A printing method and apparatus (100) for pre-exposing a digital watermark
along a length of photosensitive medium at the time of manufacture is disclosed.
The printing apparatus has an illumination source (60) for providing an
exposure illumination, first and second LCD spatial light modulators (90a,
90b) for modulating the exposure illumination to form first and second
exposure patterns according to image data, combining optics providing a single
output path for directing both first and second exposure patterns onto the photosensitive
medium and transport means for providing, during exposure, lengthwise displacement
of the photosensitive medium with respect to the output path. The first and second
exposure patterns form, lengthwise along the photosensitive medium, a latent image
with modulated stripes having a predetermined intensity corresponding to the exposure patterns.