People often ask me if LEDs have good colour rendering or are some better than others. But this assumes that there is a light source which shows objects in their “true colours”. We could then compare the LEDs with a true or accurate light source but, in fact, there is no such thing.
We normally think of filament lamps as having great colour rendering but, looked at objectively, a light source with huge amounts of red and almost no blue can hardly be described as accurate. Maybe if Edison or Swan had invented a small hydrogen - fusion reactor, like sunlight indoors, the unbalanced spectrum of the filament lamp would never have caught on!
We also use daylight to compare colours - we have all taken an item of clothing to a shop window to see what it really looks like. But even daylight changes from early morning to midday to sunset.
Your companion at a candlelit dinner will literally look different in the cool light of dawn!
The reason we don’t notice these changes in colour is that both filament lamps and daylight have a continuous spectrum and our brain automatically adapts depending on its surroundings. It’s known as colour constancy.
When LEDs were first introduced for regular commercial lighting some 30 years ago, they had very uneven/patchy spectrums and this meant that people and objects could look slightly odd – you could say they looked off-colour.
They have developed enormously since then and nowadays LEDs are used to illuminate even the finest artworks in the greatest galleries.
The general way of comparing light sources is by using a metric called the Colour Rendering Index, CRI. In simple terms, it divides the visible spectrum from blue at one end through yellow and green to red at the other end into 8 or 15 sections. The percentage “closeness” of the test light source achieved in each section is compared with the reference source. This is usually a laboratory filament lamp but sometimes a standardised version of daylight is used. All the test values are then averaged to produce a single figure known as the CRI.
For most outdoor and industrial locations, a CRI of 70 or more is sufficient. Indoors, you need to have a CRI of 80 or above. These values are enshrined in workplace legislation such as BS EN 12464-1. CRI is sometimes written as Ra, e.g. Ra 70.
It’s also worth remembering that most people cannot discriminate a difference of less than 5. I.e. a CRI of 75 won’t be noticeably different from a light source with a CRI of 70.
If colour rendering is really important, go for CRI >90 but at this level, other factors come in to play such as how Warm or Cool the source appears or what type of colours you want to compare.
A related but totally different issue is that of colour appearance; how Warm or Cool a light source appears. Lighting specialists refer to this aspect as the Co-related Colour Temperature, CCT, of the light source. It is measured in degrees Kelvin. Basically, Warm sources have a CCT of 3000K, 2700K or even less. Neutral sources are around 4000K and really Cool ones are >5000K. Generally, wildlife prefer Warmer sources at night.