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What is colour selectable lighting (CCT) ?

A colour selectable lamp or Light fitting is one in which the corelated colour temperature (CCT) of the light generated can be selected from a small number of fixed options.

CCT is measured in kelvin (K) and describes if a white light is warm (with a slight yellow hue, around 2700K), neutral (around 4000K) or cool (with a slightly blue hue, starting from around 5000K). For more details,  "What is correlated temperature CCT ?". CCT applies to white light only.

Note that colour selectable is also known as field-selectable CCT and CCT colour switching,

What is the benefit of colour selectable lighting?

Frequently, lighting manufacturers will produce a popular design of luminaire that could be used in different settings – for example in an office and in the home. However, in an office people will usually expect a CCT of 4000K-5000K (“cool” white), while at home they might prefer 2700K or 3500K (“warm” white).

To save the manufacturer (and the entire supply chain) from having to stock multiple versions of the same product they produce “colour selectable” fittings. These feature a switch on the product itself that allows the installer to choose from a small range of pre-set colour temperatures, so that the same fitting can be used in multiple different settings.

Colour selectable fittings are also beneficial in that they allow the end-user the opportunity to change their mind and alter the colour temperature after installation without having to change the complete fitting.

How does colour selectable lighting work?

All LEDs can produce just a single colour of light. Therefore, a colour selectable light fitting contains two or more separate arrays (or groups) of LEDs, with each LED array producing a different colour temperature. A switch, often located on the rear of the fitting, is used to determine which array of LEDs is to be powered on at any one time.  Commonly, a colour selectable light fitting will offer a choice of just two CCT values (eg 3000K and 4000K), though others offer a choice of three CCT values.

Note that colour selection is static. With a colour selectable fitting there is no means (short of accessing the rear of the fitting and adjusting a switch) to change the colour temperature of the white light that is being produced.

“Colour selection is usually made with a switch on the rear of, or inside, the light fitting”

What’s the difference between colour selectable and colour changing lighting?

Colour selectable lighting relates to white-light fittings. A colour selectable fitting allows the installer or user to select the correlated colour temperature (CCT) of the light the fitting will produce from a small number (usually 2 or 3) of fixed options. Typical options could be warm-white (3000K), neutral white (4000K) or cool white (5000K). Colour selectable lighting is widely used in residential and commercial projects such as offices and shops.

Colour changing lighting is LED lighting that can create a very wide range of different colours which can be changed dynamically. Colour changing lighting is often used on building facades and on feature walls in clubs and bars. The colours can be continually changing, usually controlled by a DMX control system.

What’s the difference between CCT and CRI?

CCT and CRI are both important measures of the quality of white light.

CCT (correlated colour temperature) defines how the light appears and how it might make us feel. A low CCT value (eg 2700K) indicates that the light contains a substantial proportion of the red, orange and yellow wavelengths, which we often call “warm” white. A higher CCT value (eg 5000K) would have less red and orange but more of the blue wavelengths, which we would call a “cool” white.

With CCT it is not accurate to claim that one value is better than another. If we prefer warmer or cooler whites is generally a matter of personal preference.

CRI (colour rendering index) defines how well a particular white light enables us to perceive the colour of the objects it illuminates.

The colour of the objects we see is a result of the wavelengths of light that land on them and which are subsequently either absorbed or reflected. We perceive grass to be green because even though daylight (which contains red, orange, yellow, green, blue etc wavelengths) is shining onto it, all the other wavelengths are absorbed and only the green wavelengths are reflected.

However, grass can only appear green if the white light shining onto it contains green wavelengths in the first place. If any wavelength is missing from a beam of white light then an object that should be that colour will not appear so under that particular beam of light. Grass that should be green, seen under a light source that is missing some green wavelengths, will appear to be dull, and in extreme cases, even grey in colour.

Daylight has a CRI of 100 because it contains at least some of every single one of the visible wavelengths. A good quality artificial white light that enables us to see colours well will have a high CRI, above 90. This would normally be required in a shop – especially one selling clothes. A CRI>80 is generally required in offices and classrooms. A CRI>70 is suitable in many exterior applications, such as street lighting, but any CRI lower than 70 is generally not acceptable in any setting.

Unlike CCT (where there is no objective “better” or “worse” CCT value), CRI is objective. High values are better and lower values are worse.