Patent ID: 6930759

Claim:
An apparatus for exposing a latent watermark along a length of photosensitive medium, the apparatus comprising: (a) an illumination source for providing an exposure illumination; (b) a first spatial light modulator for modulating said exposure illumination to form a first exposure pattern according to first image data; (c) a second spatial light modulator for modulating said exposure illumination to form a second exposure pattern according to second image data; (d) combining optics providing a single output path for directing said first and second exposure patterns onto the photosensitive medium; (e) a transport for providing continuous lengthwise displacement of the photosensitive medium, during exposure, with respect to said single output path; wherein said first and second spatial light modulators cyclically alternate in providing respective first and second exposure patterns to said combining optics, such that during a cycle wherein said first spatial light modulator forms said first exposure pattern, said second spatial light modulator loads the image data for providing said second exposure pattern; and said first and second exposure patterns thereby forming, lengthwise along the photosensitive medium, said latent watermark comprising modulated stripes having a predetermined intensity corresponding to said first and second exposure patterns.