An anti-masking device for an intrusion detector comprising: an interior area with an electric light source and a light detecting sensor, the electric light source and the light detecting sensor being spaced-apart from one another; a cover enclosing at least a portion of the interior area, the cover having a light-permeable area having an exterior surface comprising a jagged array in cross-section, the jagged array having a plurality of parallel spaced-apart teeth forming a series of alternating channels and projections, the light-permeable area of the cover having a thickness and the plurality of spaced apart teeth of the jagged array having a height, the thickness being greater than the height such that the jagged array extends partially into the thickness of the light-permeable area of the cover; a light guide having opposing first and second ends and an elongated body therebetween, the first end being located within the interior area adjacent the electric light source, the second end being located outside the interior area and adjacent the exterior surface of the cover, wherein the light guide is configured to direct light emitted from the electric light source outside the interior area and onto the jagged array along the exterior surface of the cover, wherein the jagged array is configured to refract the light directed from the light guide through the cover and into the interior area, wherein, during an ordinary condition of use, the light refracted by the jagged array is directed onto a first location within the interior area, the first location within the interior area being different from a position where the light detecting sensor is located, and wherein, during a masking event, the alternating channels of the jagged array are configured for being filled with a masking substance, and wherein the light passing through the light-permeable area is redirected from the first location within the interior area to the position where the light detecting sensor is located, an alarm signal being triggered by said redirection of the light onto the light detecting sensor. an interior area with an electric light source and a light detecting sensor, the electric light source and the light detecting sensor being spaced-apart from one another; a cover enclosing at least a portion of the interior area, the cover having a light-permeable area having an exterior surface comprising a jagged array in cross-section, the jagged array having a plurality of parallel spaced-apart teeth forming a series of alternating channels and projections, the light-permeable area of the cover having a thickness and the plurality of spaced apart teeth of the jagged array having a height, the thickness being greater than the height such that the jagged array extends partially into the thickness of the light-permeable area of the cover; a light guide having opposing first and second ends and an elongated body therebetween, the first end being located within the interior area adjacent the electric light source, the second end being located outside the interior area and adjacent the exterior surface of the cover, wherein the light guide is configured to direct light emitted from the electric light source outside the interior area and onto the jagged array along the exterior surface of the cover, wherein the jagged array is configured to refract the light directed from the light guide through the cover and into the interior area, wherein, during an ordinary condition of use, the light refracted by the jagged array is directed onto a first location within the interior area, the first location within the interior area being different from a position where the light detecting sensor is located, and wherein, during a masking event, the alternating channels of the jagged array are configured for being filled with a masking substance, and wherein the light passing through the light-permeable area is redirected from the first location within the interior area to the position where the light detecting sensor is located, an alarm signal being triggered by said redirection of the light onto the light detecting sensor.