Light-emitting diode (LED) Christmas lights are quickly gaining popularity in many places due to their low energy usage (about one tenth the energy used by incandescent bulbs), very long lifetimes, and associated low maintenance. Colored LEDs are also far more efficient at producing light than their colored incandescent counterparts.
There are two types of LEDs: colored LEDs and white LEDs. Colored LEDs emit a specific color light (monochromatic light), regardless of the color of the transparent plastic lens that encases the LED's chip. The plastic may be colored for cosmetic reasons, but does not substantially affect the color of the light emitted. Because the light is determined by the LED's chip rather than the plastic lens, Christmas lights of this type do not suffer from color fading. In addition, the plastic lens is much more durable than the glass envelope of incandescent bulbs.
White LEDs are similar to colored LEDs in most respects such as power and durability, but utilize a two-stage process to create the white (polychromatic, or broad spectrum) light. In the first stage, the LED actually only produces one color of light, similar to any other LED. In the second stage, some of the blue or violet-blue is absorbed by a phosphor which fluoresces yellow, imitating the broad spectrum of colors which our eyes perceive as "white". This is essentially the same process used in fluorescent lamps, except for the use of an LED to create blue light rather than excited gas plasma to create ultraviolet.
White LEDs can be used as white Christmas lights, or can be used to create any other color through the use of colored refractors and lenses, similar to the more commonly used incandescent bulbs. Color fading may therefore occur due to the exposure of colored plastics to sunlight or heat, as with ordinary Christmas lights. Yellowing may also occur in the epoxy "bulb" in which the LED is encased if left in the sun consistently.
LEDs use much less electricity (only 4 watts for a 70-light string) and have a much greater lifespan than incandescent lamps. Since they are constructed from solid state materials and have no metallic filaments to burn out or break, LEDs are also much less susceptible to breakage from impact or rough handling.
Although LEDs themselves are long-life devices, older or lower-quality strands of LED-based Christmas lights can suffer from early failure. This is particularly so with blue ones, which are the newest and most expensive, and therefore prone to cost-cutting; in addition, spares are rarely included with sets. Most LED-based Christmas lights use copper wire which connects to the aluminum-based wires of the LEDs. Exposing this combination of metals to moisture can result in galvanic corrosion inside of the lamps' sockets, causing them to stop working. Many other sets use cheaper steel leads on the LEDs, which instead rust, leading to the same result. Some newer and higher-quality sets of LED Christmas lights have each LED permanently mounted in a non-removable weathertight base to keep out rain and other moisture, helping to prevent such corrosion; however, this prevents the user from replacing defective bulbs.
Most common consumer LED lamps produce intense, deep, pure colours, versus incandescent bulbs which generally have subtler, yellow-tinted colours, often somewhat faded especially if used outside. Blue tends to be the dimmest incandescent color, but the brightest in LED, while yellow is just the opposite. Very early strings of LED lights were noticeably dimmer than incandescent bulbs, but now are often noticeably brighter. These factors combine to give LED lamps a distinct aesthetic from older incandescent strings, although white LEDs behind coloured lenses do offer the ability to provide a more incandescent-type appearance with most of the benefits of energy efficiency. However, most use coloured-chip type LEDs that produce the intense colours. This is largely due to the maturity of coloured LED versus newer white LED technology, and as the technology improves so will the ability to change the aesthetics of the lamps, at lower cost than at present. As of 2007, "warm white" LED sets are common for the first time in U.S. stores, having a color similar to a compact fluorescent light. However, this color needs to be more of an orange tint to match the color of such small bulbs, because they burn at a lower temperature. Still, this is a significant improvement from the very cold-white (and often irregularly-tinted) color of early white sets.
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