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Optoelectronics/Displays??

Exploring light sources for all projector sizes

Posted: 30 Sep 2013 ?? ?Print Version ?Bookmark and Share

Keywords:multimedia applications? pico-projectors? LEDs? lasers? light sources?

Most information, presentations and other image content are now accessed through 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 C without a focusing unit and on any surfaces, either flat or curved.

LED projectors produce images from three basic colours C red, green and blue C 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 120Hz) that it is simply perceived as a single full-colour image. The colour components of the image result from the emission times of the individual colours. The micro display switches the individual mirrors for each light colour 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 colours, in other words it has a high colour gamut.

Figure: Lumen classes for projectors.

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 colours.

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 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.

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