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Abstract

Infrared detectors, such as thermistors, thermocouples, thermopiles, pyroelectric detectors, and the like, are rendered insensitive to short wave radiation, such as the visible or near infrared, but sensitive to longer wave infrared, for example, beyond 4.5.mu. by coating the active surface of the detector with a strong reflector of visible light but which absorbs infrared at least in the range from 7.mu. to somewhat beyond 10.mu.. A preferred example of the paint, which appears white to the eye, is titanium dioxide, but this is not the only material which can be used as any white or strongly diffuse reflecting coating which absorbs in the infrared from 7.mu. on can be used. The infrared instrument for which the detector is particularly useful is an intrusion detector or burglar alarm which is activated by the infrared radiation from the intruder, which is in the range from around 7.mu. to 10.mu. and beyond. The strongly diffuse reflecting action in the visible prevents false alarms from ambient light. As is common, the detector can be mounted in a housing which protects it and which is provided with a window that transmits infrared. Improved differential sensitivity is obtained by coloring the window, which may be of a plastic such as polyethylene, with dyes or pigments which are opaque or poor transmitters for visible light.

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