to the desired print line along an ideal print path and the track 26 the light source 30 can be an incandescent or fluorescent light a low intensity gas or ruby laser or preferably a low cost semiconductor laser the beam may be directly used or may be reflected from a mirror to allow an orthogonal or offset mounting position of course other light beam control devices such as lenses fiber optics and filters can be employed to define and orient the light beam a photodetector 40 is an optical sensor producing an electrical signal when illuminated the photodetector 40 is mounted on the print head 22 to receive the illumination from light source 30 in this way a deviation of the path of the print head 22 from the path of the beam from light source 30 causes a change in the electrical signal from photodetector 40 for example a drop of intensity will be sensed if the head deviates from the illuminated path the photodetector can include multiple elements in an array or pattern to further detect the direction of deviation or to be responsive to changes in relative angle between the light path and detector the necessary resolution number of elements sensitivity and size to accomplish the task will vary according to the light source brightness path length additional optical elements interspersed in the path and other factors in a particular embodiment a print head mount 24 carries print head 22 along track 26 and includes and provides electromechanical adjustment of the position of print.