Choosing a White LED
What is correlated colour temperature (CCT)?
Unlike coloured LEDs, which are differentiated by wavelength, the appearance of white light is measured in Kelvin (K). Interestingly - the lower the CCT value the “warmer” the white, and the higher the value the “cooler” the white. It sounds a little back to front.
Choosing a white LED
Warm whites have a red or yellowish tone and are used to imitate candle light, while the cool whites appear an icy bluish white. Often the choice of colour temperature is down to personal preference however they are better suited to certain environments.
Warm whites are calming and cosy, frequently used in restaurants, hotels and at home. Cool whites are harsher and you’ll often see these used in industry and street lighting. Then there are neutral whites that fall in the middle. These are mostly used in places such as offices, classrooms and retail.
When selecting your white LED, you may come across colours such as “Quartz white” or “Firelight white”. These are evocative but they are not very specific. Take the colour temperature value given in kelvin (K) and compare it against a colour temperature (CCT) chart:
What is CRI?
Colour rendering index (CRI) measures the LED’s ability to show the colours of objects in comparison to how they would appear under an ideal light source (daylight).
If you need colour accuracy you should choose a CRI of more than 80 (90 and above are readily available). Lower CRI values can show variations from the object’s natural colour appearance. However such LEDs are still great for outdoor applications e.g. street lights – and they are generally cheaper.
Retail, particularly clothing, furnishings and drapery, are examples of where a high CRI is important. Not only can it be used to attract customers but it can also prevent returns after they get home and see it’s a completely different colour to how it looked in the shop. Lumileds have designed a range of LUXEON COB LEDs with CrispWhite technology which are suited for retail. They have a high CRI alongside making whites more vibrant and colours highly saturated.
General Colour Rendering Index (Ra) and Special CRI (Ri)
The general Colour Rendering Index looks at the first 8 colour indices (Ri). Ra is the average of the eight Ri values and is stated as CRI with a maximum value of 100. Special CRI refers to the remaining 6 colours (9 to 15) including the saturated colours and skin tones. Therefore manufacturers often aim for interior lighting LEDs to have strong R9 and R13/R15 values, as this improves the appearance of complexion, giving the skin a healthy looking glow.
Using RGB LEDs for white light
RGB LEDs include three chips (red, green and blue) which can be used to create white light. The three colours can be mixed together to create a range of different colours. This is great if you need flexibility in your design. The only downside with using them for white light is that they can’t always produce as consistent an output as traditional white LEDs, and are therefore not a substitute for high CRI Lighting Class white LEDs.
How about remote phosphor?
An alternative way to create white light is through remote phosphor. This is designed to work with royal blue LEDs (sometimes called blue pumps) to produce a white light. It also gives you the option to swap lenses, which is much quicker and easier than having to change the LED itself. Simply changing the lens can completely change the ambience of the lighting. Remote Phosphor Components.
Related Posts - LED Wavelength vs. LED Colour
CommentsAdd a comment
Is there any value (or studies) in specifying a high (80+) R13 value for outdoor streetscape lighting projects?
Because of facial better color benefits in specifying a high R13 value does the pedestrian feel:
- better comfort in window shopping?
- better safety factor because facial features are better seen?
- a more inviting environment therefore promoting shopping in that space?
- a better uniformity? Is yes, does that constitute lower FC requirements?
As a separate subject:
Is there any truth to lowering the blue band width (4 - 6K CCT) that there will be less light trespass therefore constituting 2700 K to 3000 K designs being prudent?
Measuring white light in terms of temperature comes from the black body radiation theory.
There are really no white LEDs. The Nobel Prize was given to the inventors of the blue LED.
Blue LEDs plus a luminescent phosphorus mixture which adds red a green light from a part of the blue, creates the white light.
Actually white is a human concept of our perception. Exact color rendering is impossible using 3 chromatic systems and that is well known fact since color printing and color TV.