LED Light Engines for Battery-Powered Microprojectors: 3M's MPro 120 PocketProjector

Everyone in business is familiar with video projectors; there would be no Microsoft PowerPoint presentations without them. However, meeting room projectors are big, noisy, power-hungry things tethered to wall sockets by their power cords. These bigger projectors typically use halogen incandescent bulbs for illumination. A new, small, battery-powered class of projectors is starting to appear. These are LED-powered microprojectors, and they’re starting to appear as standalone peripherals—such as 3M’s new MPro 120 PocketProjector—and as built-in peripherals in combination with other devices, such as Nikon’s Coolpix S1000pj digital still camera introduced last August. The LED light engines in these microprojectors combine red, green, and blue high-power LEDs to create a single component capable of emitting light in a wide color spectrum. The term “LED light engine” refers to a light emitter, usually high-brightness that is used where the light serves some sort of purpose other than as an indicator—the initial use for visible LEDs back when LEDs were first developed.

The 3M MPro 120 PocketProjector is a battery-powered device that can throw a 50-inch, full-color image on any convenient flat surface. Available from a number of retailers, the MPro 120 currently sells in the $275-$360 range. In general, 3M’s MPro 120 PocketProjector cannot project usable images for large audiences, although some reviewers report that they have used the projector for audiences ranging from six to 50 people. In addition, it is not designed for use in rooms that are even moderately illuminated. However, the LED-powered projector is handy for delivering impromptu presentations to a small number of people when looking over the presenter’s shoulder at a laptop screen is not practical.

 The projector’s native resolution is 640 x 480 pixels (VGA), but the PocketProjector supports VGA, SVGA (800 x 600-pixel), XGA (1,024 x 768-pixel), and WXGA (1,280 x 768-pixel) video formats through image scaling. It also houses two 0.5-W speakers in its diminutive volume (0.9 inches high, 4.7 inches deep, and 2.4 inches wide). The diminutive projector is about the size of a mobile phone. It normally operates at 8 lumens and runs for four hours in that mode on its internal, rechargeable lithium-ion battery. In the high-brightness, 12-lumen mode, the device operates for two hours on internal battery power.

The 3M MPro 120 PocketProjector

3M based the MPro 120 PocketProjector on a customized version of the OSTAR LE ATB S2W multichip LED from OSRAM. The OSTAR LE ATB S2W is an SMT multichip module that packages four high-brightness LEDS—one amber-red (617nm) LED, two green (525-nm) LEDs, and one blue (470nm) LED—in a transparent SMT package with a board footprint of only 0.19 x 0.22 inches. The glass-topped SMT package measures only 0.048 inches high. The 8-pad package brings out the anode and cathode connections for each of the four LEDs separately. Typical drive current for each LED in the standard OSTAR LE ATB S2W device is 700 mA. A Linear Technology LT3475 quad high-current LED buck-boost driver IC supplies drive current to the four LEDs in the OSTAR LE ATB S2W.

OSRAM OSTAR LE ATB S2W integrated LED array

To create the MM200 projection engine that forms the heart of the MPro 120 PocketProjector, 3M packaged a customized OSTAR LED array with a 640 x 480-pixel, ferroelectric LCOS (liquid crystal on silicon) imager from Displaytech, a specially developed light-engine enclosure, a small cooling fan, and optics. The MM200 light engine produces broad-spectrum light modulated by Displaytech’s FLCOS reflective display acting as a light valve to create a digital projector. The LCOS reflective display decodes the incoming video stream and controls the color being emitted by the OSRAM LED array through separate RGB output control lines to create a time-multiplexed color image.

 

OSRAM developed the OSTAR LE ATB S2W LED array specifically for applications that need to emit a lot of light but cannot accommodate the power or volumetric requirements of more conventional halogen or incandescent light sources. The company targets the three-color OSTAR LE ATB S2W LED array at portable projector applications. The inclusion of four differently colored LEDs in one SMT package eliminates the spinning color wheel of other color-projection technologies and reduces the volume requirements for the digital projector. You can use both the multicolor and monochrome (all the LED chips in the array are the same color) versions of the array for mood and architectural lighting.