Ulrich Hartwig

Challenges for reducing the size of laser activated remote phosphor light engines for DLP projection

Laser activated remote phosphor (LARP) is an upcoming technology for high luminance SSL light engines. This presentation outlines some of the challenges met reducing the engine’s size, so it can be retrofitted into DLP-projectors.

Laser-activated remote phosphor light engine for projection applications

Recent developments in blue emitting laser diodes enable attractive solutions in projection applications using phosphors for efficient light conversion with very high luminance levels. Various commercially available projectors incorporating this technology have entered the market in the past years. While luminous flux levels are still comparable to lamp-based systems, lifetime expectations of classical lamp systems are exceeded by far. OSRAM GmbH has been exploring this technology for several years and has introduced the PHASER® brand name (Phosphor + laser). State-of-the-art is a rotating phosphor wheel excited by blue laser diodes to deliver the necessary primary colors, either sequentially for single-imager projection engines, or simultaneously for 3-panel systems. The PHASER® technology enables flux and luminance scaling, which allows for smaller imagers and therefore cost-efficient projection solutions. The resulting overall efficiency and ANSI lumen specification at the projection screen of these systems is significantly determined by the target color gamut and the light transmission efficiency of the projection system. With increasing power and flux level demand, thermal issues, especially phosphor conversion related, dominate the opto-mechanical system design requirements. These flux levels are a great challenge for all components of an SSL-projection system (SSL:solid-state lighting). OSRAM´s PHASER® light engine platform is constantly expanded towards higher luminous flux levels as well as higher luminance levels for various applications. Recent experiments employ blue laser pump powers of multiple 100 Watts to excite various phosphors resulting in luminous flux levels of more than 40 klm.

A solar simulator design for concentrating photovoltaics

In our contribution, we discuss an optical system which is able to provide sun-like radiation on a CPV module. This system is able to realize collimated light with more than 130 klx, with an angle of incidence of less than 0.26°. Special attention is given to a uniform light distribution on an illuminated area of 8 × 8 inches. The resulting optical efficiency of the system is 33%, much better than previously achieved with Xe flash lamp designs [1]. The design is based on a P-VIP 330/1.0 lamp, the latest in a series of OSRAMs P-VIP lamp types for video projectionwhich is featuring a peak luminance of approximately 9 Gcd/m2. As a result of the enormous operating pressure of the lamp, its spectrum is similar to the spectrum of the sun and will probably enable at least a class B simulator. The device based on the described design delivers continuous, sun-like radiation. This way, features of CPV modules as efficiency, angular sensitivity or tracking behavior can be tested during development or even in production.

Fiber optic illumination by laser activated remote phosphor

For some fiber optic applications, like high-end endoscopy, light sources with high luminance are necessary. Currently, short arc discharge lamps are being used. However, more and more LED solutions are trying to compete, but they can not yet reach the performance obtainable by 300 W Xenon short arc discharge lamps. To make this field of application accessible for solid state light sources, a new approach is necessary. Diode lasers have rapidly advanced in the past years. This is particularly true for multimode laser diodes emitting at around 445 nm wavelength. Single diodes emitting more than 1 W of optical power are already available. These laser sources exhibit extremely high radiance, thus they can be focused onto very small areas. Phosphors placed near the focus can result in high luminance sources. On the basis of this idea, a device has been developed to match the performance of a state of the art 300 W Xenon lamp system. An array of laser diodes is used to illuminate a phosphor plate which converts the blue pump light into yellow light. The converted light is collected and adapted to the application by a tapered TIR rod. To achieve a color point on the Planckian locus at 6000 K, the light of an LED emitting at around 460 nm is superimposed to the converted light.

LED camera light with a compact wide-range zoom

A design concept for an extremely compact zoom optics which is suitable for illumination applications is presented. Such optics is especially useful for camera or flash lights as the illuminated area can be adjusted according to the picture content of the photo or film camera. The principle of the design is as follows: Collimated light passes through two lenses, each with a freeform surface. The freeform surfaces face each other and fit into one another perfectly. When the two lenses are merged together, they basically represent a coplanar plate: The cone angle entering the merged lenses does not change while passing them. When the plates are separated, the light is scattered at the freeform surfaces. Due to the smooth characteristics of the freeform surfaces shape, the cone angle can be adjusted continuously with the distance of the zoom lenses. The distance of the zoom lenses, which is necessary for maximum angle widening, is dependent on the size of the structures of the freeform surfaces and can be reduced to the sub-millimeter range. The compactness of the resulting device is a major advantage of the design concept. The principle of operation of the design could be shown by the construction of a prototype. It features a LED light source and a zoom range of 5° to 30° (cone angle). The luminous flux of the device is approx. 650 lm.