LED By Example

Note: This is an excerpt from Charles Platt’s newest book, The Encyclopedia of Electronic Components, Volume 2, the second in his reference trilogy. Unlike the “Learning by Discovery” emphasis of Charles’ popular Make: Electronics and Make: More Electronics, the Encyclopedia series doesn’t have step-by-step instructions for experiments, but rather, is intended to pull together the best information on modern electronics in one elegant reference guide.

On the rising primacy of the LED

At the time of this writing, products are evolving rapidly in the field of LED area lighting. A shared goal of manufacturers is to increase efficiency while reducing retail price to the point where high-brightness LEDs will displace fluorescent tubes for most low-cost lighting applications. Within a decade, as LED area lighting continues to evolve, some of these examples may look quaint.

What’s more, since 1965 the cost per lumen of light from a given color of LED has decreased by about a factor of 10, while the maximum number of lumens emitted by an LED package has increased by a factor of 20, during each decade. This is known as Haitz’s Law, named after Dr. Roland Haitz of Agilent Technologies.

The advantages (and disadvantages) of incandescent and fluorescent lamps are relatively well known. Here are some advantages of LED area lighting:

• While the life of an incandescent lamp for room lighting can be as little as 1,000 hours, LED area lighting typically claims up to 50,000 hours.
• The lifetime of an incandescent bulb is the average time it can emit light before catastrophic failure. The lifetime of an LED is the average time it can emit light before gradually dimming to 70% of its rated output. This is a much gentler, less inconvenient failure mode that does not require immediate replacement.
• Unlike a fluorescent light or incandescent bulb, the LED does not contain hot tungsten that fails as a result of erosion.
• Unlike a fluorescent light, an LED does not contain mercury, and therefore does not require special recycling arrangements that entail associated fees.
• While fluorescents can have difficulty starting in low temperatures, an LED is not sensitive to a cold environment.
• Bright LEDs are available in a wide range of colors that do not require filtering. Filters greatly reduce the efficiency of incandescent bulbs when they are used in applications such as traffic signals or rear lights on automobiles.
• High-brightness LEDs can be dimmable. Fluorescent lights are usually not dimmable, or perform poorly in this role.
• LEDs are inherently directional, because the die radiates light at an angle of 90 degrees to its plane. This makes it ideal for ceiling mounting, where as much light as possible should be directed downward. A fluorescent tube or incandescent bulb often requires a reflector which reduces the overall efficiency.
• LEDs are insensitive to cycling. The life expectancy of an incandescent bulb or (especially) a fluorescent tube is reduced by cycling it on and off.
• No flickering. Fluorescent tubes may start to flicker as they age.
• No electrical interference. Fluorescent tubes can interfere with AM radio reception and some audio devices.
• Safe from breakage. LED area lighting does not necessarily use any glass.

However, high-brightness LEDs still have some barriers to overcome:

• Cost. In the United States, before 60W incandescent bulbs were legislated out of existence, they could be sold profitably for less than $1 each. A T8 fluorescent tube, measuring 1” diameter and 48” long, currently costs between $5 and $6 (retail) but has a life expectancy in the region of 25,000 hours, and uses only 20% of the power of an incandescent bulb to generate two to three times as much light. Clearly the fluorescent tube is a more economical choice, despite the price of the electronics that must be included in the fixture to start the tube. By comparison, currently the purchase price of an LED tube is three times that of a fluorescent tube. It may last twice as long, but is not significantly more efficient, generating perhaps 100 lumens per watt while a fluorescent is typically capable of 90 lumens per watt. Prototype high-brightness LEDs have exceeded 200 lumens per watt, and should be competitive with fluorescents by 2020, but even then, migration will take time.
• Heat sensitivity. Heat reduces the light output and the lifespan of LED fixtures.
• Placement issues. Because LEDs are heat sensitive, they must be installed in locations that do not become excessively hot, their heat sinks must be correctly oriented, and they must have adequate ventilation.
• Color shift. Heat and age may cause the color temperature of an LED to shift slightly, as the color is usually derived from two types of phosphors.
• Nonuniformity. Manufacturing inconsistencies can cause LEDs of the same type to display slightly different color temperatures. Fluorescents and incandescents are more uniform.
• Lower heat output than incandescents. While this is an advantage from the point of view of efficiency, it can be a disadvantage in applications such as traffic signals or airport runway lighting where waste heat can help to keep the lights free from snow or ice.

Volume 2 of Charles Platt’s Encyclopedia of Electronic Components covers discrete semiconductors; integrated circuits, both analog and digital; light source indicators and displays; and electronic sound sources. Volume 3, anticipated for publication in May 2015, centers solely on sensors.