Abstract: A hidden or concealed lock for sliding windows, door panels and the like, employing a concealed hollow tubular locking sleeve that is positioned entirely inside of an aligned opening passing through the interior frame of one of said windows and into the frame of the other window. The locking sleeve is manipulated into and out of locking position by a small rod shaped key member that is coaxially insertable into the locking sleeve from outside the window after passing through the small opening in the window frame. Both the locking and key members and openings are made small and unobstrusive; and are located for concealment in differing locations. These features prevent an unauthorized user from either locating the lock, determining its mode of operation; or unlocking the windows or panels without access to the special key.

Abstract: An optical marking system which utilizes tags that are formed from multiple diffraction gratings, which gratings differ in orientation and number of lines (frequency) in accordance with the coded information desired on the tags. A monochromatic light source illuminates a diffraction grating tag, with reading of the coded information on the tag being accomplished by sensing the reflected light which is projected as images in a Fraunhofer plane. Retroreflective elements are provided in the optic train as a means for stabilizing the direction of reflection from the diffraction gratings such that the reflections are somewhat independent of the tilt angle of the tag with respect to the illuminating light.