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To enhance the security of a dot-matrix hologram (DMH), a method to hide an image inside the DMH is proposed. The hidden image, which does not depress the visibility and the dynamic-effect feature of the DMH, is invisible but can be reconstructed by illuminating the DMH with a specified laser beam. The reconstructed hidden image is invariable to the lateral shift between the laser beam and DMH. Two experiments are carried out to illustrate the applicability of the proposed method. © 2002 Society of Photo-Optical Instrumentation Engineers.

Dot-matrix holograms created by two-beam writers usually contain two-dimensional grating dots. Although the dot positions may be accurate, the fringe positions of the dots are randomly determined. The random fringe positions are good tools to identify a hologram. A grating position-deviation coefficient derived from the fringe position difference of two grating dots with the same pitch and the same orientation is defined and it is used as data to identify a dot-matrix hologram. Experiments have proved the proposed method practical and feasible. Because the fringe position difference between the two grating dots with the same pitch and the same orientation is difficult to reproduce, it is excellent for anticounterfeiting.

Modified light-diffusion patterns created by contact-copy interference of diffused laser lights are proposed. A conventional light-diffusion pattern is created by means of a diffuser and a noncontact photosensitive plate (a noncontact or sandwich method). The proposed method uses a contact photosensitive plate and allows implementation of new applications such as surface-relief holograms, anticounterfeiting, and light-diffusion eyemarks.