Apparatus for concentrating light rays (150 A) arriving at said apparatus, said light rays originating from at least one light source external and at a distance to said apparatus, characterized by: a plurality of openings (142 A,142 B,142 C), for enabling said light rays arriving at said apparatus to enter said apparatus, the size of each one of said plurality of openings being substantially small compared to said distance, individual beams of said light rays thereby each arriving at each one said plurality of openings substantially collimated; a single photodetector, for receiving said light rays; and a respective Fresnel lens assembly for each one of said plurality of openings, for concentrating said light rays entering said apparatus from said plurality of openings onto said single photodetector, each said respective Fresnel lens assembly comprising: a first Fresnel lens (144 A, 144 B, 144 C), located between a respective one of said plurality of openings and said photodetector, for making parallel with an optical axis (152 A, 152 B, 152 C) of said first Fresnel lens, said light rays arriving from said respective one of said plurality of openings and passing through the focal point of said first Fresnel lens; and a second Fresnel lens (146 A, 146 B, 146 C), located between said first Fresnel lens and said single photodetector, for converging said parallel light rays onto said single photodetector, wherein each one of said plurality of openings is located in front of said respective Fresnel lens assembly, on the focal plane of said respective first Fresnel lens, centered on the focal point of said respective first Fresnel lens; wherein said single photodetector is located behind each said at least one respective Fresnel lens assembly, on the focal point of the respective said second Fresnel lens. a plurality of openings (142 A,142 B,142 C), for enabling said light rays arriving at said apparatus to enter said apparatus, the size of each one of said plurality of openings being substantially small compared to said distance, individual beams of said light rays thereby each arriving at each one said plurality of openings substantially collimated; a single photodetector, for receiving said light rays; and a respective Fresnel lens assembly for each one of said plurality of openings, for concentrating said light rays entering said apparatus from said plurality of openings onto said single photodetector, each said respective Fresnel lens assembly comprising: a first Fresnel lens (144 A, 144 B, 144 C), located between a respective one of said plurality of openings and said photodetector, for making parallel with an optical axis (152 A, 152 B, 152 C) of said first Fresnel lens, a first Fresnel lens (144 A, 144 B, 144 C), located between a respective one of said plurality of openings and said photodetector, for making parallel with an optical axis (152 A, 152 B, 152 C) of said first Fresnel lens, said light rays arriving from said respective one of said plurality of openings and passing through the focal point of said first Fresnel lens; and a second Fresnel lens (146 A, 146 B, 146 C), located between said first Fresnel lens and said single photodetector, for converging said parallel light rays onto said single photodetector, a second Fresnel lens (146 A, 146 B, 146 C), located between said first Fresnel lens and said single photodetector, for converging said parallel light rays onto said single photodetector, wherein each one of said plurality of openings is located in front of said respective Fresnel lens assembly, on the focal plane of said respective first Fresnel lens, centered on the focal point of said respective first Fresnel lens; wherein said single photodetector is located behind each said at least one respective Fresnel lens assembly, on the focal point of the respective said second Fresnel lens.