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Any Color, As Long As It's White

It is widely believed that general lighting will one day become a huge market for white-light LEDs. Bill Kennedy describes some of Toyoda Gosei Optoelectronics' recent technological progress.
Why use LEDs in general lighting? The problem with incandescent lamps is well documented. Their efficiency, in terms of photon propagation, isn t very good. They produce maybe 15 lm per W of input power, with a high degree of wasted energy given off as heat. And while fluorescent bulbs can produce up to 100 lm/W, their high operating voltages require expensive transformers. At peak efficiency, fluorescent sources also produce poor color rendering, while their mercury content is an environmental concern.

LEDs offer the best of all worlds: ultra-efficient, environmentally friendly, low-power lighting. Many countries are now recognizing LEDs as a "strategic technology", as evidenced by Japan s Light for the 21st Century program, the US Government s Next Generation Lighting Initiative (NGLI), and similar schemes in Europe, Taiwan and Korea.

For these programs, white-light LEDs are the key technology. But what exactly is white light? Light possesses no specific "color" until it is processed by the human brain. What we call "white light" is a sensorial phenomenon that doesn t exist until it is processed and is perceived by the individual. There is no single kind of white light - there are in fact many kinds, each with a subtly different hue (Table 1). With LEDs a whole new type of light source has become available; one that can both mimic many of the characteristics of older lighting technologies and provide a wealth of new lighting capabilities.

Degrees of "whiteness"Technically, the color temperatures detailed in the table only apply to blackbody sources, and, as such, the figures are a specification of both the degree of "whiteness" and the spectral composition of the source. However, the term "apparent color temperature" is often used to specify the degree of whiteness of all types of light sources, including LEDs.

All possible colors, including all types of white light, can also be designated on the chromaticity chart established by the International Commission on Illumination (CIE) in 1931 (figure 1). The x/y chart, which was based on a number of research programs dating from at least as early as Maxwell s work in 1854, defines the color-matching characteristics of a standard observer. It has established a framework for the specification of colors in coordinates defined for virtually all wavelengths associated with visible light.

Superimposed upon this chart is the Plankian Curve (black-body locus), which is also used to establish a definition of white light, including equal-energy white (EEW), through x/y plotting. The information collected for this chart includes pure spectrum colors. This chart is the lighting and LED communities standard tool for the definition of chromaticity, including the many different shades of white light.

The development of high-brightness white LEDs has already led to new approaches for lighting, and will eventually significantly affect the type, style and quality of general illumination. The key here is that most traditional light sources never really provided a close approximation to EEW. Among the common types of non-solar white light that people encounter are: the cool "bluish" white from fluorescent lamps ubiquitous in many office buildings; incandescent "yellowish" white seen in residential lighting; the brilliant "blue-white" of mercury vapor and metal halide lamps seen in most warehouses, manufacturing, assembly and repair facilities; and the "yellow-white" of high-pressure sodium street lamps.

With white LEDs, emissions that approximate incandescent, fluorescent and - more importantly - EEW, can be produced. Indeed, the whole definition and quality of such light can be changed and/or redefined by LEDs.

LEDs offer a number of solutions to creating white light, ranging from bichromatic (two-color) to full-spectrum polychromatic sources. The two basic techniques for LED white-light propagation can be described simply enough, but to really define this technology, we must look at the two in more detail.

The first technology is "analog white". This is the term used to describe white light produced by efficient blue or violet GaN-based LEDs coupled to phosphors. Analog white products that provide bichromatic, trichromatic and full-spectrum polychromatic white light are in production (figure 2).
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