The luminous efficiency is an index for evaluating the efficiency of the light source, and is expressed by the ratio of the luminous flux (lm) emitted by the light source to the electric power (W) input to the light source. The unit is lm/W.
Recently, the luminous efficiency of white LEDs has exceeded 100 lm/W. As a white LED that is expected to become a new generation of light source after incandescent lamps and fluorescent lamps, the luminous efficiency of 100 lm/W which is the same as the overall efficiency of a straight tube type fluorescent lamp is attracting attention. The luminous efficiency only indicates the efficiency of the light source, and the overall efficiency (combined efficiency) of the appliance after the light source is mounted on the lighting fixture is different.
Luminous efficiency is a numerical value indicating external quantum efficiency by visual sensitivity (sensitivity of human eyes to light). The external quantum efficiency is the ratio of the number of photons emitted to the LED chip and the package to the number of electrons (current) flowing through the LED. The external quantum efficiency of a white LED using a combination of a blue LED chip and a phosphor is relative to the internal quantum efficiency (the ratio of the number of photons generated in the LED chip light-emitting layer to the number of electrons flowing through the LED chip (current)) , the light extraction efficiency of the chip (the ratio of the emitted light to the outside of the LED chip), the conversion efficiency of the phosphor (the ratio of the light emitted from the chip to the phosphor converted to different wavelengths), and the light extraction efficiency of the package ( It is determined by the product of the ratio of the LED and the proportion of light emitted by the phosphor to the outside of the package.
A part of the photons generated in the light-emitting layer are absorbed in the LED chip or are constantly reflected in the LED chip, and the LED chip cannot be output. Therefore, the external quantum efficiency is lower than the internal quantum efficiency. A white LED with a luminous efficiency of 100 lm/W has only 32% of its input power as light energy output to the outside. The remaining 68% is converted to heat.
Will increase 100lm/W in the next 3 years
Luminous efficiency has been slowly increasing at a rate of lm/W per year until 2003. In improving the luminous efficiency, the phosphor and the package are not initially changed, but an improvement is made to the chip technology. Specifically, an MOCVD crystal growth technique such as an GaN-based semiconductor crystal used for improving a blue LED chip is performed.
Since 2004, luminous efficiency has increased at a rate of 10 to 20 lm/W per year. Thus, from 50 lm/W in 2004 to 100 lm/W in 2008, it has increased by 50 lm/W in 4 years. This speed is achieved by extending the chip technology improvements that originally gathered in the film forming technology to the major adjustments in the entire LED manufacturing process. In addition, in addition to improving the chip technology, improvements have been made to phosphors.
68% for heat loss
The result of simulation of energy conversion of a white LED having a luminous efficiency of 100 lm/W was performed. The white LED achieves the same luminous efficiency as the fluorescent lamp, but only 32% of the input power can be output as light energy to the outside. The remaining 68% is converted to heat. This simulation is the result when 62 mW of electric power is input to a bullet-type white LED having a diameter of 5 mm. The white LED is obtained by using a combination of a blue LED chip and a yellow phosphor.
In the future, LED manufacturers are planning to increase the luminous efficiency of 100 lm/W achieved in 2008 to 140-170 lm in 2010 and 150-200 lm/W in 2011. That is to say, the goal of LED manufacturers that are ahead of the newly added manufacturers in terms of luminous efficiency is to increase the average annual increase by 30 lm/W or more and increase the current by 100 lm/W in three years. The upper limit of the luminous efficiency of LEDs is considered to be around 250 lm/W, and LED manufacturers are challenging to what extent they can approach the upper limit.
To challenge this cap, LED manufacturers are introducing the latest chip technology, phosphor technology and packaging technology. In terms of chip technology, internal quantum efficiency and light extraction efficiency will continue to be improved. In terms of phosphors, in addition to improving the conversion efficiency, measures are taken to reduce the attenuation caused by the scattering of the phosphor. In terms of packaging technology, materials and structures should be improved to improve light extraction efficiency.
By simultaneously investing in a variety of technologies in the field of chips, phosphors, and packaging, the luminous efficiency can be increased at a higher speed than in the past, thereby coping with the early use of lighting and the rapidly expanding backlight market.
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