Patent ID: 6842559

Claim:
An apparatus comprising: an optical filter having a predefined passband, said filter being responsive to optical radiation which impinges on said filter while traveling along a first optical path for causing a portion of the radiation from said first optical path which is within said passband to pass through said filter and for reflecting a remaining portion of the radiation from said first optical path so that it travels along a second optical path, and said filter being responsive to optical radiation which impinges on said filter while traveling along a third optical path for causing a portion of the radiation from said third optical path which is within said passband to pass through said filter and for reflecting a remaining portion of the radiation from said third optical path so that it travels along a fourth optical path, said first, second, third and fourth optical paths being different from each other, said filter having a substantially planar surface portion on one side thereof, said filter effecting said reflections of portions of radiation substantially at said surface portion, and wherein said first, second, third and fourth optical paths are each contained at angularly spaced locations within a side surface of an imaginary right circular cone having an axis perpendicular to said surface portion and having an apex substantially at said surface portion; and a redirecting section for causing optical radiation traveling away from said filter along said second optical path to be redirected to travel toward said filter along said third optical path.