In electronic input scanners for document scanning, image information is acquired by sensing light from an image at any array of photosites arranged across a path of relative movement of the array and the image. The photosites, typically photodiodes or amorphous silicon sensors, are formed on a semiconductor substrate or chip, with a number of chips butted or arranged closely together to form the array. The photosite array may provide a 1:1 correspondence of photosites to the width of the actual image (a full width array), or may rely on optics to reduce the apparent image size to correspond to a smaller array. In use, a photosite produces an output signal proportional to light intensity detected at the photosite.
As the number of photosites in the imaging bar increases to provide a full width array or greater number of photosites, the problems of manufacturability of the larger arrays begin to become prominent. In a 300-600 spot per inch (spi) full width array for scanning documents, which may include 3000-6000+ photosites, even a manufacturing process that has a 99% yield can produce bad photosites along the array. Even when the photosites are not bad, the offset and gain response may be non-uniform across the array. To allow the use of arrays not meeting the required specifications for number of pad photosites or uniformity, correction of the array output is required. Accordingly, it may be seen that correction techniques are important to the economic manufacture of the arrays. Without correction techniques, arrays not meeting specifications in these areas would be discarded.
Responsivity at the photosites is measured against a standard value. Gain is a measure of sensitivity of the photosite to light and is the slope of the curve of light intensity (x-axis) versus output voltage (y-axis). Offset indicates the voltage output of the photosite at zero light intensity, or constitutes the y-axis intercept of the light intensity curve. These values have a tendency to vary somewhat from photosite to photosite within a range of values. Uniformity is desirable to avoid a streaking response.
U.S. Pat. No. 4,698,685 to Beaverson shows an arrangement which provides a gain correction for each pixel in an array. Gain values are stored for each pixel in an electronic storage device. As data is acquired by the array and directed to an image processor, each incoming pixel value is multiplied by a selected gain correction value to produce a gain corrected output. U.S. Pat. No. 4,639,781 to Rucci et al. shows that distortions in a video signal may be corrected by applying a continuous gain adjustment to the video information generated at the pixels and dynamically changing the gain factors on a line by line basis. U.S. Pat. No. 4,660,082 to Tomohisa et al., teaches that calibration and shading correction of image data may be corrected in synchronism with input scanning by comparison to a density reference value. U.S. Pat. No. 4,216,503 to Wiggins shows deriving offset and gain values from the sensor, storing those values and subsequently using those values for signal correction. U.S. Pat. No. 4,314,281 to Wiggins et al. teaches providing a compensation signal compensating for variations in light to which the sensors are subjected and deriving the compensation signal over a group of pixels, by taking an average response from the group as the group is exposed to a test pattern. U.S. Pat. No. 4,602,291 to Temes teaches a multimode pixel correction scheme which includes correction for pixel offset and gain.
U.S. Pat. No. 4,590,520 to Frame et al. and U.S. Pat. No. 4,314,281 to Wiggins et al. teaches correction of bad photosites (sometimes, "bad pixels") in an array of photosites by detecting the pixel information from the photosites and substituting the previous pixel value. U.S. Pat. No. 4,701,784 to Matsuoka et al. teaches correcting the bad pixel by calculating the correlation coefficient and selecting a substitute value for the bad pixel based on the calculated value.