2

The frequency-doubled output of the system is at the 532 nm green

wavelength. Since the modulation speed of the solid state laser is

relatively slow, an external acousto-optic modulator (AOM) is

required. The DPSS green laser system requires precise

temperature control and is always on during engine operation,

therefore it accounts for more than half of the total 5 watts of

power needed to run the LPD engine.

The red, green and blue laser beams are individually focused, and

the three beams are combined with dichroic optical filters and

aligned into a co-linear beam, which is scanned by two orthogonal

scan mirrors to obtain a raster. A high speed MEMS mirror scans

the beam 56 in the horizontal direction, and a low speed mirror

scans the beam 42 in the vertical direction. The system is capable

of displaying XGA resolution images.

The combined laser beam impinges on the high-speed horizontal

mirror first. The fan of beams leaving the high-speed mirror is

directed towards the larger low-speed mirror, which scans in the

vertical direction. If the laser beam and the mirror's scan axis are

perpendicular to each other, a straight scan line is projected on a

flat surface. On the other hand, if there is an angle between the

mirror's scanning axis and the incoming laser beam, the scan line

follows a curved trajectory when projected on a flat screen.

There is no possible geometric arrangement that allows the beam

impinging on both scan mirrors to be perpendicular to the

respective scan axes. Therefore, the system was designed so that

the beam is perpendicular to the axis of the horizontal mirror,

thereby ensuring straight projection of the horizontal lines. Since

the off-center scan lines impinging on the vertical mirror are not

perpendicular to that mirror's scan axis, the length of the projected

scan line increases towards the top and bottom edges of the

image. This results in a pincushion distortion effect in the vertical

direction, which is electronically corrected by displaying the scan

lines near the top and the bottom of the screen within fewer

clock cycles.

Both mirrors generate feedback signals, which allow the system to

control the laser modulation in synch with the correct mirror

position. Since the high-speed mirror is scanning with a sinusoidal

velocity profile at its natural resonance, the system needs to

compensate for the changing speed of the mirror. At scan angles

pointing to the left or to the right of the center position, where the

scan velocity is lower, the laser is turned on for a longer time

period and at proportionately lower intensity. This approach

ensures that pixel size and intensity remains uniform along the

scan line. Near the ends of the scan line, where the mirror stops

and changes direction, the lasers are completely turned off.

LPD FEATURES AND PERFORMANCE

The size of the LPD engine is approximately 6.6cm x 4.6cm x 2.3 cm

(71cc or 4.3 cubic inches). The projector can support a variety of

optical resolutions, including QVGA, VGA, SGA, XGA, and HDTV

720p. The device projects 56 horizontal and 42 vertical maximum

scan angles. The average optical output of the engine is 10 lumens,

which generates about 60 nits on a 13 inch diagonal screen without

optical gain. Typical power consumption of the device is 5 watts.

The LPD engine produces images with 24 bit color depth within an

unparalleled color gamut, due to the selection of laser wavelengths

at the edges of the visible spectrum (red = 635 nm, blue = 440 nm,

green = 532 nm).

Figure 3. Symbol LPD Optical Layout

Figure 4. Pincushion Correction of LPD Image