Benefits and drawbacks of LEDs

Comments on this article are welcome; please send them toTim Whitaker, the Editor of LEDs Magazine.

Lifetime
As solid-state light sources, LEDs have very long lifetimes and are generally very robust. While incandescent bulbs may have an expected lifetime (to failure) of 1000 hours, LEDs are often quoted of having a lifetime of up to 100,000 hours - more than 11 years. However, this figure is extremely misleading; like all other light sources, the performance of LEDs degrades over time, and this degradation is strongly affected by factors such as operating current and temperature.

At present, there is no standard definition of lifetime for LEDs, although various parties have suggested that lifetime should be the time taken for the LED’s output to fall to some percentage (such as 70% or 50%) of its original value.

Standardization
The general lack of standardization in the LED field is an ongoing issue. Various standards relating to LEDs exist in areas such as automotive lighting and traffic signals. Other efforts are being conducted by bodies such as CIE, NEMA and IES.

Low maintenance
The long lifetime of LEDs reduces the need to replace failed lamps, and this can lead to significant savings, particularly in the cost of sending out maintenance crews. This also makes LED fixtures useful for installation in relatively inaccessible locations. However, if tasks like cleaning the light fixture or performing electrical checks need to be carried out regularly, then the light sources could be replaced at the same time, negating the "low maintenance" advantage.

Efficiency
LEDs are high-efficiency light sources. White LEDs with efficacies of 25 lm/W and up are commercially available, exceeding the performance of incandescent and some fluorescent sources. The directional nature of light produced by LEDs allows the design of luminaires with higher overall efficiency.

Low power consumption
The low power consumption of LEDs leads to significant energy savings that can often drive the installation of LED-based systems, for example traffic signals. National programs to develop effective solid-state lighting industries in the US and Japan have been driven by the potential energy savings associated with using LEDs.

Brightness
Although LEDs have high efficiency and consume a small amount of power, the devices produce a small total number of lumens. For example, a 60 W incandescent bulb with an efficiency of 20 lm/W produces 1200 lumens. A one-watt LED with an efficiency of 30 lm/W produces only 30 lumens i.e. 40 such LEDs are required to produce the same amount of light as the incandescent bulb.

Heat
LEDs don't produce heat in the form of infrared radiation, which makes incandescent bulbs hot to the touch. The absence of IR radiation allows LED fixtures to be positioned in locations where heating from conventional sources would cause a particular problem e.g. illuminating food or textiles.

However, LEDs do produce heat at the semiconductor junction within the device. The wall-plug efficiency (optical power out divided by electrical power in) of LED packages is typically in the region of 5-40%, meaning that somewhere between 60 and 95% of the input power is lost as heat.

Without very efficient thermal management and heat sinking this causes the junction temperature of the LED to rise, which causes the LED characteristics to change. Driving LEDs above their rated current causes the junction temperature to rise to levels where permanent damage may occur.

Cost
In many applications, LEDs are expensive compared with other light sources, when measured by metrics such as “dollars-per-lumen”. LED manufacturers continue to work towards reducing their production costs while at the same time increasing the light output of their devices.

However, the high initial cost of LED-based systems is offset by lower energy consumption, lower maintenance costs and other factors.

Small form-factors
LEDs are very small - typical high-brightness LED chips measure 0.3 mm by 0.3 mm, while high-power devices can be 1 mm x 1 mm or larger. There are many examples where the availability of small, high-brightness devices have enabled significant market advancement. The obvious example is in mobile phone handsets, where blue, green and white LEDs are now used in most models to backlight keypads and liquid-crystal display (LCD) screens.

Instantaneous switch-on
LEDs switch on rapidly, even when cold, and this is a particular advantage for certain applications such as vehicle brake lights.

Color
LEDs are available in a broad range of brilliant, saturated colors (although performance varies across the spectrum), and white devices are also available. Modules containing different-colored LEDs (typically red, green and blue, or RGB) can be tuned to a huge range of colors, and easily dimmed. RGB modules provide a much wider gamut of colors than white LEDs or other traditional white light sources, which is a particular advantage in applications such as backlighting liquid-crystal displays (LCDs).

RGB LEDs and color mixing
LED characteristics change with time, temperature and current, and from device to device. For RGB LEDs, the performance of different-colored devices changes at different rates. This can result in variation of lamp color and intensity, and poor reproducibility.

White LEDs
The color of white LEDs can be very inconsistent, although manufacturers have narrowed their binning ranges. White LEDs with the same correlated color temperature can have different color tints perceptible to the human eye.

Semiconductor processing
Fabricating LEDs is a complex high-temperature process involving the growth of crystalline layers across the surface of a semiconductor wafer. The quality of these layers determines the properties of the LED. Reproducibility is difficult to achieve across a single wafer, or from wafer to wafer, or from day to day. Some LEDs processed from a wafer will yield high quality devices, while others from the same wafer will have much lower quality and will end up in low-end applications such as children's toys.

Designability
LEDs open up many new design options, some of which were previously inconceivable.

Environmental
LEDs do not contain mercury and in many cases steps are being taken to replace lead-containing solders (used mainly to fix LEDs to circuit boards) with lead-free material, in line with European directives. The energy-efficient nature of LEDs also makes them environmentally friendly.

Drivers
LEDs are low-voltage light sources, generally requiring a constant DC voltage or current to operate optimally. Designing and implementing an effective driver is key to obtain all the benefits of LEDs.

Knowledge gap
In general, there is a gap in understanding between the LED manufacturers and the lighting community. The former group do not include the latter in their product development activities and do not provide information that is directly comparable to the information available for competing light sources. The latter do not understand a huge amount about LEDs and are unfamiliar with crucial issues such as thermal management, or why white LED performance is not highly consistent.

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