Colour Temperature and Rendering

                                                                                            from Gartner Superlux Ltd

Not all light sources emit light that reproduces colours accurately.   Although general lighting service(GLS) lamps and tungsten halogen lamps do, other sources such as fluorescent, miniature fluorescent, gas discharge lamps do not.   A good example of a poor colour rendering light sources are sodium gas discharge l;amps, such as those used instreetlights.   Colour rendering can be regarded as a percentage, where a lamp that has a colour rendering index (CRI) of 100 reproduces viewed under it perfectly.   The colour rendering properties of lamps are sometimes denoted by the prefix 'Ra'. A lamp  with a rating of Ra 100 has a perfect colour rendering ability.

Lamp type                                             Colour rendering Index (CRI)

Daylight                                                                 Ra 100

Incandescent or GLS lamp                                   Ra 100

Tungsten halogen lamp                                        Ra 100

Fluorescent tube (old type)                                Ra 55 - 65

Fluorescent tube (new tri-phosphor type)          Ra 82 - 95

Metal halide lamp                                                Ra 75 - 90

Sodium (streetlight) lamp                                   Ra 30 - 35

When items are viewed under very low colour-rendering lamps it is difficult to accurately tell their correct colour.  This is why some people take products in shops to the window, so they can recognise its proper colour when lit by natural light.

Colour temperature

The colour temperature of a light source is the colour it appears to be, in relation to the coloursof the visible spectrum.  IE, a 'white' coloured lamp can appear to be 'warm' if it yellow/red and 'cool' if it is blue/green. Colour temperature is measured in degrees Kelvin, which is denoted by a number followed by the letter 'K'.   A warm colour-temperature lamp is around 3000 degrees Kelvin, or 3000K.   A cool coloured-temperature lamp would start at around 5000K.

Range of colour temperatures.

2500k     3000K                  4000K                  5000K     65000K

      Warm                     Intermediate                       Cool

Typical colour temperatures of lamps.

Lamp Type                                                      Colour temperature

Sodium discharge lamp (streetlight)                       2000K (warm)

GLS or incandescent globes                                   2700K (warm)

Tungsten halogen lamps                                         3000K (warm)

Dichroic tungsten halogen reflector lamps              3200K (warm)

Typical fluorescent lamp                                          4000K (warm)

'Daylight' fluorescent  lamp                                      6000K (cool)

  The colour chart below shows where colour temperatures fall in relation to all colours

Lamp types - General overview

Lamp types fall into three main categories - incandescent or general lighting service (GLS), fluorescent and gas discharge, which are also called high intensity discharge or HID.

Incandescent or GLS (general lighting service) lamps

The most well recognised lamp type is incandescent. All incandescent lamps operate on the same principle,  an electrical current is forced through a wire filament that resists the electricity flow.   As the current fights the resistance, heat is generated.   When the filament gets very hot, it glows and becomes "incandescent".   There is a large range of variations of this kind of lamp.   These include ordinary light bulbs or globes, candle lamps, lustre lamps, reflector lamps and more.   These are the easiest and cheapest types of lamps to manufacture and variations of incandescent lamps have been in use for around 100 years.   GLS lamps also have a relatively short life, typically 1,000 hours.

Halogen Lamps,

These also emit light from a heated wire filament, however there are several differences from a normal incandescent lamp.   The lamp is smaller so that the (quartz) glass envelope is closer to the (tungsten) filament and a halogen gas is added (bromine) or (iodine).   For maximum efficiency Halogen lamps must operate at higher temperatures than conventional incandescent lamps.   There  are several types of halogen lamps including linear, capsule and reflector - most of which are of comparatively small size.   lamp life varies from 2000 - 5000  (commonly) depending on quality, type and construction.

Fluorescent and compact fluorescent lamps

Also called fluorescent tubes, these lamps do not emit light by heating a filament.   The tube contains mercury vapour through which an electric current is passed.   The current causes the mercury to emit ultra-violet light, which activated light-emitting phosphor compounds on the inside of the tube.   There are several variations of fluorescent lamps.   These include straight lamps (or tubes), circular, U-shaped, multi- stem compact fluorescents and more.   Fluorescent lamps areas much as six times more efficient than GLS lamps.   They also have a longer life, typically 7,500 hours to 16,000 hours.

High Intensity Discharge Lamps

HID lamps operate in much the same way as fluorescent lamps - electricity is passed through a mixture of chemical compounds, which then emit light.   There are three main types of HID lamps.   These are mercury, sodium (also called high-pressure sodium or HPS) and metal halide.   Each one of these three different lamp types exhibits different characteristics in terms of colour rendering, colour temperature and efficiency.   Gas discharge lamps are the most efficient in terms of light output for power used and also have the longest life expectancy, with some lamps lasting as long as 24,000 hours.