History of Thermal Imaging
Welcome to Part I of the latest in SECURITY SALES & INTEGRATION’s acclaimed “D.U.M.I.E.S.” series: “Thermal Imaging for D.U.M.I.E.S.” Brought to you by FLIR, this four-part series has been designed to educate readers about recent advances in technology and systems that are likely to shape this decade’s progression of the video surveillance industry. “D.U.M.I.E.S.” stands for dealers, users, managers, installers, engineers and salespeople.
This particular series delves into thermal imaging as it pertains to capturing video surveillance in low- and no-light applications. While this installment establishes a working foundation for further study by explaining what thermal imaging is in basic terms and revealing its history, successive entries will address technology specifics; incorporating thermal into system design; and application examples and solutions.
Thermal Imaging Explained
Thermal imaging was made possible by the discovery of infrared energy over two centuries ago. Thermal imagery is the use of specialized equipment to detect infrared energy and create images out of tiny differences in that heat. As the security industry continues to advance, many avenues now require more sophisticated methods in order to provide a higher degree of surveillance. This includes the ability to see in areas containing very little or no light, or areas of extreme contrast that make it very difficult to distinguish between good and bad.
In the past (prior to 1992), conventional night vision products consisted of IR illuminators or intensified imaging devices such as silicon intensified target (SIT) or intensified charge couple devices (ICCD). Intensified imagers focus existing light on the photocathode of an intensifier. The light causes electrons to be released. These electrons are then accelerated by a high voltage (about 15,000 times). The accelerated electrons are subsequently focused onto a phosphorous screen. The energy of the electrons makes the screen glow, which in turn is received by a CCD sensor producing a video image.
A major disadvantage of intensifiers was their life expectancy. For the most part the average device lasted only about 15 months. In addition, this device does require some existing light in order to function. So in a totally darkened environment light amplification would yield no image, whereas a thermal imager would. However, what happens if there is no existing light available?
The infrared range of lighting is by far the most popular. The main reason is in the cost of the equipment. However, there are drawbacks. Unlike intensified cameras, IR devices emit a light source in order to illuminate an area. The other methods rely on existing light (intensified) or energy (thermal) to produce an image.
IR illumination is an active device. Therefore, the first problem area is the wavelength of the emitted light. IR illumination operates on a completely different electromagnetic spectrum than that of thermal. The wavelength incorporated in IR illuminators ranges from 850 to 940nm (nanometers). This can produce some undesirable results depending on the material being viewed. Since IR is an active device it can leave a footprint behind. In some cases that can be a major disadvantage, especially in security and law enforcement applications.
As an active device and operating at a range not detected by the human eye, safety also becomes an issue. If any dealer or installer has discussed or implemented IR illuminators in a system, the topic of eye safety will always come to the surface. Since the human eye does not recognize the IR spectrum of light, the iris of the eye will not react. This may cause some damage to the retina. Depending on the light source, the safety requirements will change.
The older style of illuminators incorporated large lamps that could reach distances of 1,000 feet or greater, which could cause damage if viewed from closer than six feet.
This now brings us to thermal imaging. All objects over absolute zero (the temperature at which all molecular activity stops), including human beings, emit infrared energy (also called thermal energy). This infrared energy is not to be confused with the radiation emitted by IR illuminated cameras. Infrared energy is part of the electromagnetic spectrum from 0.75µm to 1,000µm; IR illuminated cameras only use part of the near IR (NIR) waveband at the bottom of that scale from 0.75-0.94µm. Thermal imaging cameras use either the midwave IR (MWIR) band of 3-5µm or the longwave IR band from 8-12µm.
Warmer objects emit more IR energy than colder objects. This emitted energy can be translated into a viewable image by the use of a thermal imager. On the monitor screen of a thermal camera set to white-hot, hotter objects appear as white on the monitor screen whereas cooler objects appear as black. Objects between these temperatures are displayed in different shades of gray or color.
Thermal imagers can perform exceptionally well in adverse weather conditions due to the fact they do not rely on light but rather emitted energy. Because the IR wavelength is longer than the visual light wavelength, thermal imagers can detect emitted energy through smoke, dust, fog, blowing sand, rain and snow.
Thermal imagers are totally passive in nature. This means they do not emit any kind of energy; they only detect IR wave lengths. Thus they do not have a footprint that can be detected by other types of covert devices.
An Electromagnetic Exploration
The infrared portion of the electromagnetic spectrum can be divided into areas that react differently and have specific applications. Following is a brief description of the basic areas and a few of their functions.
Near Infrared (NIR) — The NIR band is between 0.74-0.9µm. It is commonly used in fiber-optic telecommunication because of low attenuation losses in the glass medium. Image intensifiers are sensitive to this area of the spectrum. Examples include night vision devices such as night vision goggles.
Short-wave Infrared (SWIR) — This waveband is between 0.9-3.0µm. Water absorption increases significantly at 1.450µm. The 1.530 to 1.560µm range is the dominant spectral region for long-distance telecommunications as used in single mode fiber-optic systems.
Midwave Infrared (MWIR) — This range is located in the 3-8µm waveband, but the stretch from 5-8µm suffers from extreme atmospheric attenuation so it is not used for imaging. MWIR imagers usually, but not always, use cryo-coolers to cool the detector in order to be sensitive to thermal energy.
Longwave Infrared (LWIR) — LWIR energy is in the 8-15µm waveband. Longwave imagers are usually uncooled sensors, so they have no moving parts and drastically lower maintenance requirements than their cooled counterparts.
Far infrared (FIR) — This range resides in the 15-1,000µm area. It is used in laser technology, most recently incorporated into today’s saunas.
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