A ceiling mount intrusion detector with arbitrary direction detection capability is provided. The detector can include a pyroelectric sensor and a unitary Fresnel lens such that the unitary Fresnel lens includes a first piece of a first full Fresnel lens and a second piece of a second full Fresnel lens and such that the first piece and the second piece are spliced together. Due to the dimensions and location of the unitary Fresnel lens relative to the sensor, the sensor can output a signal indicative of an alarm condition when an object passes in front of the lens at any angle relative to the sensor.