What is Colour Rendering?


Colour rendering index [CRI] is the standard scale to compare different light sources. It is derived from the visible light spectrum as detailed below.

The  light spectrum visible by the human eye  is less than 1.5% of the entire electromagnetic spectrum.

The following picture shows the  electromagnetic spectrum. Note the small section visible by the human eye.

Frequency Wavelength
violet 668–789   THz 380–450   nm
blue 631–668   THz 450–475   nm
cyan 606–630   THz 476–495   nm
green 526–606   THz 495–570   nm
yellow 508–526   THz 570–590   nm
orange 484–508   THz 590–620   nm
red 400–484   THz 620–750   nm

The color temperature of the electromagnetic radiation emitted from an incandescent light bulb‘s light is over 97% thermal and less than 3% light, so its color temperature is essentially the temperature of the filament. Therefore incandescent lamps have a CRI of 100.

Many other light sources, such as fluorescent lamps, emit light primarily by processes other than thermal radiation. These sources are assigned what is known as a correlated colour temperature (CCT). CCT is the color temperature which to human colour perception most closely matches the light from the lamp.

The table below details a range of light sources, both natural and man-made. The table details Correlated Colour Temperature  CCT values in Kelvin [K] and Colour Rendering Index [CRI] values.

Light source
CRI CCT (K)
Low-pressure sodium (LPS/SOX) 5 1800
Clear mercury-vapor 17 6410
High-pressure sodium (HPS/SON) 25 2100
Coated mercury-vapor 49 3600
Halophosphate warm-white fluorescent 51 2940
Fluorescent light tubes or Cool White/Daylight CFL 62 5000
Halophosphate cool-white fluorescent 64 4230
Tri-phosphor warm-white fluorescent 73 2940
Halophosphate cool-daylight fluorescent 76 6430
“White”   SON 82 2700
Soft White compact fluorescent light bulb 82 3000
Quartz metal halide 85 4200
Tri-phosphor cool-white fluorescent 89 4080
Ceramic metal halide 96 5400
Candle flame, sunset/sunrise 100 1850
Incandescent light bulb 100 3000
Incandescent/halogen bulb 100 3200
Vertical daylight, electronic flash 100 5700

The picture below shows what colour temperature looks like

Lighting design is a complex subject and there are many Industry experts to help guide you through the process.

The chart below gives details of colour temperature and typical applicable locations

I work with MHA Lighting  http://www.mhalighting.co.uk/  who are passionate about lighting and go the extra mile to deliver quality products as standard.

There is much more to lighting than how many Watts the lamp is. We are here to help you make the right choices for your needs. We will work through your requirements and ensure you have the best possible lighting available.

Contact us on 01782504503 or 07901380746 to discuss your requirements. You can also e-mail ecologic@gmx.co.uk to arrange a free lighting survey or to answer any questions you may have.

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About aimhome
All aspects of energy efficiency and help with saving money spent on utility bills. Home visits undertaken to assess what is used and explain what can be done to save money. Central heating control upgrades, central heating system power flushing. Energy saving lighting surveys and installation of low energy lamps and lighting. Every day that action is not taken is an added cost in hard cash terms. Call today to see what can be done. Green Deal Ecologic Ltd, improving your world without costing the earth.

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