Halogen versus LED (Part 3: LED Technology)

A light emitting diode is a solid state device that produces light when current is passed through it. The figure to the right shows the construction of a simple LED which includes:

1.      Anode- accepts the positive power

2.      Cathode- negative power post

3.      High impact plastic- epoxy resin enclosure that protects the LED from the elements and allows the maximum amount of light to be dispersed

4.      Whisker- applies voltage to one side of the semiconductor

5.      Lens-determines beam and focus of the light output

6.      Semiconductor- determines the color of the light the LED produces

7.      Anvil- completes the circuit through the semiconductor

A LED controls current without heated filaments, and is self-contained, so there are no other moving parts to produce failure over time. LEDs emit pure monochromatic color in a narrow frequency range. Distinct colors are produced by altering the material that comprises the chip makeup in the semiconductor. White light will be produced when all parts of the visible spectrum overlap, or in other words, when red, blue, and green light are combined. However, this method is not suitable for high white light output applications; therefore recent technology uses a single LED with a layer of phosphorous on the surface of a blue chip to produce white light that compares to the color temperature of halogen lights. Color temperature is a quantitative index that measures the color characteristics of a light source in terms of degrees Kelvin. The temperature scale, below, shows a relative scale of color temperature. The latest LED white light technology broadens the spectrum of visible light into warmer white light at around 2700°K, as well as, the standard offering at 3000°K and the cool white at 4250°K.

LED fixtures use the number and placement of LEDs to control the pattern and intensity of the light output which varies from the technique that halogen systems utilize. Halogen systems us a mirrored reflector to direct the light pattern, whether spot, or flood. In fact, the nomenclature for MR-16 halogen lamps is described as Mirrored Reflector that has a diameter of 16 eighths of an inch, or 2 inches. The figure to the left demonstrates a cross sectional diagram of a halogen lamp and the one on the right shows the familiar MR-16 halogen lamps.

The LED fixture below, on the left has, 3 LEDs outputting 4.5 watts, and and the LED fixture to it's right shows a a 9 element LED fixture outputting 12.8 watts. As the notation demonstrates, the fixture is offered with beam spreads of 10°, 35°, and 60°, which corresponds to a narrow spot, a mid-flood, and a wide flood pattern. The construction of these fixtures is a one piece construction, which cannot be taken apart, and therefore is not subject to moisture entering the fixture; the major source of failure for halogen fixtures. Halogen fixtures must have a removable lens cover to facilitate lamp changes, and over time, water breaches the seal, collects in the base of the fixture, and causes arcing in the lamp base, resulting in eventual failure of the lamp and base.

Life span for LED lamps differs slightly in its calculation and definition from that of halogen systems. LED fixtures, for exterior applications, have a driver integrated into the fixture circuitry. LEDs are current devices, and exceeding the maximum rated current will result in excessive heat, which is detrimental to the LED circuitry. The driver ensures that the proper amount of voltage and current are presented to the semiconductor, to protect it, and maximize the LED’s operational life cycle.

LEDs are solid state devices that are not subject to catastrophic failure, when protected, with the driver. Operating life is determined by the degradation of intensity over time. When the light output of the LED reaches 50 percent of its original intensity, it is said to have reached the end of its operating life. An LED, that has reached this point, will still continue to output light but its output will also continue to diminish.  This characteristic also occurs in halogen lamps, especially if they exceed their average lamp life. Degradation in halogens occurs due to dirt accumulation, or general fading of the diachronic coating. Generally, LEDs have an operating life of 40,000 to 50,000 hours, and are not subject to failure when exposed to shock and vibration while in operation, as in the case of halogens, and can be cycled on and off without excessive degradation.