Understatement of the year: there are a lot of LED-based lighting products. In the past two years alone, at least a dozen new companies have been formed explicitly to manufacture LED lighting products for professional stage and events applications. Meanwhile, in the general illumination market, LED lighting is creating an even greater amount of attention. The white LED is the star player there, and the incentive is partly the energy savings that LEDs can offer, but also the reduction of mercury if LEDs can replace fluorescent lighting. LED lighting is clearly here to stay.
This article aims to demystify LED lighting and put it in perspective. What are its strengths and its unique traits? In what applications and circumstances can LED lighting provide a better solution than other light sources?
How LEDs Work
Let's look at the fundamentals of the LED itself before we get into the nitty gritty details. First of all, an LED (Light Emitting Diode) is not a miniature lamp with a colored cap. There are no filament wires as found in an incandescent lamp, no electrodes with an arc gap, like in a discharge lamp. LEDs are substantially different from any other type of light source. They are semiconductors (like transistors), and this makes them suitable for large scale mass production and for direct control by microprocessors. This should enable LED lighting to be very cost effective compared to other lighting technologies.
The light from an LED is practically a pure, single color. A green LED produces green light only, a red LED only red light, and so on. It is the material of the LED chip that determines the color. LEDs are driven by direct current, and the amount of current determines the brightness. The brightness of the LED is proportional to the current flowing through the LED: more current, more light. In order to create a full range of colors including white light, we need to use a selection of different colored LEDs that can be combined in various proportions to create a wide color palette. Typically this is done using red, green, and blue LEDs.
Yes, LEDs producing white light are also available. They use a technique borrowed from fluorescent lighting. White LEDs are actually blue or ultraviolet LEDs with a coating of phosphorescent material that shifts some of the light into green and red which, together with the original blue light, creates white light.
With those basics covered, let's look at all the virtues attributed to LEDs and how they hold up to closer scrutiny.
LEDs have no moving parts, no filaments to break, no arc gaps to erode, and no gases to deteriorate. They use very little power too. No wonder they are rated for up 100,000 hours. Imagine, that's 11 years if operated 24 hours a day, seven days a week. Unlike other light sources, the life of an LED is not determined by an outright failure to work. LEDs rarely go out with a bang; they gracefully decline over time. The single most influential parameter is the operating temperature of the LED. Heat is the #1 enemy of LEDs but more on that later.
Most likely, your LED lighting fixture will be going (but not strong) after those 100,000 hours. Productive working life would perhaps be a better description. LEDs should be allowed to retire after a certain age.
What sort of lifetime can you realistically expect? When a large number of LEDs are packed tightly together, as they invariably are in LED lighting fixtures, the working environment is far from ideal, and this affects the life of the LEDs. Nevertheless, a well designed LED-based lighting fixture should achieve at least 40,000 to 50,000 hours. That is still very impressive and more than enough for most real world applications; 24/7 operation at full power is rarely required anyway. Let's say there's three shows a day, each show at two hours, seven days a week. That's 42 hours of operation per week. If we conservatively assume 40,000 hours of lifetime, that translates into more than 18 years of operation.
This is based on the assumption that the power supplies, cooling fans, and other electronics inside the LED fixture are also rated similarly to the LEDs. In all likelihood, the power supply is the weakest link. This may be a good argument for choosing external, easy to access power supplies for LED lighting, at least for fixed installations.
The working life of a properly designed LED lighting fixture will far exceed its economic life. Most likely, you will be replacing your LED lights with better and brighter models long before the life of LEDs become an issue.
Hand in hand with the long life comes the issue of maintenance. Is it reasonable to assume that, since no LED replacement is required, an LED-based light can be installed and left alone for several years?
This is a tricky one, since maintenance is typically not only light source replacement. It also involves cleaning lenses, reflectors, etc. due to dust circulating in the air. Fortunately, modern LED fixtures often use sealed optics, but there is always at least one exposed optical surface. After all, the light does has to leave the fixture! This surface, be it a protective cover or a lens, will need cleaning from time to time. The frequent use of smoke and haze machines in performance spaces further adds to the contamination of the air and produces deposits on all lighting fixtures, helping to bind dust and other debris in the air. In the stage and events lighting market, the lights will be subject to frequent rigging and de-rigging anyway, and any decent rental shop and lighting department in a theatre or studio will also maintain inventory on a regular basis.
Maintenance, therefore, is drastically reduced compared to conventional fixtures, but cleaning and general upkeep is still required, just like any other fixture.
LED lights have one very strong and unique selling proposition: they are very rugged. With no moving parts, fragile lamps, or dichroic glass filters, no other color-changing lighting fixture comes close to an LED-based unit in terms of ability to withstand abuse and harsh environments. This is a major benefit in the architectural market, but the ruggedness is, of course, equally welcome in theatre and touring applications. LED lights can be built into set pieces and other hard-to-access positions. A current favorite application for LED lights is as truss toners. Mounted inside aluminum trussing, the small dimensions, rugged nature, and directional beam of LED lights really shine in this environment.
The convenience and creative freedom of being able to tune the color to suit the requirements is a major attraction of a color-mixing lighting fixture. This and the possibility to seamlessly fade between any two colors over time have made fully color-mixing moving lights a must have today. With color-mixing LED lights, we have a similar situation. Three or more colors of LEDs are combined to provide a wide range of colors.
Additive color mixing has been around for a long time using normal incandescent light sources and color filters. But this is not a very efficient way to mix colors. Each of the three sources is white light that gets filtered down to red, green, and blue. With LEDs, additive color mixing has made a real comeback. LEDs produce pure single colors. No color filters with their significant associated losses are needed.
Typically, LED lights are quoted as having 16 million colors. Yes, with three colors, each controlled by a standard DMX channel with 256 steps between 0 and full, just over 16 million combinations of red, green, and blue are possible. But as with moving lights with CMY color mixing, the range of colors is determined by which and how many colors are used, not simply by the resolution of the control system.
The visible spectrum — the colors we can see — is described in the CIE diagram (Figure 1). Typical color points of LED fixtures (red, green, and blue) are indicated in this diagram. These three points describe a triangular area, the color gamut. By careful adjustment of the three colors' intensity, any color within this triangle can be produced. Choosing LEDs at the extremes of the CIE diagram would seem like a good way to maximize the range of possible colors. Yes, you could do that but at the expense of brightness and with great difficulty at mixing up less saturated hues such as pastels and skin tones, not to mention white.
Regardless of the colors chosen, there will be a number of colors that the RGB-type LED fixture will have trouble creating. This is the same issue that we have with CMY color mixing in moving lights, though in the case of LEDs, we are working with additive rather than subtractive color mixing.
Some manufacturers have chosen to go beyond three colors, some even using up to seven. This does expand the color range but at a price in terms of needing more LEDs and complexity of control. However, if a wide color range and great pastels are of great importance to you, these may be worth the extra cost and control overhead.
If different types of LED lights are used together — say floods and spots — it is probably wise to choose from the same manufacturer if you expect them to match, not only in static colors but during color crossfades. This is because there is no industry standard for the LED colors used or the translation between control input value and brightness.
So color mixing without moving parts or filters is a big plus for LED lighting. Be aware, though, that is subject to the same limitations as with color mixing in moving lights.
Dimming — The Good, The Bad And The Plain Ugly
Like any other lighting fixture used in a performance environment, the intensity, or brightness, must be controllable from zero to full. Unlike incandescent lamps, LED fixtures don't need external dimmers. Just like automated lights, they require fixed power and a control signal, typically plain DMX.
Surprisingly, not all manufacturers of LED lights have figured out how to smoothly control the intensity. In some cases, the control characteristics of the individual colors can vary, resulting in a shift in color when the LED light fades in and out. Perhaps this is indicative of the fast nature of the LED lighting business. Many companies are entering the field, but few have a good understanding of what the users really value. Anyway, it's perfectly possible to design a cost effective control and drive circuit that controls the intensity of the LEDs in a smooth consistent way.
And as with automated lights, never plug an LED into a dimmer. There's more than a fair chance it will be permanently damaged. LED dimming is not yet standardized by ESTA or other industry working groups. It'll take some time before this will happen, and so you really need to test drive the lights to make up your mind if the dimming is good enough for your application.
LED light sources are hailed as being very efficient; they provide a lot of visible light for the energy the use. They do, but you need to be careful about comparing apples with oranges.
Let's say you compare a high efficiency white LED with a typical household lamp. In this case, the LED will be about five times more efficient. White LEDs are breaking new efficiency records on almost a weekly basis, it seems. This is not surprising, considering that there is a lot of business (and money) available for LED lights that can replace incandescent and fluorescent lighting in homes and offices. But in our market, white LEDs are not generally useful. We are more likely to be considering replacing tungsten-halogen lamps and color filters with RGB color mixing LED fixtures.
So let's see how these compare. As our reference for the conventional fixture, I have chosen the well known ETC Source Four PAR EA with a 750W lamp. It's similar in optical design to currently available LED lights. The LED alternative will remain anonymous but is an established fixture from a large manufacturer specializing in LED lighting.
To compare with red, green, and blue, I have chosen normal plastic filters, in this case from the Lee Filters range. Note that the efficiency is calculated based on the transmission percentage for these colors at the wavelength of the corresponding LED color and not based on the general full spectrum transmission average.
I also used correction factors to factor in the non-linear spectrum of the tungsten halogen lamp: 2.4 for Red, 1.0 for Green, and 0.4 for Blue. The tungsten lamp is very strong in the red end of the spectrum and correspondingly weak in the blue end. The correction factors chosen should help figure that in for a fair comparison.
As you can see (Figure 2), at white light, the efficiency of the conventional fixture holds up well next to the LED fixture. In red, the Source Four PAR is really strong. This is not surprising, considering the amount of red energy present in a tungsten source. But in green and blue, the LED has almost double the efficiency of the tungsten source.
The blue is clearly the weakness of the conventional, tungsten-based fixture — again, not really surprising as there simply isn't that much blue present in the white incandescent light to begin with.
On an efficiency basis, therefore, in lumens per watt, LED compares very well to incandescent lighting in white, excels in blue and green, but falls a bit short in red. Overall, today's RGB LEDs are on par with the best incandescent lights but are likely to pull ahead in a not too distant future.
But efficiency on its own is useless. We cannot light with it. We need absolute levels of light, and it is here that LED lighting is facing its greatest challenge. Our LED fixture is rated at 50W and produces 665 lumens at full white. Our 750W tungsten-based ETC Source Four PAR achieves 10,000 lumens. This means we would need to use 15 LED lights to match the white light output of the tungsten light. In red, the LED fixture falls far behind due to the huge amount of red in the tungsten light. Here, we would need to use close to 30 LED lights to balance the output of the conventional fixture.
On the other hand, in blue, you would only need eight LED fixtures, since the efficiency of the LED light is double that of the conventional fixture with its blue color filter.
So, while LEDs are very efficient, the lumen output of LED lights is still not great enough to enable them to replace high wattage conventional tungsten-based fixtures on a general basis. That said, they are viable alternatives for a lot of applications where massive amounts of lumens are not required.
In The Shadows
In order for the LED fixture to produce a lot of light, large numbers of LED sources must be used. Typically, an LED fixture uses a certain quantity of red, green, and blue LEDs mounted on a common surface and arranged to give a nice mix of the individual colors. But getting LEDs to mix well together without additional optics is tough. And since two or more LEDs can't be in exactly the same position inside the fixture, multiple shadows in multiple colors will appear. There are some clever optics that can combine the output from red, green, and blue into one beam, but again, at the cost of reduced light output.
The availability of high brightness “power LEDs” has meant that most LED fixture manufacturers are now using a smaller number of high brightness LEDs instead of a larger number of low brightness ones. Ironically, this is actually counterproductive from a color mixing point of view. The ideal, from a color mixing perspective, would be to have an infinite number of LEDs, as this would give a perfect blend of the colors.
Does it matter? It depends on the application. If the object being illuminated is very close, flat, and clearly visible, like a cyclorama, it sure does. On the other hand, using LEDs as truss toners or accent lights is much more forgiving. There is a wide range of applications in between these two extremes, and I don't believe one LED fixture fits them all. Some manufacturers are offering additional clip-on lenses or diffusers to improve blending the LEDs together. This can work well but at the price of some loss of light output. It might be worth experimenting with normal diffusion filters from color filter manufacturers. The small amount of radiated heat means these filters can be taped directly to the front of the LED fixtures.
Multiple LEDs, therefore, are needed to produce enough light for most applications. The blending together of the individual LEDs into a single homogenous beam remains a tough nut for the manufacturers to crack.
The Heat Is On
A significant feature and attraction of LED fixtures is that they radiate very little infrared heat (IR) together with the visible light. There is also a minimal amount of ultraviolet (UV) content from an LED fixture. This is of great benefit to applications such as museums, where there is great concern about long term exposure to heat and UV and the proximity of fixtures to the public.
A substantial amount of heat is produced inside an LED, and it must be removed and dissipated. Heat management is of critical importance, as LEDs must be kept within a certain temperature range, or the performance and life expectancy will suffer. Unlike tungsten lamps that perform best at literally red-hot temperatures, LEDs like to keep it cool and are very touchy about their environments. The heat stays in the LED chip itself rather than being radiated out along with the visible light. The beam of light stays cool, but heat is, nevertheless, generated.
Well designed LED lighting fixtures typically have substantial heat sinks or fans to help the airflow. Another reason for keeping the working temperature of the LEDs under control is that the individual colors are affected differently by heat. Some LEDs get dimmer, some stay the same, and others get brighter (though not dramatically so). Enjoy the cool light generated by LED fixtures, but be aware that heat is nevertheless generated and needs to be managed by the fixture.
There are also some lesser known advantages of LED lighting. LEDs are fast, very fast. If the electronics are so designed, LED lights can strobe at a very high rate. LED video screens typically run their LEDs at rates of 1,000 to 2,000Hz, so a strobe rate of 15 flashes per second is a breeze. Some control consoles and media servers enable video content to be translated into DMX data for further distribution to LED lights, turning the lights into very bright pixels. These displays are becoming increasingly common, and pixel mapping for media servers is proving to be very popular.
LED lighting has come a long way since its first steps in the late 1990s. While it has some way to go before becoming an alternative to your ellipsoidals, PARs, and followspots, it has taken a slice of the market that will grow as more people discover and experience it and the products improve. There are things that LED lights let you do that no other type of lighting fixture can. They are a welcome addition to the designer's toolbox. More efficient LEDs, a better understanding of the technology and issues, as well as a large number of manufacturers competing for the business should bring us better and more cost effective LED lighting products for many years to come.
Mats Karlsson is product manager for creative light imaging at Barco and can be reached at firstname.lastname@example.org.