Today's Lesson

Let's Talk White Light

Figure 1. White light passes through a prism and is separated into
its component wavelengths.

So what makes white light so special?

Aside from the fact that white light is what we use everywhere for conventional lighting (which does make it, somewhat important), unlike coloured light, it is not a single wavelength.

Instead it is a combination of visible light frequencies (in a perfect world, all of them) known as a spectrum. You want proof of this, just look at rainbows. When white light passes through certain materials (glass/crystal/raindrops), each frequency of light is bent slightly differently, the end result of which is a rainbow (as shown in Figure 1). The different ratios of each frequency will set the colour temperature of the white light, as you can see in Figure 2.

You mentioned in the introduction to the LED that white LED's don't create white light directly. So how do they work?

First you need to realise that the nature of a LED only allows a single LED die to produce a single colour (in a narrow band around a single frequency), so obviously it's not possible for a single LED to produce all the colours that make up white light.

White light can be generated by LED's in two ways. First is through colour mixing. Individual red, green and blue dies are used and are driven at a ratio to create white light. This method isn't all that common for actually trying to achieve white light (though obviously it is used for RGB colour mixing). The main reasons for this are cost, efficiency and quality (the white light can be somewhat "spotty", possibly have colour fringing effects, and would generally have a poor CRI due to the narrow spectrum).

The second method is similar to how a fluorescent tube works, but rather than first creating UV light, a white LED is based on a blue LED thus ensuring there is no UV content in the light output). This can be seen in Figure 2 as the spike at about 450nm. On top of this is placed a yellow emitting phosphor (which creates the rest of the spectrum).

Figure 2. A comparison of the specturm of warm, neutral and warm white LED spectrums.

So white LED's are made from a blue one, why aren't the lumens levels of blue LED's as high as white LED's then?

Remember back to where the definition of lumens was given and recall that they are "spectrally sensitive". The eye isn't as sensitive to shades of blue, and therefore the lumens measurements for a blue LED is lower than if the exact same LED was then converted to a white LED.

White LED's are made from blue LED's. White with a blue tinge is a cool white. So how do we get warm white LED's?

The exact colour and thickness of the phosphor used to convert from a blue LED to a white one will change the spectrum of the final white light, and therefore alter whether it is warm, cool or neutral (you can see a comparison of this in Figure 2). As you can understand, the microscopic nature of placing the phosphor can lead to variations in the final colour. This is the primary reason behind there being more bins in white LEDs than in any colour.

The improvement of the placement of the phosphor is an area of ongoing research by LED manufacturers. I do know of at least one lighting manufacturer that is now making luminaires using blue LEDs and a separate phosphor disk for the entire luminaire. The advantage of this approach being that a specific colour temperature can then be targeted (as the wavelength of the LEDs is known beforehand, as is the spectral conversion of the phosphor disk). Just note that this is only an emerging technology, and isn't widely practised as yet.