Powerful light sources for all projector sizes
Most information, presentations and other image content are now found on small mobile devices such as smartphones and notebooks. The small displays on these devices impose restrictions on their use for multimedia applications. Embedded or pico-projectors remove these restrictions and enable all the content to be presented in the appropriate size. So even with small devices it is possible to provide presentations, share web content or offer slide shows. LEDs and, increasingly, lasers are the light sources of choice for these miniature projectors. Both are extremely compact, offer impressive efficiency and provide high luminous flux.
These semiconductor light sources are penetrating more and more into other projection applications. This is thanks on the one hand to the enormous increase in output and wide variety of these light sources, and on the other hand to the fact they are better adapted to meet projection requirements. The advantages of semiconductor light sources are obvious. They generate their light extremely efficiently, produce very little heat, need less cooling as a result, and are very small so they take up very little space in the device. What’s more, they last a very long time. All these factors enable highly compact projection units to be designed, including embedded projectors for smartphones. Lasers as light sources always produce a sharp image – without a focusing unit and on any surfaces, either flat or curved.
LED projectors produce images from three basic colors – red, green and blue – and direct the light to a micro display via filters or mirrors that allow only certain wavelengths to pass through or be reflected (dichroic filters). On the screen a red image, green image and blue image are produced one after the other at such a high refresh rate (typically 120 Hz) that it is simply perceived as a single full-color image. The color components of the image result from the emission times of the individual colors. The micro display switches the individual mirrors for each light color for different lengths of time to produce the correct amount of red, green and blue for each pixel. LEDs generate monochromatic light so the projected image contains saturated colors, in other words it has a high color gamut.
The light emitting surface of the light source must be appropriate for the micro display
A projector operates at maximum efficiency if the light emitting surfaces of the LEDs perfectly match the size and type of micro display and optical system because the micro display only receives light that hits it up to a certain angle of incidence. To design a brighter projector therefore it is not enough simply to use larger or more numerous LEDs. The micro display itself must also be adapted. This aspect is important in view of the different package shapes for the lighting unit. The simplest solution is to use discrete RGB LEDs (3-channel illumination) with their light being passed through three dichroic filters. The system can be made smaller and at lower cost if a single component can produce several colors.
Projector performance classes:
· 15 lm for embedded smartphone and pico projectors
· 300 lm for pocket and video projectors and for head-up displays
· 1000 lm and higher for semiprofessional applications such as home cinema
· 2000 lm and higher for professional applications
With its Osram Ostar Projection package platform Osram Opto Semiconductors has developed an LED product family which is specially tailored to projection applications and which can cover projector performance classes up to around 2000 lm. Osram Ostar Projection Cube, for example, is ideal for embedded projectors, and Osram Ostar Projection Compact 1 x 2 for head-up displays. Osram Ostar Projection Compact 2 x 2 mm has been designed for beamers up to 1000 lm, and Ostar Projection Power can meet the requirements of beamers that need a luminous flux of more than 1000 lm. All Osram Ostar products are based on the latest ThinGaN and Thinfilm semiconductor technologies.
Fig. 1: Lumen classes for projectors
Embedded and pico projectors can now be used successfully in any ambient light conditions thanks to the Osram Ostar Projection Cube. The green LED produces 110 lumen per watt and is therefore much more efficient than any other green LEDs available up to now for such applications. The brightness of embedded projectors in smartphones can be almost doubled, which means that a luminous flux of up to 25 lumen is now possible. High luminance from a small surface area is made possible by a special encapsulation which sharply defines the light emitting surface . The light is therefore emitted from this defined surface without package reflections. The LED is only 0.6 mm high so extremely low-profile projection units are possible – ideal for modern smartphone and camera designs. The LED does not have a glass cover, which means that external optics can be placed very close to the light emitting surface. This all contributes to the efficient use of the light.
Projectors in head-up displays and projection applications with similar lumen requirements benefit from the Osram Ostar Projection Compact 1 x 2. The current versions with a green and a blue chip can be pulsed at 6 A; the version with the amber chip can be pulsed at up to 4.5 A. The LEDs typically achieve 464 lm in amber, 1260 lm in green and 4W optical output in blue at 460 nm. The thermal resistance of the blue component is 3 K/W, which means that heat can be easily dissipated.
For compact beamers up to 1000 lumen
Compact, high-performance projection solutions can be easily achieved on the basis of the Osram Ostar Compact 2 x 2. The LED offers the right ratio of light to size. With twice the light output of the single-chip version, this is ideal for micro display diagonals between 0.4 inches and 0.55 inches. A projector output of 400 lm can be achieved with just one set in red, converted green and blue. Sets of LEDs can be combined for larger imager diagonals. With a suitably matched micro display these LEDs can also cover the high projector brightness levels needed for office applications.
Fig. 2: Osram Ostar Projection Cube
Osram Ostar Power Projection has been designed for high-performance projection systems greater than 50 inches, for home cinema applications and for professional applications. It can be operated with a maximum of 36 A and produces light of impressive brightness. Typical values are 3300 lm in red, 7500 lm in green and an optical output of 25 W in blue. While the dimensions of 45 mm x 25 mm seem large for a standard LED the thermal resistance is only 0.5 K/W. The LED is suitable for all types of cooling, including water cooling.
Large projection surfaces requiring luminous flux values of 2000 lm and more are more the domain of semiconductor lasers. These have a high efficiency of 27 percent and heat up only slightly, which in turn contributes to a longer life. There are various options for using them in professional projectors. In most cases, red LEDs are combined with blue high-power lasers. The color green is produced when blue lasers cause a special phosphor to luminesce. Red light can also be produced in this way, so if necessary red LEDs can be left out of the projection light source and the unit can be made even smaller.
Fig. 3: Green laser from Osram Opto Semiconductors
Osram also offers direct blue and, more recently, green lasers. Direct green lasers open up new possibilities for pico projector because they eliminate the costly frequency doubling of infrared laser light. Their wavelength of 515-530 nm provides precisely the right green for projection applications. For an optical output of 50 mW and efficiency of 6% the laser diode has a package diameter of only 3.8 mm, so projection units can be made significantly smaller. The high emission quality of the laser also provides the basis for high image resolutions. This is particularly important for pico projectors that project the laser light with a micro- electro-mechanical system (MEMS) without any further optics
With its broad portfolio of LEDs and laser diodes specially tailored to projection requirements, Osram Opto Semiconductors can cover virtually the entire spectrum of projectors. Users benefit from more than 30 years of accumulated know-how and high-quality application support.
About the authors:
Dr. Stefan Morgott studied physics at the University of Ulm before doing a doctorate on optical semiconductor amplifiers at the Fraunhofer Institute for Applied Solid-State Physics in Freiburg. He joined Osram Opto Semiconductors in Regensburg in 2000, working in infrared laser diode applications and measurement technology for eight years. Since 2005, his work has focused on application possibilities for visible LEDs with an emphasis on projection and entertainment lighting.
After becoming a master craftsman in radar mechanics, Wolfgang Schnabel went on to study telecommunications engineering at Osnabrück University of Applied Sciences. He then spent several years as a sales engineer with Siemens AG before transferring to the same post at Infineon Technologies in 1999. Schnabel has been employed as the Marketing Manager LED in the Industry division of Osram Opto Semiconductors since 2001. His work mainly focuses on the field of projection.