Abstract: A lens segment used in passive infrared motion detectors is disclosed. A preferred embodiment has semi-circular rings separated by grooves. The semi-circular rings are offset about a separation axis passing through their centers or optical axes. The separation axis effectively divides the lens segment into two subsegments causing projection of two cross-sectional areas of coverage, one offset from the other. This increases the size of the projected cross-sectional area of coverage preventing a rodent near the lens from substantially filling the projected cross-sectional area of coverage, thus avoiding detection of the rodent.