Measurements of the diffraction characteristics of one-dimensional surface-
relief gratings of locally varying profile are compared with rigorous diffr
action theory. These gratings result from the superposition of two linear s
inusoidal gratings of uniform depth for which the relative phase between th
e two gratings varies slowly with position. The resultant surface profile e
xhibits a relatively large-period variation in profile form. The periodic v
ariation in diffraction efficiency that results yields a visual moire patte
rn that has interesting asymmetry and polarization properties that alter as
the viewing conditions are changed; the gratings Lean be exploited by diff
ractive optically variable devices for document security. (C) 1999 Optical
Society of America.