

When a black surface is heated enough and begins to emit light it starts to glow a dull red. As more heat is applied it glows yellow and than white and finally blue through the spectrum. As illustrated on the previous page as a piece of metal is heated the colour of light emitted by the metal will be changed as its temperature increases.

Higher colour temperatures (5,000K or more)  cool colour (bluish white)

The middle temperatures (3,700-5000K)  neutral colours (nearly white)

The lower colour temperatures (2,600-3,700K)  warm colours (yellowish white through red)

Colour Rendering Index CRI (Ra) is a measure of the ability of a light source to reproduce the colours of different objects faithfully in comparison with an ideal or natural light source. A lamps value on the Ra scale is measured from 0 to 100 on the CRI.

It needs to be remembered that visible things give off light in two different ways. Some emit light as a light source and others simply reflect light that is already made. A leaf for example when white light falls on it absorbs some of the spectrum colours and reflects or transmits others. This is why a leaf looks green as it absorbs almost all the colours in sunlight except one — green and reflects this, so green is the colour that is seen.