Last week, I talked about how the absence of sensation could be a useful way of understanding how we sense the world. This week we can discuss how the PEARL lighting system helps us to understand about light and colour.
The PEARL lighting system uses Light Emitting Diodes (LEDs) to create the light. These are extremely energy efficient and they also provide a really good way to create coloured light. This is because the diodes emit light as a frequency of vibrations, which then accord with various wavelengths of light. The visible spectrum - the light we can see - ranges from a wavelength of about 380 nanometres, which is a very deep purple, with wavelengths just a little longer than ultraviolet light (which we cannot see), to about 780 nanometres, which is a very deep red and just short of infrared (which we also cannot see). If we display all the wavelengths together, we will deliver white light - it was realising that white light is made up of different wavelengths that started Isaac Newton on his scientific research on light.
An LED is semiconductor that, when energised, vibrates at a particular frequency that is in the visible spectrum, i.e. it emits light. If we select LEDs with certain frequencies, we can find a frequency range that delivers a particular wavelength band, that shows up as a particular colour. If you buy an LED light, this selection will probably have been made for you, maybe to give you white light or some other colour (e.g. the red LEDs used in brake lights for road vehicles).
In the PEARL lighting system, we selected LEDs so that they would give us a set of colours, which, when mixed, could provide white light, or which could give us particular colours. We have 11 wavelength groups, and a twelfth one which is a bright white colour. Of course, LEDs do not deliver precisely the same wavelength, so we had to select ones to give us an appropriately precise distribution for each colour, and a range that overall would give us a controllable white colour when they are all blended together. our wavelength range goes from 405 to 750 nanometres, so covers almost all the visible spectrum for humans.
There are interesting issues of course. First of all, there are two broad types of LED chip. one is based at the shorter wavelength end of the spectrum, and gives mainly blue-ish light, and the other is centred on the longer wavelength end of the spectrum, and gives a reddish light. As the wavelengths increase from short to long, the light becomes greener, but after a peak, the light intensity becomes weaker. Conversely, as the wavelengths decrease from long to short, the light moves from red towards orange, yellow and green, but as it does so the intensity reduces. As a result, it is impossible to have a 'yellow' LED with any power, because for both chip groups, that part of the visible spectrum can only be reproduced, given the chip chemistry, at a very poor light intensity. If you see a 'yellow' LED, it is almost certainly a white LED with a phosphor coating painted on it.
With our range of LEDs, we can of course create yellow - by blending blue and green (an maybe a bit of red if we want it to be a more orange-looking yellow). As a result, with our 11 colopurs (plus bright white), we can create almost any colour in the visible spectrum. Televisions, cameras, smartphones - any device with a colour display - creates the range of colours by blending red, green and blue light in the same way. The difference is that the PEARL system is blending 11 colours, instead of 3, so we have a much richer blend capacity.
Why do we do this? Well, it means that we can create light with any blend of colours within it. This means that we can simulate the different colours of daylight - including the light dominated more by short wavelength light in the morning, or more longer wavelength light in the afternoon. We can do this at different levels of intensity irrespective of the colour. This means that we can simulate daylight from anywhere in the planet for any time of day (or night). Of course, we can also simulate artificial light, such as street lighting, as needed.
What does this all look like? It is a bit difficult to show you the almost infinite subtlety of the lighting we can produce, because the device you are using to see this post is limited by its 3-colour RGB graphics capability. But we can show you what happens when we switch from one wavelength to another in sequence. We showed this as part of a demonstration to a group of visiting students and this is shown here:
This is just about the lighting technology of the lights themselves. How we use this in experiments is of course of great importance, and we will look at this another time.