1. Field of the Invention
The present invention relates to the field of linear optical sensor arrays, particularly those using dual output channels servicing interleaved sensing elements, and scanners utilizing such arrays.
2. Prior Art
Scanners for scanning documents, x-rays and other images are now well known and widely used for various purposes, with the market therefor now being large and steadily increasing. One common type of scanner utilizes one or more linear arrays together with associated lighting and optics to view a single line of an article at a time, with the article being moved relative to the linear sensor array in a direction orthogonal to the length of the array at a uniform speed so that the image may be scanned line by line to provide two dimensional image data.
In such scanners there is normally a substantial variation in sensitivity exhibited by the output signals of the various sensor elements along the linear array, in part because of some variations in the array itself, but primarily due to variations caused by uneven lighting and particularly lensing which gathers more light from the center of the scan line than from the ends thereof. Thus, the apparent sensitivity across the array will generally be maximum at the center thereof and curve downward at each end thereof to some substantially reduced apparent sensitivity, typically with some asymmetry and roughness in the apparent sensitivity versus position on the array due to variations in the array, non-uniform lighting, dirt on mirrors and other components in the optical system, etc.
Thus, it is common to take a background reading of the sensors with the system (lighting, etc.) turned on but without an article in scanning position to measure these characteristics immediately before a scan. This measured background output for each sensor element is then stored and values computed so as to provide multipliers for each sensor element which, when used to multiply the output corresponding to that sensor element during scanning of an article, will normalize the apparent sensitivity across the linear array (or arrays), at least to the third order. Thus a uniform shade of grey across an article being scanned will ideally provide a uniform output across the array irrespective of the fact that the edge regions of the article being scanned are undoubtedly subjected to lower illumination than the center region and in spite of sensor element to sensor element sensitivity variations within the array, etc.
In practice, there are various sources of error which can vary from time to time and particularly scanner to scanner, thereby diminishing the performance a scanner user can be assured of obtaining upon the use of a scanner.
One source of error which may heretofore not have been recognized is noise in the reference readings used to normalize the apparent sensitivity of the sensor array along the length of a scan line. Aside from the fixed variations, the present inventors have found time variations in these apparent sensitivities. While one can imagine various causes for such short term variations, the net effect is that the typical method of taking the reference readings and normalizing the sensitivities across the array tends to lock in such variations as they existed at the time the reference readings were taken. Since errors in the reference readings will result in errors in the apparent sensitivity of the output of individual sensor elements, such errors will be replicated across entire lines of the image orthogonal to the array (e.g. in the direction perpendicular to the array scan line), resulting in streaking of the image, particularly apparent when part or all of the digitized image is displayed on a monitor. Image processing enhancements (e.g. sharpening filters) increase the appearance of streaks.
Another source of error which has been found by the inventors relates to the charge coupled display device (CCD) frequently used as the linear array in such scanners. One such sensor is the CCD 191 6000 element linear image sensor manufactured by Loral of Milpitas, Calif. Such devices have a row of image sensor elements with two transfer gates adjacent the sensor elements, one on each side thereof, which do a parallel transfer of interdigitated charge packets accumulated in the sensor elements to A and B transport (shift) registers, again one on each side of the row of sensor elements. These shift registers each shift the charge packets to a respective gated charge detector/amplifier, each of which provides an output signal from the linear detector circuit. Thus the outputs from the row of sensor elements appear alternately on the A and B channels, requiring the recombining (interleaving) of these signals into a single channel to provide the typical scan line data stream for storage and display.
Because of the two output channels, there is the possibility of the two channels being of different amplitude. In general, these may be trimmed so that, by way of example, the amplitude end points in the output range for the two channel correspond. However, because of potential non-linearities in the linear image sensor itself, in the external supporting electronics and/or caused by a combination of the two, there is an opportunity for such non-linearities to result in differences between the A outputs and the B outputs at various exactly equal illumination intensities within the normal operating range of the image sensor when in fact the outputs should be exactly equal. Further, because of the cost of such linear image sensors, it is not practical to use only linear image sensors selected from production which exhibit minimal or no detectable deviation between the two channels. The effect of such deviation is again to cause streaking in the digitized image in a direction orthogonal to the linear array, particularly for the grey shades of maximum difference between channels and when a portion of the digitized image is enlarged and displayed on an expanded scale. Further, it has been found that the transient response of the A and B channels is often unequal, which causes streaking in the portions of the image within which substantial changes in grey shade are occuring. Heretofore, to the best knowledge of the inventors, there has been no solution to this problem other than for manufacturers to work on product improvements to reduce the effect on future sensor arrays.