NOVELTY - The detector has first and second hemispherical caps (1-1, 1-2) including opposite concavities and a plate arranged in an equatorial plane (5) of the hemispherical caps. The hemispherical caps are reflective on a portion of opposite surfaces. The plate is placed at distance from a center of an equatorial plane. A diverging light generator (AB) is directed toward the first hemispherical cap. A light detector (CD) is directed toward the second hemispherical cap. The hemispherical caps are assembled in a sphere and spaced apart from each other by a ring (3).    USE - Optical absorption gas detector for an imager detector i.e. infrared imager detector (all claimed), for detecting gas in an atmosphere. Uses include but are not limited to carbon dioxide, carbon monoxide, methane and toxic gas e.g. xylene or toluene, released by paint.    ADVANTAGE - The detector is simple to manufacture and tolerant to misalignments among an emitter, reflective surfaces and a receiver. The detector forms source-filter-receiver systems collectively so as to limit manufacturing cost of the detector. The detector performs metallization on a section of surfaces located at latitudes greater than 60 degrees relative to the equatorial plane so as to limit loss of rays. The detector performs metal deposition to make inner surfaces of hemispherical cap reflective to be performed, so that the reflective surface is slightly rough to blur an image of filament on the receiver, thus increasing tolerance to manufacturing imperfections. The detector provides large positioning tolerance due to fact that an emitting portion and a receiving portion of an emitter-receiver assembly are assembled head-to-tail in a single block, so that the emitter and the receiver displace together, thus compensating for consequences of possible mispositionings.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for an imager detector.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of an optical gas detector tolerant to misalignments.    Diverging light generator (AB)    Light detector (CD)    Hemispherical caps (1-1, 1-2)    Ring (3)    Equatorial plane (5)    Openings (9)    Emitter-receiver assembly (11)