Application of additional mirrors for rectilinear laser scanning of wide formats

If an inexpensive postobjective laser scanner is used for high-resolution scanning of wide formats, the focus either describes a curved scanning line or departs from the scanning plane. To eliminate these drawbacks, it is shown that the laser beam, if first deflected by a rotating polygon mirror, may be positioned by an additional mirror just in front of the scanning plane to rectify the curve. The shape of the additional mirror, which depends on the properties of the deflection unit, was determined in solving a differential equation. Unfortunately, the resulting mirror shape follows a complicated curve which is difficult to manufacture. Further calculations, however, showed that the deviation of the focus path from a straight scanning line can be made small, even for large scan widths, if circular and elliptical additional mirrors are utilized.