Abstract: An anti-masking device for an intrusion detector and method for detecting a masking substance is disclosed. The device can comprise a cover enclosing an interior area housing an electric light source and a light detecting sensor. The cover can have a light-permeable area having an exterior surface with a jagged array including a plurality of parallel spaced-apart teeth forming a series of alternating channels and projections. A light guide can be provided with opposing first and second ends. The first end can be located within the interior area adjacent the light source and the second end can be located adjacent the exterior surface of the cover. The light guide can be configured to direct light outside the cover and onto the jagged array. The jagged area can be configured to refract light from the light guide through the cover and into the interior.