Tech Trends

Thermal imaging a hot technology

Infrared (IR) energy for sensing and imaging - either as a complement or as an alternative to visible energy - has for years been used in many applications, including physical security, military and law enforcement. But technological advances in both active and passive infrared are bringing the technology closer to the mainstream.

Infrared energy resides in the electromagnetic spectrum in the frequency band above microwave energy and below visible light. Since many readers may be familiar with light in terms of wavelength, I will discuss IR in these terms. Where visible light runs from 390 to 750 nm (violet to red), near-IR picks up and covers the range from 750 nm-2500 nm (2.5 æm or microns), mid-IR from 2.5 -10 æm, and far-IR from 10 æm to approximately 1 mm. According to NASA, humans at normal body temperature radiate most strongly in the infrared at a wavelength of about 10 microns.

Active infrared employs the use of an infrared light source in the wavelength range of 850-940 nm. Practically speaking, most objects emit little radiation in the near-IR, however, one must use auxiliary IR lighting, or illumination, to create light reflections off the object. Just as we need auxiliary lighting such as flashlights to see people or objects in the dark, devices need IR "flashlights," called illuminators, to see in the IR.

These illuminators can be light bulbs, such as quartz halogen, LEDs or lasers, with a trade-off between cost and performance (range).The performance advantages of active IR include illumination of features that have no heat emission characteristics (e.g., license plates), transmission through translucent materials, and the potential for low-cost and better performance in video compression, due to improved signal-to-noise ratio resulting from a clearer, better lit, scene.

While illumination enhances the performance of these systems, it does have the potential drawback of being "non-stealth." Look for continuing improvements in both illuminators and detectors, including lower cost, higher-performance light sources; improved optics; better detector performance; and enhanced signal processing, particularly using CMOS technology which allows each pixel's signal to be processed individually.

Passive infrared relies on heat sensing from the surveillance target, where feature discrimination is based on temperature differences. The military has used this technology for decades, generating video images from targets miles away based on their heat signatures. Passive sensing can be used in the 3-5 æm mid-IR or 7-12 æm long wavelength IR (LWIR) ranges (water vapor in the atmosphere tends to absorb the energy of wavelengths between about 5 to 7 microns).

Because mid-IR can be more affected by airborne particles and because humans emit heat around 10 microns, most passive IR systems, including PIR intrusion sensors, work in the LWIR. A significant advantage of passive IR in this range is that elements in the atmosphere that obscure or block visibility in the visible and the near-IR, such as fog or smoke, are almost transparent to LWIR. Even across distances of 500m and greater, the signal is not susceptible to any considerable attenuation.

Technological advancements in detectors now promise to bring passive IR imaging more into the mainstream. Historically, detectors have required cooling to allow a voltage difference or resistance change to occur when impacted by photons. This is commonly provided by thermo-electric coolers, a type of solid state heat pump.

The microbolometer, a detector based on micro-electromechanical (MEMS) technology, detects changes in temperature caused by the incoming 8-12 æm heat radiation which affects its electrical resistance. This requires no thermo-electric cooling (TEC-less), thus reducing size and cost. Amorphous silicon (a-Si) and vanadium oxide are the two most common materials used in the microbolometer's outermost layer. The semiconductor industry's expertise in silicon processing has provided the means to process the a-Si detector with increased uniformity and lower cost. The a-Si microbolometer has shown high sensitivity, capability to produce images at greater than 60 Hz frame rate, resolutions to XGA, and performance in harsh environments.

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