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A monolithic image sensor for a reading aid for the blind

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2 Author(s)
Salsbury, P.J. ; Fairchild Semiconductor, Palo Alto, Calif. ; Meindl, J.D.

A compact optical-tactile reading aid that allows a blind person immediate access to virtually all printed reading material used by sighted people has been developed. A singular feature of this instrument is a silicon monolithic image-sensing array that serves as the "retina" of the reading aid: signals from the retina are used to control an array of piezoelectric stimulators that form tactile images of printed characters. The image sensor consists of a 24×6 matrix of bipolar phototransistors integrated on a chip approximately 3×5 mm in size, and is designed on the basis of electrical, optical, psychological, and psychophysical considerations. The column-isolated structure incorporates a 2 to 1 aspect ratio to provide sufficient resolution and field of view for unambiguous letter-by-letter detection using a minimum number of sensor elements. A one-dimensional scanning technique minimizes fixed pattern noise and reduces the complexity of the total system. The individual 150×300 µm photosensing elements are optimized for maximum responsivity, uniformity, and dynamic range. The doping profile, featuring a 2 µm base depth and 12 µm epitaxial thickness, is based on spectral response and on a detailed electrical analysis of array operation. Output nonuniformity is minimized, and electrical crosstalk is virtually eliminated by designing for hFE≥ 200 and CTC/CTE≥ 5. Array performance correlates well with the predicted characteristics and has led to improved reading speeds greater than 50 words per minute.

Published in:

Proceedings of the IEEE  (Volume:58 ,  Issue: 9 )