| 摘要 |
PURPOSE:To detect plural detection areas by one Fresnel lens by reflecting a heat ray which is incident on the Fresnel lens in a direction different from the optical axis of the Fresnel lens by a reflecting mirror and thus irradiating a heat ray detecting element. CONSTITUTION:The reflecting mirror 3 uses a plane mirror and arranged in parallel to the optical axis X of the Fresnel lens 2 which converges the heat ray on the heat ray detecting element 1 so that the heat ray in the direction of an optical axis X is incident on the lens 2. Consequently, a new detection area B is obtained in addition to a detection area A from the optical axis X. The mirror 3 while held parallel to the optical axis X is moved up to a position 3', so the detection area B is changed to the detection area B'. Therefore, detection areas B and B' in different directions from the detection A in the optical axis direction of the normal Fresnel lens are also detected by the heat ray detector, and consequently the detection area is widened. |
