| 摘要 |
A passive-type infrared detector 1 includes an infrared sensor 3, a pair of lens elements (optical elements) 4R and 4L, a pair of mirrors 5R and 5L and a casing 2. The lens elements 4R and 4L are used to define detection areas a+ and a- opposed substantially 180° to each other for the infrared sensor 3. The mirrors 5R and 5L are used to direct infrared rays of light from the detection areas towards the infrared sensor 3. When this passive-type infrared detector 1 is installed at a position intermediate of an alert region, a combination of the infrared sensor 3, one of the lens elements 4R and one of the mirrors 5R is effective to monitor a range from one end of the alert region to the intermediate position whereas a combination of the infrared sensor 3, the other of the lens elements 4L and the other of the mirrors 5L is effective to monitor a range from the opposite end of the alert region to the intermediate position. Therefore, one half of the alert region may be a detection distance. Consequently, the lens elements 4R and 4L can have a reduced focal length, allowing the passive-type infrared detector 1 of a reduced outer size to monitor the long alert region.
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