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What’s available, how much, where from? Looking Into LEDs Light Emitting Diodes (LEDs) are everywhere. They’re available in all colours of the rainbow and range from quite dim indicators through to spectacular-brightness lights suitable for finding your way in the dark. G one are the days when LEDs were just used as replacements for dial lamps and indicators. I remember (probably back in the early ’70s) when a then-much-younger Dick Smith used to advertise what was a pretty pedestrian multi-band radio receiver with, wait for it, a “LED Lamp Indicator” which moved up and down the dial as you tuned it. Wow! (Of course he sold thousands!). That was only about a decade after LEDs were first commercially produced. Even then, they were still relatively expensive and were rather dim by today’s standards. LEDs have certainly come a long way, even in those thirty-ish years. But let’s go back even further and have a look at where LEDs came from. A brief history of LEDs LEDs have been around in one form 8  Silicon Chip Ever wondered what’s inside a LED? By Ross Tester or another for almost one hundred years (although there is some uncorroborated evidence that the phenomenon was first noted back in 1861). In 1907, Joseph Henry Round observed a very dim yellow glow from a piece of Silicon Carbide (SiC) when subjected to small electric currents. This was in the form of a “Cat’s Whisker” diode used commonly in ensuing decades as a detector for crystal set radio receivers. (Galena was commonly used but Silicon Carbide also worked). Jump forward twenty years: German experimenters working with phosphor materials made from Zinc Sulphide doped with Copper (ZnS:Cu) also produced a dim light – unfortunately, too dim to be of much practical use. Then in 1936, a report was published by George Destriau on the emission of light by Zinc Sulphide (ZnS) www.siliconchip.com.au At left (facing page) is an array of high brightness and special effect LEDs, LED fittings and collimators/diffusers. On the left are some of the Luxeoon Star range of 1W, 3W and 5W high brightness LEDs (from Prime Electronics or ATA). In the middle, what look like standard LEDs are in fact microprocessor-controlled flashing coloured LEDs, also from Prime Electronics, who also supplied the lenses. The various LED fittings are from Jaycar. powder and an electric current. Destriau is widely credited with inventing the term “electroluminescence”. While limited experimentation was no doubt going on over the years, it wasn’t until the 1960s that the first LEDs, only somewhat as we know them today, were produced, following British (and perhaps German) research of the previous decade. These were based on the semiconductor Gallium Arsenide (GaAs). Unlike modern GaAs LEDs, which produce visible (usually red) light, the first laboratory LEDs produced only infrared light and even then had to operate at supercool levels to work – usually by immersing them in liquid nitrogen! Professor Nick Holonyak Jr. (1928 - ) is credited with developing the first practical visible-spectrum (red) LEDs in 1962. These were produced using Gallium Arsenide Phosphide (GaAsP) on a GaAs substrate. Using GaAsP allowed much more efficient red LEDs to be made and also allowed orange light for the first time. Another decade on and Gallium Phosphide (GaP) LEDs were producing a pale green light. The first yellow LEDs were actually dual GaP chips in one package, one producing red and the other green. True yellow-emitting LEDs using Silicon Carbide (SiC) were produced, at least on an experimental basis, a little later. However, these suffered a similar problem to those first SiC yellow LEDs – low light output. In the 1980s, superbright LEDs started to appear, using Gallium Aluminium Arsenide Phosphide (GaAlAsP) – first in red, then in yellow, then in green. Around the turn of the decade, the first of the ultrabrights started appearing, using Indium Gallium Aluminium Phosphide (InGaAlP) in orange-red, orange, yellow and green colours. Blue LEDs started to appear in the early 1990s. The first used Silicon Carbide, but this gave way to the much www.siliconchip.com.au These close-ups of the 1W ultra-white LEDs from Oatley Electronics show some of the structure deep inside the LED. These are rated at 20 lux, drawing 300mA at 3.6V. They retail for about $15 each. brighter blue of Gallium Nitride (GaN) around the middle of the decade and the even brighter Indium Gallium Nitride (InGaN) blue and green LEDs late in the decade. The next big breakthrough came with white LEDs which, in fact, do not have diodes emitting white light. They are in fact a high-intensity blue LED with the chip coated in a fluorescent phosphor. The phosphor absorbs the blue light and then fluoresces at the chosen colour – white is the most common but virtually any colour can be obtained using this technique. The most recent LED development, only now beginning to appear on the market, is the ultraviolet LED which produces no visible light but causes other colours to fluoresce (you’ve probably seen this effect with socalled “black light” fluorescent tubes at a disco). So as we can see (pardon the pun), LEDs have gone through virtually the entire “light” spectrum from infrared through all visible colours to ultraviolet. Experimentation is continuing to see just how far up the spectrum diodes (no longer called LEDs!) can be made to emit. Really bright LEDs The story of LEDs is not just about colour. As we have noted, superbright and then ultrabright LEDs have changed the way we think about the devices: first a curiosity in the laboratory, then an indicator device on a panel; later fashioned into shapes conveying messages (numeric and alphanumeric displays); then combined into arrays of colours, capable of displaying colour pictures; and of course, most recently into lighting devices in their own right. Already, many LEDs are so bright that you risk eye damage by looking straight into them. Lasers carry warnings about eye damage – it won’t be too long before many LEDs do too! The development of LEDs as lighting devices continues – already they are being sold to replace many other forms of domestic, industrial, automotive and outdoor lighting. It is perhaps only the relatively high price of the devices that prevents their wider usage. That will change in time, probably sooner rather than later. With more efficient use of electricity high on the agenda in many areas (eg, California with their blackouts last winter) the high efficiency of LEDs as lighting devices, not to mention their longevity, is being viewed with more and more interest. Measuring light output It’s not easy to compare apples with apples when it comes to the brightness of LEDs, or any other light source, because so many different units are applied – invariably, the ones which make the device look best to the manufacturer (or their PR firm!). High brightness LEDs are generally rated in Watts, similar to traditional Back and front of the Luxeon Star 3W LEDs from Prime Electronics and ATA. The back is all heatsink, designed to be bolted to a larger heatsink. These are also available in 1W and 5W models, ranging in price from $15.95 to $54.45. April 2004  9 This particular Luxeon Star/O is a 1W version of those on the previous page, here fitted with a built-in 20° reflector. They’re a little over $20 each from Prime Electronics. Collimating lenses are designed to mount directly onto all standard Luxeon Star 1W, 3W and 5W LEDs. They are available in wide, medium, narrow and elliptical beams. light bulbs, or in Lux, while ordinary (ie garden-variety) LEDs are usually quoted in mCd (millicandela), the candela being one of the seven base SI units. One problem is that the candela as a unit for coloured LEDs is that it is based on light at a particular frequency (540 x 1012Hz) – green. That’s fine for green LEDs (well, LEDs of that particular green) but what about red LEDs which will have little, if any green output? A second problem is that the light output is defined as being in a particular direction. When a LED “viewing angle” is quoted, this is defined as the point off-axis where the light ouput is reduced by 50%. All LEDs are directional; as a general rule the higher in brightness, the more extremely directional they become. There are several types of lenses and collimators available which reduce this, to an extent. To add further to the confusion, someone comes along and rates their LEDs in Lux, which is a measure of illuminance, (defined as lumens per square metre squared) – not a measure of lamp brightness at all! As you can see, there’s much more to the light output of a particular LED than noting its claimed “output”. At SILICON CHIP, we’ve seen 2000 and 3000mCd white LEDs that, at least to the naked eye (and that’s what matters!) easily outperform LEDs rated at 10000 and even 15000mCd. There is no such thing as a perfect light source – one where all of the energy applied is converted to light. In an incandescent bulb (a lamp with a glowing filament), for example, there is a lot of energy lost as heat. “Cool” flourescents and even LEDs lose energy as heat, though nowhere near as much as filament bulbs. The light output of a bulb is measured in lumens per watt. Incandescent bulbs, the most common form of light, are cheap to buy but are inefficient, generating from about 16 lumens per watt for a domestic tungsten bulb to 22lm/W for a halogen bulb. Fluorescent tubes are more efficient, from 50 to 100lm/W for domestic tubes. While they allow large energy savings, they require special starting and driving circuitry and are bulky and fragile. LEDs have fallen somewhere between incandescent and fluorescents in terms of efficiency - up to 32lm/W – and are more robust than either. Until now, they have been expensive, although their cost is falling. If you want to delve deeper into the way LEDs (or any other light sources) are rated and measured, there are literally thousands of pages of information available on the ’net. Google a few key words such as LED brightness output and ratings and you’ll see what we mean. Looking into LEDs We deliberately chose the title to this short article because these days, that’s something you must not do. Many of today’s ultrabright LEDs are more than powerful enough to cause you pain; perhaps even damage to your retina. Multiple LED arrays fitted in miniature bayonet cap (left) and miniature Edison screw (right) assemblies, intended to replace standard torch globes. These are just some of the versions available from Jaycar and retail for a little under $30 each. 10  Silicon Chip LED INTENSITY The unit of measure commonly used to describe LED intensity is the millicandela (mcd). 1000mcd = 1cd. Candelas measure how much light is produced as measured at the light source in a specific direction. The unit of measure commonly used for most other light sources is the Lumen. Lumens measure how much light actually falls on a surface. How do you convert lumens to mcd? There is not an exact conversion as they are different types of measurement but here is a rough conversion: If you divide the number of lumens by 12.57 you can get an approximate equivalent in candelas but this itself can be misleading as there is no qualification of direction. There’s been an adage around since the invention of Lasers: never look a laser in the eye. We’d also apply that to LEDs. Types of LEDs Several SILICON CHIP advertisers regularly feature a variety of LEDs. We’ve already covered the standard, high brighness, super bright and ultra bright models – available in virtually every colour of the rainbow (and then some!). But now there are even LEDs with a built-in microprocessor chip to drive various colour displays from the same LED – fading, for example, from red to green to blue and then various combinations of those colours, in various flash and fade sequences. Of course, they’re significantly more expensive than “standard” LEDs but that will change over time, just as the price of all other LEDs has fallen. Other LEDs of significance (to this article) are combinations of LEDs in various fittings, designed to replace standard globes or lamps. For example, there are now several MES or MBC (miniature Edison screw or miniature bayonet cap) fittings with highbrightness LEDs fitted. Another innovation is a replacement for 20W and www.siliconchip.com.au Halogen replacement highbrightness LED fitting from Jaycar. These sell for around $30 each. 50W halogen bulbs, now used by their millions in home, office and shop lighting. While architects love ’em, we have always been critical of their energy wastage (they run very hot) and their proven ability to cause fires if combustible material is too close. The heat has also been a big problem in closed shop window displays, where it has little chance of dissipating. Now you can buy a fitting, the same size as the halogens, consisting of twelve high brightness LEDs. We haven’t done tests to see how the light levels compare but these look very promising. And we would expect them to last significantly longer than halogen bulbs. This short look at current LED development is by no means exhaustive – there are plenty more suppliers around and there are also many more types. We’ve just scratched the surface of the subject here – and we haven’t even mentioned some of the work being done in street signs, traffic lights, etc. Again, there’s a wealth of information on the ’net if you SC want to delve into LEDs further! Supplier Websites: Prime Electronics Oatley Electronics Jaycar Electronics Alternative Energy Assoc. Outdoor Bright sun Hazy day Cloudy bright Cloudy dull Very dull Sunset Full moon Starlight Indoor Operating theatre Shop windows Drawing office Office Living rooms Corridors Good street light Poor street lighting www.primelectronics.com.au www.oatleye.com www.jaycar.com.au www.ata.org.au Illuminance (lux) 50k - 100k 25k - 50k 10k - 25k 2k - 10k 100 - 2k 1 - 100 0.01 - 0.1 0.001 - 0.01 Luminance (cd m-2) 3k - 6k 1.5k - 3k 600 - 1.5k 120 - 600 6 - 120 0.06 - 6 0.0006 - 0.006 0.000006 - 0.00006 5k - 10k 1k - 5k 300 - 500 200 - 300 50 - 200 50 - 100 20 0.1 300 - 600 60 - 300 18 - 30 12 - 18 3 - 12 3-6 1.2 .006 How bright are ‘things’? This table gives you some idea of the level of luminance and also the illuminance in lux. When you consider that ultrabright LEDs are quoted as around 20 lux, there is still a fair way to go . . . www.siliconchip.com.au April 2004  11