Smoke detectors

A smoke detector of the type that depends on the scattering of light by smoke particles comprising:

• a chamber configured to receive smoke particles from an environment in which said smoke detector is required to detect smoke;
• a source of light configured to project light therefrom such that light that is received by said chamber is scattered by said smoke particles;
• an optical element said optical element having
• a light receiving surface in the form of an objective lens configured to receive light scattered by said smoke particles,
• said optical element having an optical axis defined as 0° being normal to said light receiving surface of said objective lens;
• and
• said optical element, upon receiving said scattered light, configured to selectively differentiate between required scattered light that is derived from a subset of predefined directions with respect to the angle of incidence upon said optical element and scattered light that is derived from other directions in order to thereby filter out the light that is not required and thereby redirect said required scattered light onto a photodetector that is configured to detect at least some of said received scattered light;
• said smoke detector characterized in that:
• said optical element comprises a primary prism to provide said differentiation, said primary prism having a first primary prism surface, a second primary prism surface, and said optical element having a secondary prism surface,
• wherein each of the first, second primary and secondary prism surfaces are flat,
• said first primary prism surface and second primary prism surface being located on a rearward side of said optical element, said rearward side being opposite said light receiving surface;
• said secondary prism surface being disposed substantially diametrically opposite a point of intersection of said first primary prism surface and said second primary prism surface;
• said prism thereby configured to substantially transmit said received light to said photodetector for a range of angles of incidence relative to said optical axis of up to ±25° and substantially reject incident light that is received from angles of incidence outside of said range ±25°.