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Thermal Imaging

Written by Eugene Nielsen

http://www.raytheoninfrared.com http://www.leta.orgThe high cost and a general lack of knowledge about the usefulness of thermal imaging have been major factors limiting its use by law enforcement. Recent technological advances have resulted in significant reductions in cost. Today, thermal imagers are available for less than a tenth of what they cost a decade ago. In addition to being more affordable, today’s thermal imagers are smaller, lighter and provide greater performance than their predecessors. 

A thermal imager measures minute temperature differences that can’t be seen by the naked eye. It takes those measurements and creates an infrared (IR) picture. Thermal imagining not only allows one to see in no or low light conditions, it has numerous other law enforcement applications.

The Law Enforcement Thermographers’ Association (LETA) currently recognizes 11 official law enforcement applications for the use of thermography. A federal or state court must have accepted the IR images as evidence in a case before the particular application is recognized by LETA. These uses include search and rescue, fugitive searches, vehicle pursuits, flight safety, marine and ground surveillance, perimeter surveillance, structure profiles, disturbed surface scenario, environmental law enforcement, officer safety, and hidden compartments in vehicles. 

Fugitive Searches and Search and Rescue

Thermal imagers can be invaluable tools in fugitive searches and search and rescue. People and animals are excellent radiators of thermal energy. Thermal imagers can be used to quickly search large areas during darkness or in daylight. Thermal imaging is excellent in finding people hidden in foliage. Searches can be conducted without blind sweeps involving numerous officers or without giving away the searcher’s position.

Vehicle Pursuits

Vehicles radiate considerable heat during and after use. Heat is radiated from the engine, tires, brakes and exhaust. Thermal imaging allows officers to easily track a vehicle from the air or another vehicle. A recently driven vehicle can be detected from the residual heat.

Flight Safety

Aircraft-mounted thermal imagers are effective tools to enhance flight safety during nighttime operation, or through obscurants such as smoke from wildfires. Hazards, such as power lines and other obstacles, can easily be detected.

Marine and Ground Surveillance

Thermal imagers allow one to navigate under conditions of total darkness. They allow officers to conduct surveillance undetected.

Perimeter Surveillance

Thermal imagers allow officers to more effectively surveil and secure perimeters. They reduce the manpower necessary to surveil and secure perimeters.

Structure Profiles

One of the earliest applications in which thermal imagers have proven to be especially helpful is in the detection of indoor marijuana-growing operations. Indoor growing operations require the use of high-intensity lamps. These lamps generate heat, which must be exhausted from the building to maintain acceptable growing conditions.

In June 2001, the United States Supreme Court found that the use of a thermal imager on a residence was a “search” under the Fourth Amendment. The Court found that police officers must have a search warrant before taking thermal images of a residence. (Kyllo vs.  United States)

Disturbed Surface Scenario

Whenever a surface is altered or disturbed, the IR characteristics of the surface are also altered, although it may not be noticeable with the naked eye. For example, turned earth radiates heat differently than compacted earth. The change in radiance of the soil allows thermal imagers to detect buried items. 
Environmental Law Enforcement

Thermal imagers can be employed to track pollutants to their source. Pollutants (such as chemicals, oil and waste matter) radiate or emit heat differently than the soil or water surrounding them. The airborne emissions from illegal nighttime burning operations can be monitored. Thermal imagers also allow dumpsites to be covertly surveilled under conditions of total darkness.

Officer Safety

Thermal imagers can be a valuable officer safety tool during ground operations, especially at night. Thermal imagers can be used to covertly locate threats day or night, such as hidden suspects, guard dogs, obstacles, trip wires. Thermal imagers can be employed through visible obscurants, such as dense smoke or dust.

Hidden Compartments

Thermal imaging is useful in detecting hidden compartments in walls or floors, or in vehicles used for transporting drugs, contraband or people. An adjoining wall or bulkhead will cause a change in the thermal characteristics of a surface that can be detected by the thermal imager.

LETA recognizes that new law enforcement applications for thermography will continue to expand. Thermography is already being successfully employed in other law enforcement applications, including traffic accident investigation, tracking suspects, linking discarded evidence to a suspect, fire investigation, and crime scene investigation.

How Thermal Imagers Work

All natural or man-made objects that aren’t at absolute zero temperature emit electromagnetic radiation of many different wavelengths. The hotter an object becomes the more infrared (IR) radiation that’s emitted as a result of the thermal agitation of its molecules or atoms. The spectral distribution or wavelength depends on the nature of the body (i.e., its relative effectiveness as a radiator? called emissivity) and upon its temperature. Blacker colors and duller surfaces usually have a higher emissivity and radiate more effectively. Lighter colors and shinier surfaces have a lower emissivity and radiate less effectively.

The human eye can only detect electromagnetic radiation within a narrow band of wavelengths, known as the visible spectrum. The human eye is mostly blind to energy below 0.4 micrometer or above 0.7 micrometer. The IR region of the electromagnetic  spectrum contains wavelengths that range between 0.7 micrometer and one millimeter, making it invisible to the naked eye.

Modern-day thermal imaging devices operate in either the mid-wave IR (three to five micrometers) or in the long-wave IR (eight to 12 micrometers) regions. By sensing the IR energy that’s emitted by objects, thermal imagers generate a real-time image that provides a thermal signature of a scene. By measuring very small relative temperature differences, invisible heat patterns are converted by the thermal imager into clear, visible images that the human eye can see. Thermal imagers are usually quite sensitive and can detect temperature variations smaller than 0.1 degree C.

Operationally, a thermal imager utilizes optics to focus the IR energy that’s emitted by objects in a scene onto an IR detector. The IR data from each of the detector’s elements is then converted into a standard video format, allowing the image to be viewed on a standard video monitor or recorded on videotape. Since a thermal imaging system senses heat and not light, it can be used in full daylight and at night. Thermal imagers are totally passive devices. There isn’t any emission of light or RF energy to give away one’s position.

There are two classifications of IR detectors: photon detectors and thermal detectors.  Photon detectors produce an electrical response as a direct result of their absorption of IR energy. Thermal detectors produce an electrical response due to their experiencing a temperature change as a result of the absorption of IR energy. The electrical response results from a temperature dependence of some material property.

Photon detectors can be very sensitive, however their sensitivity depends on their temperature. It’s necessary to cryogenically cool the detectors to maintain a high level of sensitivity. Cooled thermal imagers typically use either Stirling coolers or liquid nitrogen.

Thermal detectors are generally not as sensitive as photon detectors, however they provide a good level of performance at room temperature. This eliminates the need for cryogenic cooling.

Thermal Imaging vs. Image Intensification              

Thermal imaging systems provide the capability to see better than the unaided human eye in daylight, at night, and in most weather conditions. Not only do thermal imagers allow you to see in total darkness, they allow you to detect critical objects or targets, such as humans, animals and vehicles in any lighting conditions. 

Current night vision technology falls into two major categories: image intensification (I2) and thermal (IR) imaging. Because they sense heat, thermal imagers have a much greater range of applications than I2 devices.

I2 devices operate by amplifying ambient light thousands of times to create an apparent daytime image. Ambient light is passed through a photocathode, a thin piece of glass that emits electrons when struck by light (photons). The electrons are amplified electronically in a vacuum tube. The amplified electrons strike a phosphor screen, which converts them back into light energy. The phosphor screen gives the images their familiar greenish hue.

Because I2 devices rely on amplifying available light, they are often ineffective in very low-light conditions without the use of supplemental IR illumination. The level of light that’s required is a function of the I2 technology that’s employed. The efficiency range of supplemental IR illumination is dependent on the sensitivity of the I2 device, the ambient reflectivity of the object being viewed, and the output power of the IR illuminator.

Image intensification technology has evolved through four generations? Generation (Gen) I, II, III and IV. The sensitivity (ability to operate effectively in less light) increases substantially with each generation. Operational life of the intensifier has also increased.

The “star light scope” familiar to many Vietnam veterans was a Gen I device. Gen I devices utilize an image intensifier tube that consists of a single- or three-stage image input tube which amplifies dim input images by accelerating electrons. Developed in the early 1960s, Gen I technology is obsolete in the United States, although many Gen I devices are now being imported from the former Eastern Bloc and marketed to consumers.

While capable of high resolution, Gen I devices are prone to blooming and streaking when bright lights are encountered. They also have problems with distortion and short-lived intensifier tubes. They don’t have the light gathering capabilities of subsequent generations.

Gen II and Gen III devices incorporate a microchannel plate (MCP) in the image intensifier tube. An MCP is a small, metal-coated glass disk with a multitude of holes (or channels), which have secondary emission characteristics; that is, they emit additional electrons when struck by a single electron. MCPs generally have between two and six million holes. The number of holes is a major determinant of their resolution. The introduction of the MCP revolutionized I2 devices. MCP intensifiers provide greater light amplification and a clearer image without the distortion of Gen I intensifiers. 

The Gen III image intensifier is characterized by a Gallium Arsenide (GaAs) photocathode, which provides maximum sensitivity, and an ion-barrier film coating that greatly increases tube life. The sensitivity of the GaAs photocathode extends into the near-IR region where night-sky illumination and contrast ratios are the highest.

Gen IV technology has recently become commercially available. Although the term Gen IV is utilized by manufacturers, the term Gen IV isn’t utilized by the military. The military instead refers to Gen IV intensifiers as Filmless and Gated image intensifiers. Gen IV technology improves the operational effectiveness of the intensifier due to a filmless MCP which improves the signal-to-noise ratio for higher image quality under low light conditions and a gated power supply which improves image quality and reduces halo under bright light conditions. These improvements also increase the effective viewing range of the devices.

I2 devices require a certain level of visual contrast to distinguish objects in a scene even with adequate light. Objects of similar colors may blend together or become indistinguishable when viewed by an I2 device.

The level of visual contrast that’s required is dependent on the technology that’s employed. Viewing contrast increases when differences are maximized, making objects more discernible. Most natural backgrounds enhance IR emittance differences more readily than visible light. Gen III devices, with their high IR response, provide a sharper and more informative image than previous generations of I2 devices.

Although thermal imagers sometimes lack the resolution of I2 devices, thermal imagers provide a more detailed image under field conditions that are less than ideal. Thermal imagers don’t require visual contrast to discern objects. While fog and other forms of precipitation do degrade IR images, thermal imagers can see through dust, clouds, smoke, haze, light fog, light rain and most camouflage. 

Personnel, equipment and other objects can be separated from cluttered backgrounds and foliage. Thermal imagers present information about the surroundings that would not be available with any degree of light amplification.

Thermal imagers can often provide operators with the capability to see their targets at much longer ranges than would be the case with devices, which rely on image intensification. With the detection range being dependent on object size and thermal contrast, current-day hand-held and small arms systems allow operators to often see man-sized targets at ranges of one-half mile or more. This ability adds a critical margin to survivability on the battlefield.

Thermal imagers are unaffected by bright lights. They don’t “bloom” or shut down like I2 devices when they’re exposed to bright light since they don’t detect light, just IR heat energy.

From a tactical perspective, thermal imaging allows operators to perform their duties with greater efficiency and safety. Unlike I2 devices, thermal imaging can be used to see through total darkness, adverse weather and battlefield obscurants. I2 devices provide an intensified view of what you would see with the naked eye. All of the shadows and other places to hide are still there. Since thermal imaging detects heat, it will detect targets in many conditions where a night vision device may miss them.

Thermal imaging isn’t without its own limitations. Although thermal imaging has numerous tactical applications, image intensification is still the best method of night vision when high resolution imaging is required, such as where the absolute identification of specific persons is necessary. For this reason, law enforcement SWAT teams rarely use thermal weapon sights. Another serious limitation of thermal imaging is that it cannot be used to see through glass. Image intensification is the only type of night vision that works for this purpose.

Cost remains the major factor that’s limiting the use of thermal imaging. This is changing.  Efforts are continuing to further reduce costs and enhance performance by improving manufacturing efficiencies and making breakthroughs in ferroelectric detector and optics technologies.

 

Eugene Nielsen provides investigative and tactical consulting services and is a former officer. He may be reached at esnielsen@us

Published in Law and Order, Mar 2004

Rating : Not Yet Rated


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