Laser-based scanning picoprojectors are hobbled today by the short supply of green lasers, which debuted only last year. Nevertheless, many picoprojector makers believe lasers will be the endgame, since the devices promise to be brighter and more energy-efficient than LEDs.
Lasers scanners require only a single microelectromechanical system (MEMS) mirror, which scans images onto the screen pixel by pixel in a manner reminiscent of e-beam raster scanning of cathode ray tubes. One approach being tried at bTendo Ltd. (Kfar Sava, Israel), however, adds a second micromirror so that vertical and horizontal scanning are handled separately.
According to bTendo, cost, complexity, scan times and image quality can all be improved by using two mirrors in the projection head instead of one. Its main rationale for going with two mirrors is that the vertical and horizontal scan rates differ markedly. For instance, for an 854 x 480-pixel display, the horizontal motion must move through 854 steps for each line, while the vertical scanner motion only ticks once.
Thus the specified requirements for each mirror are different and, according to bTendo, can be best optimized by manufacturing them separately.
Microvision Inc. (Redmond, Wash.) and Maradin Technologies Ltd. (Caesarea, Israel) both have pursued biaxial mirrors, which use two orthogonal gimbals to allow a single mirror to perform both the vertical and horizontal scanning functions. The bTendo approach uses two uniaxial micromirrors mounted on independent orthogonal hinges.
As a result, according to bTendo, each micromirror can use smaller, less expensive electrostatic actuators (instead of electromagnetic versions, which require bulky magnets) and can be processed in a single step on a CMOS fab line. The mirrors themselves can achieve higher yields, since their flatness need only be perfect in one plane (rather than two planes for biaxial micromirrors).
The pincushion effect, whereby straight lines appear curved at a display's edges, is also reduced by the two-mirror solution, according to bTendo. - R. Colin Johnson