Optical system for airport semi-flush approach lights

Claim 1

An optical system suited for an aircraft guidance lighting fixture, comprising a lamp, a reflector, a mirror and an exit prism, said lamp being of the high intensity incandescent type, having an elongated filament of finite length, said lamp and said reflector being mounted in said fixture such that said reflector surrounds said lamp, said reflector having a plurality of ellipsoidal surface portions of differing eccentricities, such that the surface portions are axially displaced relative to each other to reflect light from said lamp in the same general direction, the major axes of each of the ellipsoidal surface portions being coincident and passing through the axis of said elongated filament, a focus of each ellipsoidal surface portion being located on said filament and spaced apart equidistantly from each end of said filament, the ellipsoidal surface portion having the greatest eccentricity being located nearest the filament of said lamp, and the conjugate foci of said ellipsoidal surface portions being coincident in space said mirror lying on said major axes at an adjustable angle of substantially forty five degrees to said axes arranged to reflect light from said lamp through said prism and thus substantially at right angles to said major axes.