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Optical Design & Engineering

Optical mouse goes powerless

A non-electrical optical mouse acts as a passive reflector for an open-path spectrometer.

From oemagazine September 2003
31 September 2003, SPIE Newsroom. DOI: 10.1117/2.5200309.0007

Figure 1. In the optical mouse, an IR source in the keyboard transmits a beam to the passive mouse. A grating disperses the signal, which passes through the chopper wheels at the mouse ball. Corner-cube retroreflectors return the signal, which passes back through the optical path to reach the detector. HinesLab Inc.

Until now, all computer mice have been electro-optical devices. In the conventional ball mouse, electrical energy powers LEDs that shine light across 3-mm gaps through radial slots in rotating chopper wheels to illuminate photodetectors. Rolling the ball turns the chopper wheels, which in turn modulate the signal received by the photodetectors. The detected signal is carried to the computer by wire, or transmitted by radio signal in the case of a cordless mouse, to create cursor movement on the computer screen.

Our non-electrical optical mouse design eliminates the need for batteries and for platform compatibility. The light mouse is an opto-mechanical device containing no electrical parts. The mouse controller consists of an electro-optical device integrated into the edge of the keyboard. In concept, the design expands the 3-mm optical gap in the conventional mouse to a total of about 25 cm (12.5 cm from the controller to the mouse, and an equal 12.5 cm in the return path from the mouse to the controller). The controller contains photodetectors and a broadband infrared source such as the type of small incandescent lamp used in pocket flashlights or five IR LEDs of different wavelengths.

The light mouse acts as the reflector portion of an open-air spectrometer. The broadband IR source at the keyboard reflects off of a convex cylindrical surface and fans out horizontally across the surface where the mouse is used (see figure). Light enters the mouse through side openings near the front and travels to a reflective diffraction grating. The grating separates the light into a minimum of five narrowband wavelengths that propagate to molded off-axis parabolic reflectors. Two wavelengths pass to each chopper wheel to monitor cursor movement, and the fifth wavelength tracks the mouse button.

The parabolic reflectors focus their respective wavelengths onto mechanical devices that act like shutters to optically modulate the beams. In the case of cursor motion, tracked by the first four wavelengths, the shutters are the chopper wheels. The fifth wavelength is focused on a mouse-button-activated aperture that acts as a shutter. When the mouse button is activated, the shutter opens to allow the beam to pass through to the retroreflector and back through the system, indicating a mouse click. A molded patch of corner-cube retroreflectors behind each chopper wheel returns any signal that passes between the spokes of the choppers to essentially deliver a modulated data stream to the diffraction grating, where it recombines with the other wavelengths. The multispectral beam passes out of the mouse to travel to the detector in the edge of the keyboard. The corner-cube retroreflectors can be molded in the mouse, or micro-corner-cube retroreflective sheeting can be die cut and attached.

Inside the keyboard controller, returned light passes into the spectrometer section, where the design needs to detect the five returning narrowband IR signals. A second reflective diffraction grating in the controller module separates out the individual signals, which are focused onto five separate photodetectors. Using the five photodetectors instead of a linear detector helps control cost. Two of the photodetectors sense the presence of lateral mouse motion, and two sense longitudinal motion; the fifth detector captures the mouse-button signal. The photodetectors are spatially offset to detect the five specific wavelengths formed by the return signals dispersed by the diffraction grating in the keyboard controller.

The key optical elements of this design (flat and off-axis parabolic mirrors) are molded as part of the base, and then flashed with aluminum to increase reflectivity. The diffraction grating is molded and aluminized in the same process as that used to make music CDs. This approach reduces part count and cost while increasing reliability. Unlike conventional optical mice, the passive optical mouse of this article requires no electronics, cord, or batteries, works on any surface, and makes the issue of platform compatibility a moot point. oe

Steve Hines
Steve Hines is an inventor, optical engineer, and founder of HinesLab Inc., Glendale, CA.