| 摘要 |
A security device comprises a substrate having a transparent region (1). At least one optical element (2, 3) is provided in part of the transparent r egion, the optical element causing an incident off-axis light beam transmitt ed through the optical element to be redirected away from a line parallel wi th the incident light beam whereby when the device is viewed in transmission directly against a backlight, the presence of the optical element cannot be discerned but when the device is moved relative to the backlight such that lines of sight from the viewer to the transparent region and from the transp arent region to the backlight form an obtuse angle (.alpha.) at which redire cted light is visible to the viewer, a contrast is viewed between the part o f the transparent region including the optical element and another part of t he transparent region. When the security device is viewed in reflection unde r diffuse lighting conditions either no contrast can be discerned between th e two parts or a different contrast can be discerned between the two parts.< /SDOAB> |
