Zap! When laser high beam light is emitted from a car head-light, it shines twice as far into the night as its LED counterpart. The laser-pumped phosphor light sources used for this purpose can generate light of the highest intensity on a few tenths of a square millimeter. Installed in state-of-the-art digital projectors, these light sources illuminate cinema screens with a diagonal of 20 meters and more. The lighting technology is considered to be trendsetting in research and industry. Its trump card: maximum luminance in the smallest spaces.
Luminance indicates how brightly the eye perceives luminous surfaces. It is measured in candela per square meter (cd/m²), the intensity of a surface per unit area. A clear 100-watt bulb reaches 10 million cd/m², the glaring midday sun around 1.6 billion. Modern laser-phosphor light sources can achieve significantly higher values today. This makes them ideal for concentrated illumination of limited areas – ideal not only for search or stage lights and projectors, but also for machine vision applications, microscopes or fiber optic devices such as endoscopes.
The trend is headed towards ever higher luminance levels with miniaturized technology. SCHOTT is accelerating this journey with an advanced material: fluorescent ceramics. What do they achieve when used for laser-pumped phosphor light sources? This light technology requires the conversion of laser light. It hits a phosphor and stimulates it to glow. For example, blue laser light is converted into yellow light. SCHOTT uses the same kind of doped crystalline phosphor that every LED light source contains for this purpose. “But unlike the conventional organically embedded phosphor powders, our material is prepared as completely inorganic fluorescent ceramics,” explains Dr. Volker Hagemann, Senior Manager Applications at SCHOTT Advanced Optics. “It is more resistant to aging, allows more light to be generated in a minimal amount of space and enables even higher luminance levels with the best heat management.”
Thanks to the bulk of the converter being made of solid ceramic, it can withstand temperatures of over 1,000 degrees Celsius, while phosphors in organic bonds such as silicone can only withstand temperatures of under 160 degrees Celsius. This and the excellent thermal conductivity are decisive factors in increasing performance because higher and higher luminance also leads to higher and higher temperatures.
SCHOTT has developed converter materials for yellow and green light on this basis, especially for laser-phosphor wheels in digital projectors. Here, the converted laser light is color-filtered to cover the entire color space of digital projection. The rotation of the phosphor wheels ensures their cooling and enables luminances of over 2.5 billion cd/m². The wheels can thus withstand more than 500 watts of laser light.
Static converters that enable extremely compact designs are the latest SCHOTT innovation. The ceramic material is soldered onto a heat spreader in order to dissipate the heat sustainably and to achieve the highest efficiency and reliability. The front and back of the ceramics are given special anti-reflective and metal coatings for optimal light control and heat dissipation. The converter components withstand a high permanent irradiance of more than 50 watts per square millimeter and enable luminances of up to 1.5 billion cd/m². The latter applies to hair-thin material thicknesses of 100 micrometers, which enable particularly good heat dissipation.
Material and process developers have refined a sophisticated industrial production process from nanopowder to sintered ceramics. Today, we can manufacture tailor-made products for various laser applications,” says Dr. Hagemann. SCHOTT has been working on tapping into this new class of materials and its applications for years – and it remains exciting. “We are constantly working on increasing customer benefits, whether through thinner converter plates, new geometries, coatings or new classes of materials.”