Learning Center / LEDs for outdoor lighting / Advantages and disadvantages of LEDs
1.1 Technical introduction : Physical function
Like a normal diode, the LED consists of a chip of semiconducting material impregnated, or doped, with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. The possibilities of LED light cover a wide spectrum, from the infrared LED in your television’s remote control to the sterilizing ultraviolet LED light used in the medical field.
1.2 Advantages of using LEDs
> LEDs produce more light per watt than do incandescent bulbs; this is useful in battery powered or energysaving devices.
> LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
> The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.
> When used in applications where dimming is required, LEDs do not change their color tint as the current passing through them is lowered, unlike incandescent lamps, which turn yellow.
> LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.
> LEDs, being solid state components, are difficult to damage with external shock. Fluorescent and incandescent bulbs are easily broken if subjected to external shock.
> LEDs can have a relatively long useful life. Reports estimates 60,000 hours of useful life, though time to complete failure longer.2 Fluorescent tubes typically are rated at about 30,000 hours, HID and MH are rated anywhere between 10,000 and 24,000 hours and incandescent light bulbs at 1,000–2,000 hours.
> LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent or HID bulbs.3 This provides extra safety for any area illuminated by LEDs. Even if the LEDs dim over time, they never fail completely like HID sources before needing to be replaced. LEDs need to be replaced only after they reach 30% lumen depreciation (17-20 years for quality LEDs).
> LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds; Philips Lumileds technical datasheet DS23 for the Luxeon Star states "less than 100ns." LEDs used in communications devices can have even faster response times.
> LEDs can be very small and are easily populated onto printed circuit boards.
> LEDs do not contain mercury, unlike compact fluorescent lamps.
1.3 Disadvantages of using LEDs
> On an initial capital cost basis, LEDs are currently more expensive, measured in price per lumen, than more conventional lighting technologies. The additional expense partially stems from the relatively low lumen output, combined with the cost of the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass other sources. In December 2007, scientists at Glasgow University claimed to have found a way to make Light Emitting Diodes brighter and use less power than energy efficient light bulbs currently on the market by imprinting holes into billions of LEDs in a new and cost effective method using a process known as nanoimprint lithography.4 Around the same time, in Montreal Canada, Lumec inc. developed an LED light engine that consumes 20% to 30% less energy than HPS (high pressure sodium) and 40% to 50% less than MH (metal halide) while delivering comparable photometric performance, if not better, than HID lights.
> LED performance largely depends on the ambient temperature of the operating environment. Driving the LED hard in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. This is especially important when considering automotive, outdoor, medical, and military applications where the device must operate over a large range of temperatures, and is required to have a low failure rate. The most heat resistant LEDs available commercially, such as those used by Lumec inc. In their light engine, the LifeLEDTM can function at optimal efficiency from -40°C to +50°C(-40°F to 122°F).