Replacing incandescent lamps with LEDs
Currently, there is interest in high efficiency, long-life, light emitting diode (LED) lamps for use in factories, institutional, and commercial applications, because the costs of electricity for lighting and labor for bulb replacement are significant. The goal of the LED manufacturers is to build a very high-brightness white LED that is economical and efficient enough to be used for illumination. To gain widespread acceptance as a legitimate light source for general lighting, LEDs must be able to economically and reliably deliver illumination levels of white light of a quality within today’s acceptable standards.
Theory of operation
An LED is a PN junction semiconductor that emits photons when forward biased. The emission of light occurs when minority carriers recombine with carriers of the opposite type in the band gap of the diode. The wavelength of the emitted light — which determines its color — varies according to the semiconductor material.
LEDs are processed in wafer form similar to silicon integrated circuits, and broken out into dice. The simplest packaged LED is the indicator lamp. Typically, LEDs have a mean time between failures (MTBF) of more than 100,000 hr.
Today’s ultrabright LEDs exceed the light output of incandescent and halogen lamps. They don’t have the maintenance requirements associated with filament lamps. LEDs can be dimmed using a pulse-width modulation (PWM) circuit, which delivers energy in pulses of varying duty cycle.
History of LEDs
The first reports of a device with properties similar to LEDs dates back to 1906 when Henry Round reported electroluminescence while experimenting with carborundum. However, LEDs didn’t become commercially available until the early 1960s. Texas Instruments sold an infrared (IR) device for $130 and GE distributed red LEDs through the Allied Radio catalog for $260. They were expensive and sold in low volumes.
IBM used LEDs as on-off indicator lights on circuit boards in a mainframe computer constructed around 1964, which marks the first time LEDs were used to replace incandescent lamps. LEDs used less power, could be mounted directly on the circuit board, and had a much longer life expectancy, which made using LEDs attractive from a maintenance perspective.
In the mid 1980s, the U.S. military began gradually replacing tungsten filament indicators with LEDs, and they began appearing in elevator cars. As with the IBM application, LEDs were designed into pieces of equipment. They were mounted on printed circuit boards (PCBs), mounted in equipment panels and face plates using specific mounting bezels with wires soldered to their leads, and plugged into sockets made specifically for LEDs.
LED performance made a leap in the early 2000s. Companies started manufacturing flashlights using LEDs instead of the traditional incandescent bulb. As improvements were made in brightness and color, LEDs moved farther into tungsten territory. They appeared in traffic signals, home entertainment, and decorative lighting.
Today, LEDs are used in many industries from automotive to architectural lighting applications. Industrial plants are discovering the benefits of replacing traditional bulbs with LED lamps. For example, hundreds of incandescent lamp part numbers now have direct LED-based replacements. Most LED suppliers have extensive cross-reference literature and databases. Standard lamp bases are available, allowing LED lamps to replace incandescent lamps without having to retrofit equipment.
Flashlights continue to get brighter. Some currently available flashlights suitable for industrial use boast as much as 1800 foot-candles (fc) of white light. LED floodlights, work lights, and luminaires for general-purpose lighting applications are available as well.
LEDs have enjoyed continued success because they use considerably less power and last much longer than tungsten filament incandescent bulbs. LED lamps use only 10% to 20% of the energy consumed by equivalent incandescent lamps. An average LED life span can exceed 100,000 hr — more than 11 yr.
LEDs are solid-state devices, which make them virtually immune to electrical and mechanical shock — unlike incandescent lamps, which have filaments that are very susceptible to electrical and mechanical shock. Electrical shock comes from constant on-off transitions, transients, and surges; mechanical shock comes from bumping, jarring, and other forms of vibration. Also, LEDs produce very little heat, making them an attractive alternative to incandescent lamps in applications where heat is an issue, such as biotechnology, chemical, and food processing.
LEDs had to overcome physical and technological issues to get where they are today. The primary hurdles have been drive current, packaging, color, and price. Although these issues have been addressed, they still exist to some degree. Drive current directly affects LED lamp output and lamp life. LEDs are inherently robust. They are capable of delivering high output at high current, as long as heat is extracted properly.
Packaging issues include thermal management, current handling capability, and color. Advanced device packaging allows adequate heat dissipation and increased current capacity. Packaging also affects color, which is extremely important in applications that require white light. Use of LEDs as illumination sources requires white light with a degree of “warmth.” This requirement must be met if LEDs are to make any headway in replacing incandescent lamps for general-purpose illumination. Fluorescent lighting addressed this issue. And it appears that LEDs are rising to meet the challenge as well.
The cost-effectiveness of LEDs depends on the application. Today, the system price is high for replacing conventional incandescent lamps with LED-based technology. However, for established LED applications, such as control panel indicators and annunciator lamps, LEDs are more cost effective. Although the unit price is higher, the lower power consumption and longer lamp life help offset the initial purchase price. Some plants can justify the higher cost of LEDs for this application based on lower maintenance costs alone.