As discussed in “Light Emitting Diodes (LEDs) for General Illumination, Update 2002, An OIDA Technology Roadmap” (co-sponsored by the U.S. Department of Energy, the National Electrical Manufacturers Association, and the Optoelectronic Industry Development Association), artificial lighting technologies are substitutes for sunlight in the 425-675 nm spectral region where sunlight is most concentrated and to which the human eye has evolved to be most sensitive. The history of lighting can be viewed as the development of increasingly efficient technologies for creating visible light inside, but not wasted light (i.e., outside of that spectral region).
The three traditional technologies are fire, incandescence, and fluorescence; solid-state lighting (SSL) constitutes a new, fourth technology. These four technologies can be differentiated by the
- material type (gas or solid) that emits the light
- light-emission spectral bandwidth (broadband blackbody or narrowband) and,
- fuel (chemical or electrical) used to create the light.
These difference, in turn, have consequences in each technology’s fundamental efficiency.
Fire: Chemically fueled blackbody emission
Incandescence: Electrically fueled blackbody emission
The first lighting technology is fire. This technology involves burning a chemical fuel (usually a combination of gases, solids or liquids) to heat a gas or solid that emits broadband blackbody light. Because the light is broadband blackbody, most of which lies outside the visible spectrum, fire is an inherently inefficient light source.
Moreover, because the fuel is chemical and must be transported into the reaction zone, the transport mechanism itself (typically convective flow) can make it difficult to achieve high temperatures. Hence, most of the emitted light lies outside the visible spectrum, with the peak of that blackbody spectrum in the invisible infrared.
The history of fire can be viewed as attempts to control the mechanism for fuel transport and burning to increase the temperature of the emitting gas, and to enhance visible-light emission. Hence, the evolution from open fires (1.42 million years ago), to torches, to wax candles, to oil and kerosene lamps. The culmination of fire can be thought of as gas-fired lamps, first introduced by William Murdock in 1792, in which the fuel is converted into a continuous incoming stream of gas before being burned.
Discharge/Fluorescence: Electrically fueled narrowband emission from gases
The second lighting technology is Incandescence. This technology involves using electricity as the fuel to heat a gas or solid that emits broadband blackbody light. Because, as with fire, the light is broadband blackbody, most of which lies outside the visible spectrum, incandescence is also an inherently inefficient light source.
However, because the fuel is electrical, and can be transported more easily into a small emitting zone than can chemical fuels, the emitting zone can be very hot. The peak of the blackbody spectrum can be arranged to be near the visible portion of the spectrum, hence incandescence efficiency can be much higher than that of fire.
The history of incandescence can be viewed as an attempt to increase the temperature of the emitting filament while maintaining an acceptable lifetime; as demonstrated by the evolution from lamps that utilize the electric arc then to carbon-filament and finally to metal-filament lamps. Incandescence culmination can be thought of as the tungsten-filament lamp with a trace amount of lifetime-enhancing halogen gases.
Solid-State Lighting: Electrically fueled narrowband emission from solids
The third lighting technology is discharge/fluorescence. It technology involves using electricity as a fuel to excite (but not heat) a low-pressure gas that emits narrowband atomic line emission. This primary narrowband light can be used as is, or it can be absorbed and re-emitted as secondary light at different (longer) wavelengths through fluorescent or luminescent materials.
Because the light is narrowband, and can be concentrated in the visible portion of the spectrum, discharge/fluorescence is much more efficient than incandescence. Indeed, the highest-efficiency lamp of any type is the sodium lamp, at 200 lm/W, which emits narrowband yellow light at 589 nm.
However, because the primary light is narrowband, it does not fill the visible spectrum, and appears colored. For general lighting, it is necessary to convert this narrowband emission into semi-broadband emission that optimally fills the visible spectrum and gives the appearance of white light. The history of discharge/fluorescence for general lighting has been driven by efforts to develop luminescent materials that can perform this conversion while surviving direct exposure to reactive gas plasma discharges. Hence, the evolution from early fluorescent lamps, which had a greenish, low-quality color, to modern fluorescent lamps with phosphor blends and relatively good-quality color.
The fourth and most recent lighting technology is SSL. This technology uses electricity as a fuel to inject electrons and holes into a solid-state semiconductor material. When the electrons and holes recombine, light is emitted in a narrow spectrum around the material’s energy bandgap. Because the light is narrowband, and can be concentrated in the visible portion of the spectrum, it has, like fluorescence, a much higher light-emission efficiency than incandescence.
However, as with fluorescence, because the light is narrowband, it does not fill the visible spectrum with light, and appears colored. Thus, SSL’s evolution must eventually include overcoming similar challenges associated with converting the narrowband emission into semi-broadband emission that fills the visible spectrum to give the appearance of white light.
Unlike in fluorescence technology, the wavelength of the narrowband emission can be tailored relatively easily, hence can be adjusted either to increase the quantum efficiency, or to minimize the quantum energy (or Stokes-shift) inefficiency associated with its conversion to semi-broadband emission. This technology is potentially even more efficient than Fluorescence.
Both inorganic and organic semiconductors are being considered for this new lighting technology. Inorganic semiconductors, which are much further developed, are the focus of our SSLS EFRC.