In image reproduction or projection technology, mixed light sources, i.e., sources such as white light sources for electromagnetic radiation of different wavelengths are used as light sources for backlighting, among other things. One can mention, for the sake of example, LCD projection or image reproduction as a field of application. Mixed light serves there for brightening the liquid crystal matrix situated in the image reproduction system or projection system. This matrix has a plurality of individual liquid crystals. Depending on the applied control voltage and the wavelength of the light passing through the crystal, the crystals become transparent. This liquid crystal matrix is generally referred to as a light modulator. By illuminating the light modulator with a mixed light, the image generated by the driving becomes visible and achieves a perceptible contrast.
The use of digital light processing (DLP) technology is also conceivable as a light modulator. In this case, microscopic mirrors (digital micromirror devices, DMD) that are mounted on a chip are used. These mirrors are movable and have two stable end states, between which it is possible to change within a few microseconds under the force exerted by electromagnetic fields. Light is directed by the inclination of the individual micromirrors either directly to the optical system, or to an absorber. Different degrees of brightness of the individual image pixels can be generated by pulse-width modulated driving of the mirrors. Unlike in LCD technology, the mirrors in this case operate wavelength-independently. The so-called primary colors red, green and blue are sequentially switched onto the CMD chip to generate color images. The position of the mirrors determines the color component in the image. Because of the more direct light path as compared to LCD technology, and the lack of polarization of the light, higher output light powers are generated than with an LCD projector.
To date, semiconductor-generated mixed light has been generated by the combination of so-called primary colors. These primary colors are electromagnetic radiation with the colors red, green and blue generated by an emitting semiconductor element, e.g., an LED. So-called mixed light originates from the superimposition of these three colors. It has a multiplicity of different wavelengths in the visible range, for example, in the range from 380 nm to 780 nm.
To generate these different wavelengths, the active layers of the PN-junction in the semiconductor component are doped differently, for example. The resulting energy level difference, in the energy level diagrams, also referred to as bandgaps, lead to emission of light of different wavelengths. The wavelength of this light is directly dependent on the energy level difference, and is adjustable to an extent by means of the doping.
The color green has a significant role in the thus-generated mixed light, since the human eye is most sensitive in its absorption properties to the color green. It is accordingly especially necessary in image reproduction systems to generate the color green as authentically and stably as possible.
The semiconductor components, especially for use in projection systems, are operated at a high current density. Thereby a high radiation flux density, or a high beam intensity, is achieved. This high current density causes an effect known in technology as wavelength drift in conventionally manufactured LEDs.
The primary wavelength that is emitted changes because of the higher current density. Conventionally manufactured green-emitting InGaN-LEDs, in particular, have a high dependency of the emitted wavelength on the operating current. When this current is elevated, the operating wavelength, which preferably lies between 520 nm and 560 nm, shifts to a shorter wavelength, which is referred to as “drift.” Thus, for example, a green-emitting InGaN-semiconductor component will illuminate with a bluish cast when operated at high current density.
This effect has two substantial disadvantages. First, the desired wavelength changes for a mixed light source operated at a high current density. For the white light source mentioned as an example, the wavelength drift will produce a bluish-appearing light source. Because of the high sensitivity of the human eye to the color green, a subjective color impression arises. The already mentioned driving of a light modulator in projection systems is modified in the sense that the spectral distance between green-emitting and blue-emitting semiconductor components is reduced and the separation of the two colors, even with the aid of wavelength-selective dielectric edge or bandpass filters or mirrors in the light modulator, is made more difficult and cost-intensive.
An additional crucial disadvantage in case of wavelength drift is a clearly worsened energy efficiency and an associated deteriorated effectiveness.