|Fig. 1: Red, pure green and blue LEDs of the 5mm diffused type. (Source: Wikipedia Commons)|
Several types of lamps have been developed during the history of lighting industry including incandescent lamps, Mercury vapor lamps, Neon lamps, Fluorescent lamp and recently solid-state lams or light emitting diodes (LED).
In incandescent lamp a current is passing through a highly resistive filament and heating it up which yields to blackbody radiation of filament in visible frequencies. This type of lamps have luminous efficacy around 13 lm/W.  In Mercury vapor lamps an electric arc is used through vaporized mercury to produce light. Mercury lamps have luminous efficacy of 35 to 65 lm/W.  Neon lamps are also one of the gas discharge lamps like Mercury vapor lamps, which contain neon gas at low temperature. Fluorescent lamps are gas discharge lamps, which use phosphor atoms to convert the wavelength of light. Mercury atoms are excited by electricity in fluorescent lamps, which generate ultraviolet photons in relaxation to their steady state. The ultraviolet photons are converted to visible photons by use of phosphor atoms.
All type of lamps, which we mentioned above, had significant progress in last century in material, design light quality, energy efficiency and manufacturing efficiency, but it seems that they are reaching saturation in efficiency, which is not good from energy consumption prospective. 
According to the DOE (Department of Energy) report of 2010 US lighting market characterization, about 19 percent of US total electricity consumption is used for the residential, commercial, industrial, and outdoor lighting.  Incandescent lamps are the main source of lighting in residential sector. However the efficiency of incandescent lamps is very low, their chip production cost makes them the most popular source of lighting in houses.
|Fig. 2: Light emission spectrum of white LEDs which shows a peak near wavelength of 465nm corresponding the GaN LED emission peak and one broad peak between 500-700 nm corresponding to phosphore atoms emission. (Source: Wikipedia Commons)|
Solid-state lighting is one of the rapidly growing technology and main replacing candidates for incandescent and fluorescent lamps. Until recently LEDs were mainly used in electronics and had a very small contribution in lighting industry. People used LEDs in toys and indicators before the demonstration of high efficiency white LEDs.
The efficacy of white LEDs increased incredibly fast during the last decade from less than 10 lm/W to 150 lm/W. Recently it is shown shown that white light emitting diode with luminous efficacy near 249 lm/W is achievable. The reported value for efficacy is about 18 times that of an incandescent lamp and triple of tri-phosphor fluorescent lamp.  The structure used in this work consists of a blue LED chip as the excitation source covered by a layer of phosphor atoms. The blue light emitted by LED is partially absorbed by phosphor atoms. The excited phosphor atoms will emit green and red light during their relaxation to steady state. The combination of green, red and blue light will produce a white light to the human eye. The external quantum efficiency of blue LED used in this work is 84.3 % driven at low current of 20 mA.
However the efficiency of solid state lams have improved during the last decade it still needs further improvement to be used as the main source of lighting. Nowadays, total power efficiency of commercial LEDs is les than 30%.  This low efficiency is not only due to one main dominant loss process but rather the result of many loss processes, which are acting serially and cumulatively. The first loss mechanism is "efficiency droop" phenomenon, which reduces the external quantum efficiency of blue LEDs as the driving current increases. Low quantum efficiency of commercial LEDs compare to the reported value of efficiency which people got in the labs is a result of high driving current used in them. Second loss mechanism is nonradioactive decay of excited phosphor atoms and Stokes-deficit losses (high energy blue photons are converted to low energy green and red photons). The third mechanism is the spectral match to human visual system.  The net power conversion efficiency will be the product of these three efficiencies. It is unlikely to get to high power efficiency by using blue LED plus phosphor layer because of the outlined inefficient processes. One of the possible solution to this problem is removing the phosphor layer and produce white light by mixing red, yellow, green and blue LEDs emission. This type of light mixing cannot be done efficiently these days because of the low efficiency of the green LEDs. 
If in future the commercial solid-state lamps can achieve 50% power efficiency they would provide the lighting source with following characteristics: 
Long lifetime light source
Environmentally friendly light source
Natural and tunable light source
Solid-state lamps will become the main source of lighting in next decade and will reduce the energy consumption associated with lighting substantially. 
© Majid Esfandyarpour. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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