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General Knowledge
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What is a LED (light emitting diode)?
A light emitting diode (LED) is a semiconductor diode that emits light of one or more wavelengths (colours). A diode is a device which electrical current can pass in only one direction. The diode is attached to an electrical circuit and encased in a plastic, epoxy resin or ceramic housing. The housing usually consists of some sort of covering over the device as well as some means of attaching the LED to a source of electrical current. The housing may incorporate one or many LEDs.

How efficient are LEDs?

The efficiency of LEDs is improving all the time. Current development lab data for warm white LED now exceeds 130lm/w. Lab data for LEDs do not have any correlation to real-life operating conditions and therefore cannot compare. The efficiency of the LED will depend on the operating conditions, especially heat. Some manufacturers of LED and LED systems will provide data based on optimum operation which may not be realistic. Other factors also affect the overall system efficiency, including driver performance, optical efficiencies, losses through additional components, etc. Also, phosphor conversion suffers from a loss of energy (known as the Stokes shift) when the phosphors convert light from a shorter to a longer wavelength, which reduces the overall device efficiency.

How is a LED tested to obtain lifetimes?
The lifetimes of traditional light sources are rated through established test procedures. As an example, for compact fluorescent lamps (CFLs), a statistically valid sample of lamps is tested using an operating cycle of 3 hours ON and 20 minutes OFF. The point at which half the lamps in the sample have failed shows the average life for that lamp. For 10,000 hour lamps, this process takes about 15 months. For LEDs life, switching is not a determining factor, so there is no need for the on-off cycling. But full life testing for LEDs is impractical due to the long expected lifetimes. Indeed, even with 24/7 operation, testing an LED for 50,000 hours would take 5.7 years, and products would be obsolete by the time they finished life testing.

Operating the LED or system at rated current and voltage for 1,000 hours as a “burn-in period.” This is necessary because the light output actually increases during the first 1,000 hours of operation, for most LEDs. The LED is operated for another 5,000 hours. The radiant output of the device is measured at 1,000 hours of operation; this is normalised to 100%. Measurements taken between 1,000 and 6,000 hours are compared to the initial (1,000 hour) level. If, for general lighting applications, 70% of initial lumens have not been reached during the 6,000 hours, the data is used to extrapolate those points.

Most manufacturers of high-power white LEDs estimate a lifetime of around 50,000 hours to the 50% lumen maintenance level, assuming operation at 350 milliamps (mA) constant current and maintaining junction temperature at no higher than 90°C.

What affects the lifetime of a LED?
The primary cause of lumen depreciation is heat generated at the LED junction. As LEDs do not emit heat as infrared radiation (IR), the heat must be removed from the device by conduction or convection. Without adequate heat sinking or ventilation, the device temperature will rise, and continuous high temperature operation will cause permanent reduction in light output and premature lifetimes.

How to produce white light using LEDs?
An individual LED produces a single colour of light. To produce white light, light spanning the visible spectrum (red, green and blue) must be generated in the correct proportions. Methods to generate white light using LEDs can be classified into two approaches.

1. Wavelength Conversion. This approach converts some, or all, of the LED output into visible wavelengths in the following ways:
Blue LED + yellow phosphor. Some of the blue light from an LED is used to excite a phosphor which re-emits yellow light. The yellow light mixes with some of the blue light leaking through, resulting in the appearance of white light.
Blue LED + several phosphors. Similar to the above method, except that the blue light excites several phosphors, each of which emits a different colour. These different colours are mixed with some of the blue light leaking through, to make a white light with a broader, richer wavelength spectrum. This gives a higher colour-quality light than the above method.
Ultraviolet (UV) LEDs + red, green and blue phosphors. The UV light from an LED is used to excite several phosphors, each of which emits a different colour. These different colours mix to make a white light with the broadest and richest wavelength spectrum. This gives the highest colour-quality light, again albeit at a slightly higher cost.

2. Colour Mixing. This method uses multiple LEDs in a single package, and mixes the light to produce white light. Typically, the package contains at least two LEDs (blue and yellow) and sometimes three (red, blue, and green) or four (red, blue, green and yellow). Because no phosphors are used, no losses associated with wavelength conversion are incurred; hence this approach has the potential for the highest efficiency, but quality of light can be poor.

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