Visible light may be accurately positioned or "steered" by at least three classes of electro-optical devices; galvanometric scanners, resonant mirrors, and rotating polygon mirrors.
Galvanometric scanners reflect light off of a mirror which may be rotated about an axis through a small angle by application of an electrical current situated in the presence of a fixed magnetic field. The electrical current generates a magnetic field proportional to the current and the interaction of the two magnetic fields causes the electrical lead conducting the electrical current to twist relative to the fixed magnetic field. This twisting motion can be advantageously linked to a small mirror by which incident light may be steered.
Resonant mirrors similarly reflect light off of a mirror which is caused to move by the application of an electrical signal. Here, however, the electrical signal drives a subcomponent such as a voice coil from an audio speaker. The voice coil in turn pushes against a spring-mounted hinged mirror. The mirror typically has attached to it a counterweight such that the resonant frequency of the counterweight and mirror assembly acts like a tuning fork. The mirror can then be made to oscillate at a resonant frequency to steer incident light in a periodic fashion. These two classes of devices are typically used in applications such as flying spot scanners and laser printers.
The rotating polygon mirror is a multi-faceted mirror rotated at high speed by a precision motor. As each facet subtends a light beam from a laser source, it scans it through an arc with a typical included angle up to about 120 degrees. The mirror elements are generally facets cut onto the periphery of a disk, though other shapes are sometimes encountered. The laser polygon is most often used in laser xerographic printer systems as the optical scanner, converting digital inputs into patterns of light on a photoreceptor surface. The patterns are subsequently developed and printed onto paper.
Galvanometric scanners, resonant mirrors, and polygon mirrors have disadvantages when used to steer light. All three classes of devices are relatively large, are expensive, and are susceptible to shock and vibration. These limitations preclude their use in many consumer applications and where component size is a constraint. Galvanometric scanners and resonant scanners are generally slow responding and are also typically susceptible to changes in the scanner's motion. This precludes their use in most mobile environments.
Another electro-optical device known to those skilled in the art is a spatial light modulator such as a deformable mirror device ("DMD"). While DMD's have been used in some light steering applications they suffer from the disadvantage of being too small for many applications. A typical DMD mirror is on the order of 12.times.12 .mu.m.sup.2. A useful light steering device should be in the range of 0.5.times.0.1 in.sup.2. In attempting to increase the size of DMD's a significant problem arises as a result of the mirror distortion. Typically, DMD's are supported at two edges. Due to the increased size of such a large DMD, they will distort under the influence of the applied operating voltage, making them useless.
Therefore, a need has arisen for a method and apparatus for steering light which is very compact, inexpensive, power efficient, and suitable for use in a non-stationary environment.