Optical arrangement for a passive infrared motion detector

Claim 1

In a passive infrared motion detector having an infrared detector unit and means for directing infrared radiation from at least two separate directions to be monitored on said detector unit, the improvements comprising the means for directing including a single means for focusing radiation from more than two directions simultaneously and more than two separate, flat reflecting surfaces, said single means for focusing radiation including a single concave mirror having an optical axis and a deflecting mirror being arranged on the optical axis of the concave mirror and at an angle therein to deflect light focused by the concave mirror at an approximately a right angle to the optical axis and onto the infrared detector unit so that the focused radiation from said concave mirror travels to the detector unit in a folded path with two portions extending substantial at right angles to each other, said flat reflecting surfaces being a plurality of individual separate flat reflecting surfaces with at least one being arranged on each side of the optical axis of the concave mirror, said reflecting surfaces being arranged with at least one reflecting surface associated with each direction to be monitored to reflect the radiation simultaneously from said direction onto said concave mirror with at least two of the reflecting surfaces having planes extending at different angles to the optical axis.