The use of Light Emitting Diode (LED) technology in the lighting industry is a relatively new phenomenon. This is primarily because it is only in recent years that high intensity devices have become available.
What are the benefits of LED Lighting?
- Low power consumption compared to conventional lighting
- No ultra-violet output. The UV component of conventional lighting can cause damage to fabric.
- Very little heat is produced in the light output, reducing the cost of building air conditioning and allowing lighting to fit into positions too small for conventional lights.
- Lamp life is very long; most LED manufacturers estimate 100,000 hours.
- Ecologically friendly.
- Light weight manufacture.
- Coloured light can be produced by controlling the power to each primary colour, so no power is wasted.
There are two key areas where this technology influences the lighting industry: Illumination and Effects.
Illumination generally calls for the use of white light. LEDs cannot produce white light; they can only produce a specific colour of the spectrum.
The LED is a semiconductor device made from a combination of chemically polarised semiconductors. The chemical composition is chosen to define the energy of the electrons that pass across the boundary between the two types of semiconductor. This electron energy is converted to light as electrons flow though the device. The electron energy defines the wavelength of the resultant coloured light.
So how can LED technology be used to produce white light? There are two possible approaches. The first was pioneered by Nichia in Japan in 1996: a blue LED is coated with a white phosphor. When blue light hits the inner surface of the phosphor, it emits white light.
The second method of producing white light is to use additive mixing of the three primary colours red, green and blue. The resultant white has a very ‘peaky’ spectrum and is aesthetically displeasing. This can be improved by adding in non primary colours such as amber, but as yet this approach is rather limited in its success.
White LED based luminaires were slow to gain market acceptance. There were numerous reasons, key amongst them was that the early white LEDs markedly changed colour as they aged. This was due to the white phosphor degrading over time. There have been significant improvements in white LED manufacturing over the last few years and this problem is now much less significant.
Another key objection to LED white light is its colour temperature. This has clearly been at the fore of development in recent years as numerous manufactures have developed colour temperature tuneable white LEDs. For example, Luxeon now offer their white Rebel range from 2,700K to 10,000K.
The approach of tuneable colour temperature fixtures is likely to become more popular in the next few years. It is achieved simply by using LEDs of different core colour temperature and modulating their output to ‘mix’ the required colour temperature.
Effects lighting is an area where LED lighting has found an unassailable niche. Effects lighting invariably calls for colour – it is here that additive mixing of red, green and blue excel.
The concept of mixing the light output of LEDs was probably first implemented in 1979 by Jerry Laidman at a company called Sound Chamber. The product named ‘Saturn’ involved a spinning propeller. Each of the three wings of the propeller was constructed of circuit boards fitted with red, green and yellow LED’s. (Blue LEDs had not yet been invented!)
Each of the LEDs was controlled by pulse width modulation (PWM) allowing the intensity of each individual LED to be controlled. With the propeller spinning, the product could generate a huge number of colours. Jerry is now with Lighting & Electronic Design (L.E.D.) in Las Vegas.
The next technology jump occurred in 1993, with the invention of the blue LED by Nichia.
In early 1994, Artistic Licence prototyped what is believed to be the first full colour mixing design using red, green and blue LEDs. The design used pulse width modulation of each colour channel, with a Zilog Z8 microprocessor receiving the colour request via the relatively new DMX512 protocol. The principal worked, but the LED brightness and cost was such that the design could not yet become a product.
By 1997, Nichia had very high brightness blue and green LEDs and Hewlett Packard (Agilent) were producing very high brightness red LEDs. This was the year that the brightness – cost ratio crossed the critical line on the graph. It was now possible to produce products using the concept.
Arguably the introduction of the Luxeon high power LED by Lumiled (now Philips Lumiled) in 2001 heralded the introduction of LED as a credible light source for entertainment lighting. Lumiled continue to push the technology harder and have achieved significant improvements in power density with the introduction of the ‘Rebel’ product.
Reducing chromatic aberration
The push to develop physically smaller devices, such as the Rebel, is seen across a wide range of LED manufacturers. This is one way in which a key limitation of LED colour mixing – chromatic aberration – can be overcome. Chromatic aberration is the technical term for the nasty coloured fringes often seen around the shadows cast by LED light. The reason is that the clusters of Red, Green and Blue LEDs used to mix the colour are not a point source of light. That in turn means that the light from each of the primary LEDs draws a slightly different line around an object and casts the shadow in a different position. Reducing the physical size of the LED allows the product designer to place the LEDs closer together and minimise the effect.
An alternative approach to solving this problem is to have a single LED that contains the Red, Green and Blue so close together that they are effectively a point source of light. Bringing the three LEDs close together in one high power package causes significant problems with getting the heat out of the device. This is one reason why point source RGB LEDs tends to be available only at lower power levels. Also the major players in high power LED are concentrating their efforts on improving their white LED products because that’s where they perceive real money lies!
Some high power LED manufacturers are having success with the point source concept in high power form. PhotonStar provide miniature arrays with significant power output in their ‘UNO’ series. The offerings of Avago Technologies in the form of their 3W ‘Moonstone’ LEDs are also being seen more frequently in entertainment luminaires.
Packing the LEDs closer together is not the only way to deal with the problems of chromatic aberration; it can also be improved optically. Two key approaches have made their way to market in recent years. Sophisticated plastic lenses that attach to the top of the individual red, green and blue LEDs and collimate the light into a single beam are being used in a number of products. Previously this approach required the luminaire manufacturer to design and tool the plastic lens at significant cost. Numerous companies are now offering off-the-shelf solutions, e.g. Polymer Optics.
A more lateral approach to the problem is the use of holographic lenses. A holographic lens is a thin section of plastic, often polycarbonate, which behaves as a complex set of optics. It can be thought of as recording ‘real’ optics as a hologram. The clever part is that the optics recorded into the hologram can be as complex as the designer desires, collimating and evening the field of an array of LEDs.
This approach to light mixing and focusing is set to become more prevalent and more efficient in the coming years due to the recent invention of the polarised LED. Polarised light simply means that the light waves are all oscillating in the same plane. Martin Schubert discovered that the light generated by conventional LEDs is polarised but that the optics destroyed the polarisation. His innovation was to design a new type of optics that capitalises the intrinsic polarisation of LED light. This is important because polarised light is easier to manipulate with lower loss of energy, particularly using holographic techniques.
Growth areas for LED
One particular area of growth is in exterior wall wash fixtures – for many years this has been the domain of large and power hungry conventional technology. Another key change in product type is with moving heads. Historically most LED colour mixing fixtures were static; largely because the light output could not compete with their larger moving head cousins. We have seen this change as numerous manufacturers, including major players such as Martin Professional, decided that LED based moving heads are now credible. The Alison Moyet tour in 2009 was an excellent example, with a set almost wholly based on LED moving heads.
Perhaps the most noticeable change is the continued and accelerating convergence of lighting and video technology for entertainment. Not so many years ago there was a clear differential between the two; the video screens were hung over the stage or PA towers and were very much the domain of the video crew. Not so any longer: video and lighting are very much integrated. Consider the U2 360 Tour. The centre part of Mark Fisher and Willie William’s design contained a curved video screen that expanded vertically during the show. During some numbers it was used conventionally as a video screen, in others it was used as a source of coloured light and integrated with the stage lighting. This is a theme that progressive designers like Willie Williams and Pete Barnes continue to push.
So where is the technology going?
Other LED technologies are starting to have an impact, notably OLED. An organic LED (OLED) is different to a conventional LED in that the material that emits light is, as the name suggests, organic. Organic means that the light emitting material contains carbon, not that it is grown in a sustainable manner! The organic material is sandwiched between two electrodes, one of which is transparent allowing the light out of the device. The technology is still being developed, but we are starting to see OLEDs in high-end luminaires. They are also appearing in set design by way of illuminated panels and signage.
Other exciting technology includes quantum dots, a major theme in nanotechnology research. They are photoluminescent semiconductor nanocrystals that emit a different coloured light to that which energises them. Conceptually they operate in the same way as the phosphor on a white LED although the physics is completely different.
Of key interest to our industry is the fact that quantum dots can be tuned to output light of variable colour from the visible to infrared. The technology received a new impetus from the accidental discovery by Michael Bowers of Vanderbilt University that a particular type of nanocrystal (Cadmium Selenide) gives off a warm white light of similar rendering to an incandescent light. It is likely that this technology will soon replace phosphor based white LEDs. Companies such as Evident Technologies have commercialised low power coloured LEDs using quantum dots along with white LEDs ranging in colour temperature from 2,250K to 8,000K.
Quantum dots are also seeing their first appearance in the form of highly efficient, ‘active’ light filters. QD Vision, a commercial spinout from MIT, is shipping quantum light filters. These are essentially glass plates with a film of quantum dots printed on top. The type of quantum dot defines the colour and quantity of colour that will be added to the light passing through the filter. For example, a plate printed with red quantum dots can be used to add a small amount of red to the white light passing through it, so warming the colour temperature.