All objects with a temperature above absolute zero emit infrared radiation. The wavelength of peak emission depends on the object’s temperature, described by Wien’s displacement law:<\/p>\n
\u03bb_max = b \/ T<\/em> (where b \u2248 2898 \u03bcm\u00b7K)<\/p>\n For the human body at approximately 300K (27\u00b0C\/80\u00b0F), the peak emission wavelength is around 9.7 \u03bcm. Most of the emitted energy falls within the 8-14 \u03bcm atmospheric window.<\/p>\n PIR sensors are specifically designed to be sensitive in the 8-14 \u03bcm range for several reasons:<\/p>\n Pyroelectric materials generate a temporary voltage when subjected to changing temperature. In a PIR sensor, two pyroelectric elements are arranged in a differential configuration:<\/p>\n This common-mode rejection reduces sensitivity to uniform temperature changes (e.g., ambient warming).<\/p>\n Most PIR sensors have a silicon window coated with a filter that only transmits IR in the 8-14 \u03bcm band. This filter:<\/p>\n Ordinary glass is opaque to infrared in the 8-14 \u03bcm range. This is why PIR sensors cannot detect motion behind a window.<\/p>\n While the filter helps, some sources still emit in this band:<\/p>\n When designing with PIR sensors, remember:<\/p>\n The 8-14 \u03bcm sensitivity band is a carefully chosen compromise that maximizes human detection while minimizing environmental interference. Understanding this helps in proper sensor placement and application selection.<\/p>\n","protected":false},"excerpt":{"rendered":" The Infrared Spectrum and Human Body All objects with a temperature above absolute zero emit infrared radiation. The wavelength of peak emission depends on the object’s temperature, described by Wien’s displacement law: \u03bb_max = b \/ T (where b \u2248 2898 \u03bcm\u00b7K) For the human body at approximately 300K (27\u00b0C\/80\u00b0F), the peak emission wavelength is […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6],"tags":[],"class_list":["post-1005","post","type-post","status-publish","format-standard","hentry","category-reference"],"blocksy_meta":[],"yoast_head":"\r\nWhy 8-14\u03bcm?<\/h2>\n
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Physics of Pyroelectric Detection<\/h2>\n
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Filtering: The Silicon Window<\/h2>\n
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Practical Implications<\/h2>\n
Why PIR Sensors Don’t Work Through Glass<\/h3>\n
False Trigger Sources<\/h3>\n
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Comparison with Other Motion Sensors<\/h2>\n
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\n \nSensor Type<\/th>\n Detection Principle<\/th>\n Spectral Range<\/th>\n<\/tr>\n<\/thead>\n \n PIR<\/td>\n Passive IR (heat)<\/td>\n 8-14 \u03bcm<\/td>\n<\/tr>\n \n Microwave<\/td>\n Doppler radar<\/td>\n RF (5.8 GHz, 24 GHz)<\/td>\n<\/tr>\n \n Ultrasonic<\/td>\n Sound reflection<\/td>\n 40 kHz<\/td>\n<\/tr>\n \n Thermopile<\/td>\n IR intensity<\/td>\n Broadband IR<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Design Considerations<\/h2>\n
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Conclusion<\/h2>\n