Current optical scanner such as digital facsimile scanners incorporate a galvanometer which scans a document to be reproduced as a facsimile or reproduced as signals which are transmitted to a remote location and then reproduced as a facsimile. The scanning motion consists of a right to left or left to right scan across the document followed by a retrace, as is well known. The start of each scan generally creates an uncertain start point.
Thus, it becomes important to align information from subsequent scans with the information generated by each preceding scan. In order to do this, prior art optical scanning systems utilize a target, such as a diode, across which the document scanning beam is swept. As the beam strikes the target diode, a signal is generated which may be utilized to align a subsequent beam with each preceding beam.
Generally, the leading edge of the signal generated when the target diode is struck by the light beam was utilized to enable various electronics within the optical scanning system. The signal generated by a scanning line across the document is divided into picture elements (pels) which are read as either a high or low digital signals that represent a finite space. Due to the digital character of the target signal, the method described above creates two distinct disadvantages.
First, if the grating is not exactly aligned to coincide with the target, there is an inherent error of plus or minus one grate of the grating due to the digital character of the grating signal. The tolerance between the alignment of the grate and the target is generally greater than one grating space. Keep in mind that a grating space is generally larger than a picture element and that lateral adjustment of either the target or the grate, in most systems, is impractical. Thus, it is inherently difficult if not impossible to adjust an optical scanning system to be more accurate at the start of scan than plus or minus one grating element. Further, the long term drift of the system makes such an adjustment, even if practical, impossible to retain over an extended period of time.
The second disadvantage of enabling the optical scanning system electronics on the leading edge of the target is that the target has a finite width. Therefore, a given number of picture elements corresponding to the target width are not available to carry data. The result is that the actual number of data contained picture elements is less than the apparent number.