LEDs are commonly used in aesthetic, effect, or specialty lighting applications, including architectural highlighting. Most traffic lights and exit signs, for example, now use red, green or blue LEDs.
However LEDs are fast becoming the standard lighting for many countries as they begin to phase out the use of incandescent light bulbs. This is due to the fact that LEDs are superior in both power savings and flexibility of use.
Lumen output is only part of the story and can be misleading. To fully evaluate an LED product one needs to review the overall system efficiency, optical control, thermal management of the LEDs, and know at what point in time the fixture will reach 30 percent lumen depreciation. Products with good optical efficiency and thermal management will be able to deliver more lumens, on average, than traditional HID products.
As the Department of Energy in the United States concluded in its Solid-State Lighting Commercial Product Testing Program:
“Until the field of SSL technologies and supporting knowledge matures, any claims regarding performance of SSL luminaires should be based on overall luminaire efficacy (i.e., from testing of the entire luminaire, including LEDs, drivers, heat sinks, optical lenses and housing), to avoid misleading buyers and causing long-term damage to the SSL market.”
Super-bright white LEDs have the advantage of minimal lumen depreciation, better optical efficiency and high lumens per watt. This means these LEDs can be used to replace traditional HID luminaires. LEDs also have a vastly longer lifespan than traditional lamp sources. The fixture design also must be designed to leverage these inherent advantages of LEDs. A Total Systems Approach is needed for an LED product to bring all these features together.
LED fixtures also have an environmental advantage in that they contain no mercury, last longer and produce less waste, and they are made from fully recyclable materials. Furthermore, the extruded aluminum heat sink is manufactured using 77% post-industrial recycled material.
When you average delivered lumens over the course of 50,000 hours, you’ll see that LED outperforms a 400 watt MH (metal halide) lamp operated in a horizontal position. (60,000 hours is used for this comparison to show three full life cycles of the HID.)
The MH’s lumen depreciation, as well as optical and ballast losses, quickly reduce output of the HID system. Note that there are three re-lamps over 50,000 hours.
Conversely, LED has significantly better lumen maintenance and a more efficient driver. Also note that the LED fixture typically doesn’t need re-lamping from zero to 50,000 hours.
Result: the LED’s average delivered lumens is 74% higher than HID over 50,000 hours.
Junction temperature is the temperature at the point where an individual diode connects to its base. Maintaining a low junction temperature increases output and slows LED lumen depreciation. Junction temperature is a key metric for evaluating an LED product’s quality and ability to deliver long life.
The three things affecting junction temperature are: drive current, thermal path, and ambient temperature. In general, the higher the drive current, the greater the heat generated at the die. Heat must be moved away from the die in order to maintain expected light output, life, and color. The amount of heat that can be removed depends upon the ambient temperature and the design of the thermal path from the die to the surroundings.
Heat management and an awareness of the operating environment are critical considerations to the design and application of LED luminaires for general illumination. Successful products will use superior heat sink designs to dissipate heat, and minimize junction temperature. Keeping the junction temperature as low as possible and within manufacturer specifications is necessary in order to maximize the performance potential of LEDs.
The lifespan of an LED is vastly longer than that of incandescent, fluorescent or HID lamp sources, generally lasting 50,000 hours or longer. Although the LED never really burns out, product lifespan is measured by lumen depreciation.
The Illuminating Engineering Society (IES) currently recommends calculating the life of an LED as the point at which the LED reaches 30 percent lumen depreciation.
Remember, a 50,000-hour rating is not equivalent to lamp life rating. LED life is rated where it has reached 30 percent lumen depreciation. At 50,000 hours an LED would still be operating, but at a decreased lumen output.
Based on how long a fixture is illuminated per day, here’s what 50,000 works out to:
24 hours a day 5.7 years
18 hours a day 7.6 years
12 hours a day 11.4 years
8 hours a day 17.1 years
LEDs are used more and more as incandescent light replacements. There are at least 4 possible advantages to using LEDs compared to incandescent sources:
1. longer lifetime, i.e. years rather than months of service
2. significant power savings
3. reduced heat output
4. deep saturated hue (color) of output without filters.
With the current state of the art, incandescent bulbs actually put our more visible light per watt than most LEDs. So, if you want white light or if you don’t care about color of illumination, incandescents are somewhat more efficient than any combination of LEDs, and they remain more efficient than the newest ‘white LEDs’. However, if you want single color illumination, LEDs can be much more efficient, since, in that case, much of the light output of the incandescent source is absorbed by a filtering system. If we include fluorescent fixtures, here is the general picture of light source efficiency:
- As of today, fluorescent fixtures remain the clear winner in terms of output in lumens/watt. Halogen incandescents are second. LEDs are third.
- LEDs win the race if what you want is light of a single color available from efficient LEDs. The losses involved in filtering fluorescents and incandescents are such that LEDs win out here. Examples include stoplights and auto taillights.
- LEDs provide the longest lifetime by far.
- If your supply is some low DC voltage, driving circuits for LEDs are extremely simple and cheap compared to same for fluorescents.
- White LEDs, although they do not presently provide as many lumens/watt as incandescent bulbs, can be useful because of the long life and low heat output.
For reference, fluorescent fixtures put out about 30 lumens per watt. Incandescent bulbs put out about 8 lumens per watt.
The easiest way to measure total power output is to position the LED very near to a large photodetector.
You should be able to get large area photodiodes from: Vactec, EG&G, Hammamatsu, IPI Centronic, Optodiode, Silonix, among others. Here’s one way to do it: Noonday sun at your latitude irradiates the surface of the earth with about 800 Watts/sq-m. This is equivalent to 800/10,000 = 80mW/sq-cm. Since Si efficacy for sunlight is about 0.5Amps/Watt, you should get around 40milliAmps for each square cm of solar cell. Supposing that you get a smaller value, then you can assume that your particular solar cell is less sensitive than 0.5A/W and calculate a calibration value appropriately.
Example: you obtain a 1″x1″ solar cell, i.e., about 6.5sq-cm. At midday on a clear day, you measure about 200mA short circuit current max as you orient it toward the sun. You calculate 80 * 6.5 = 520mW incident on the cell, so you were expecting more like 260mA. This means that your cell has more like 0.38A/W efficacy.
Considering that LEDs emit their energy in a narrow spectral band, it may seem surprising that White LEDs are available. There are presently 2 kinds of LEDs that have a visual white appearance. The earlier type actually consists of red, green, and blue LED chips packaged closely together. This kind of array can produce many colors, but with the appropriate currents in each chip, the combination appears white.
A newer kind of white LED consists of a blue LED combined with special phosphors that give off a broadband glow when excited by blue light.
There are some factors that one has to consider:
Manufacturer’s claims vary wildly about the efficiency of LEDs and can often be quite misleading. The simplest way to ensure that you are comparing “like with like” is to look at the efficiency in Lumens per Watt. That is, how much light do you get for each Watt of electricity used. Today (early 2010), LEDs that are in production range in efficiency from about 20 to 60 Lumens per Watt. New developments should increase this to around 80 or 100 Lumens per Watt over the next few years. This compares with around 10-15 Lumens per Watt for old-fashioned incandescent lamps and 50-100 Lumens per Watt for fluorescent tubes.
- Colour rendering
Early LEDs were mostly a very much higher colour temperature (ie a bluish light) than the typical 2700K colour temperature of a warm white incandescent lamp. This has led many people to think that LEDs gave a “cold” light. LED technology has however advanced a great deal in the last couple of years and some types of LED are available in a range of colour temperatures from warm white (2700 or 3000K) up to daylight (6000K and beyond). LEDs with a higher colour temperature do however tend to be slightly more efficient than those with a lower colour temperature so some companies offer these as a way of making their products seem brighter. To replace standard incandescent or halogen lamps, a colour temperature of around 2700K to 3300K is preferred. Some companies use non-standard terms when describing the colour of LEDs such as “natural white” or “office white” which cannot be readily compared with other products. To ensure that you are comparing like with like, always check the colour temperature.
- Colour rendering
The colour rendering of LEDs varies quite markedly between brands. For some applications this is not particularly important but for ambient lighting it is important to check the quality of light emitted. The LEDs that we offer for ambient lighting all have good colour rendering, comparable to that of a good quality fluorescent tube. Many manufacturers do not however quote a colour rendering index for their LEDs so we are rarely able to quote specific figures.
- Heat output
LEDs give off relatively little heat but do run reasonably warm and often incorporate a heat sink to dissipate any excess heat and maintain the LEDs at optimum operating temperature. Compact lamps such as miniature spot lights can thus still get reasonably warm whereas larger bulbs and candles certainly cool enough to touch when operating.
- Rated Life
The one area where LEDs really score highly is in their rated operating life which typically ranges from 20,000 to 50,000 hours. This is 20 to 50 times longer than a typical incandescent lamp so LEDs are ideal for use in areas where maintenance is difficult or long life is important.
Like the traditional fluorescent tubes, LED tubes generally have a two-pin fitting at both ends of the tube. There are many variations of fluorescent tubes, namely:
- T2 (7mm)
The ultra slim T2 (7mm) tubes use a fitting called a W4.3 with 4.3mm wide push fit fitting.
- T4 (12mm)
The undercabinet T4 (12mm) tubes use the G5 fitting with 5mm between the two pins.
- T5 (16mm)
The small T5 (16mm) tubes use the G5 fitting with 5mm between the two pins.
- T8 (25mm)
Standard size T8 (25mm) tubes use the G13 fitting with 13mm between the two pins.