KANAGAWA - Even as laptop computers have grown thinner and lighter, portable document scanners remain a hefty handful. Awkward to use, they bump into the binding area of opened books and obstruct from view much of the document being scanned. By combining fiberoptics with carefully sized rectangular apertures, researchers at NEC Corp. have beat the size limit for such devices and built a full-color document scanner hardly larger than a ballpoint pen. The scanner operates at a resolution of 200 dots per inch (dpi) and has the potential to reach 400 dpi.
Document scanners for personal computers are based on one of four concepts: a single imaging lens, a unit-magnification lens array, a coherent fiberoptic array, or proximity sensing. Upon attempts at miniaturization, all suffer from the same problem—how to get light down to the document and back to a sensing array in a compact space and without resolution loss. All contain inherently bulky off-axis light sources. In the case of the fiberoptic array, off-axis light must pass at a high angle from fiber to fiber to reach the document, creating contrast-limiting stray light.
Although built around coherent fiberoptics, the NEC device achieves space-saving on-axis illumination by use of a photodiode array that contains a series of light-admitting rectangular apertures in each photodiode (see figure on p. 40). A prototype built by the researchers contains 864 100-µm-square photodiodes spanning a 110-mm range and a coherent array made of 15-µm-diameter optical fibers that has a numerical aperture of 0.9. Because the illumination is axial, the coherent array can be fabricated with light-absorbing glass between the fibers, reducing stray light. The scanner has a width of only 13 mm.
Full-color prototype
The prototype device contains a linear light source made up of red, blue, and green light-emitting-diode (LED) chips. During a scan, each LED color is switched on and off sequentially and the photodiode outputs monitored to capture a full-color signal. The researchers aim to replace the linear light source with an inexpensive and commercially available transparent rod illuminated on its ends by LEDs. The rod contains a groove or a strip of white paint that scatters light along its length, providing uniform strip illumination. "The edge-lighting scheme is our choice for consumer applications," says Ichiro Fujieda, research manager at NEC. The researchers have built such a light source into a second prototype.
Experimental results with the scanner in contact with a test pattern show contrast-transfer-function (CTF) values of approximately 0.65 for a pattern having 2 line pairs/mm and 0.4 for a pattern having 4 line pairs/mm. These results are independent of image color. Although even a small gap between scanner and test pattern causes these values to degrade, contact between scanner and paper is easy to maintain in practical use. The researchers have also built a third, slightly larger, prototype scanner that contains both a 200-dpi full-color photodiode array and a 400-dpi monochrome array; it attains a monochrome CTF of 0.6 at 8 line pairs/mm.
