InfraredThermography.com
A site dedicated to infrared thermography and related information.
Ph: 250-579-2141 Fax: 250-579-5418 email wayne@infraredthermography.com
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InfraredThermography.com A site dedicated to infrared thermography and related information. Ph: 250-579-2141 Fax: 250-579-5418 email wayne@infraredthermography.com
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Infrared Thermography vs The Visible By Wayne Ruddock, Advanced Infrared Resources, 177451 Bastanchury Rd, Suite 100D Yorba Linda, CA, Ph 250-579-2141 Introduction: There is often confusion when considering the similarities and differences between what we class as visible light and infrared. There is also much confusion between what is called infrared night vision, and infrared thermography. While there are some similarities, they are far outweighed by the differences. This article is designed to consider both the similarities and the differences. To this end we will start with a short summary of electromagnetic radiation. Electromagnetic Radiation: Electromagnetic radiation is the emission of energy from a source, which could be a solid, liquid or a gas. This radiation is given off in the form of alternating electric, magnetic waves produced by the acceleration and deceleration of charged electrical particles. Although the electromagnetic spectrum is comprised of many different types of electromagnetic radiation there are similarities that must be recognized. As mentioned all electromagnetic radiation is produced by the motion of charged electric particles. A second point is that all electromagnetic radiation, unhindered by gasses, travels at the speed of light. As the intensity of the radiation increases, the wavelength becomes shorter and the frequency becomes higher. On the other hand as the intensity decreases, the wavelengths become longer and the frequency lower. The main difference between the various classes of electromagnetic radiation is the wavelength and frequency, as well as the way it is produced and the "equipment" used to detect it. The chart below depicts the many classifications of electromagnetic radiation and their relation to one another.
The Visible Spectrum: As can been seen in the previous illustration the visible area of the electromagnetic spectrum or the area where our eyes see, occupies the band width from .38 - .72 microns. These wavelengths are associated with temperatures of 525 degrees Celcius or higher. If there are no energy sources at, or above this temperature, our eyes would not see anything. Most of what we do see is not emitted energy from these high temperature sources but a reflection of this energy off lower temperature objects. The filament in an incandescent lamp is around 3,000 degrees Celcius. The sun's surface is around 6,000 degrees celcius. A flourescent lamp is now rated in temperature. You will find a number such as 4,500 K on many flourescent lamps. This represents the temperature in Kelvin degrees. 4,500 Kelvin is equal to approximately 4,227 Celcius. Remove all of these high temperature sources and our eyes would register nothing. In other words we do not "see" when there are no high temperature sources present. Our human eyes do not "see" in the dark. Standard video cameras work on the same basis as our eyes see and are governed by the same laws. The detectors in these cameras are of two main designs. They are either CCD devices, CCD stands for charge-coupled device or CMOS devices, CMOS stands for Complementary metal-oxide semiconductor. To produce an image, they still require a high temperature source, which produces energy in approximately the .38 - .72 micron wave length. The lenses in these cameras can be made rather inexpensively from a type of glass, as glass is transparent to visible light. A video camera system can be purchase for a few hundred dollars. The new digital still cameras sold by many retailers today depend on the same principles as video cameras. Rather than store moving images on a film they store single images on a digital media such as an internal memory chip or a compact flash card memory. Standard 35 mm film cameras also require energy from a high temperature source for exposure of the film. What we see recorded on pictures from a 35mm camera is mainly reflected energy from the field of view. Their lenses are also inexpensive and the new single use cameras can be purchased for around $10. The bottom line is, no high temperature source (525C or higher), no visible images. The Infrared Spectrum: The infrared portion of the electromagnetic spectrum can be divided into two main areas by viewing device type. There are the light intensifier products commonly known as night vision and there are the thermographic infrared devices, which are often called thermal viewers. In both cases you do not view the scene directly but rather the operator of the device views a representation of the scene, or an electronically produced image on a screen or CRT. Night Vision: In general most night vision devices are electro-optical devices, which amplify available light in low light scenes such as night time conditions. The lens(s) on these systems, focuses the small amount of light particles known as photons that are reflected off of objects from star light, moonlight or other low light sources onto the device's image tube. This photo cathode tube converts the photons into electrons. The power supply in the system amplifies these electrons thousands of times and sends them to the phosphor screen. Once they strike the screen, they produce the green colored image, which the person using the device sees. This screen produces a representation of the scene that the device is viewing. Most so called night vision systems or light intensifiers do not "see" in total darkness. In cases where there is no low light to amplify, these devices would see nothing. To see in total darkness these systems require an infrared illuminator. This is a device which gives off infrared energy that is usually not visible to the unaided human eye. This energy then reflects off of the objects in the field of illumination and this reflected energy is now "visible" to the night vision system. With night vision type systems we view mainly high temperature, short wavelength energy from a natural or artificial source reflected off lower temperature objects. These systems can be fairly inexpensive to expensive. There are four types of night vision systems available. They are Gen 1 (generation 1), Gen II (generation 2), Gen III (generation 3), and Gen IV (generation 4). Gen IV devices are not very available today due to their newness to the market and military restrictions. The price for these devices ranges from a $300. to over $6,000., depending on the generation type and accessories. Infrared Thermography: The infrared portion of the electromagnetic spectrum is from 1 - 1000 microns. All objects above absolute zero (- 273 C) emit electromagnetic radiation in the form of infrared rays. This energy is emitted from the first 1/1000 of an inch of the surface of an object. Infrared thermographic instruments are non-contact, non-intrusive systems, that see only radiated energy from the 1/1000" of most objects. Contrary to popular belief they do not see through or into most common objects in the world today. In general, thermographic instruments, known as either short wave (2 - 6 micron), or long wave (8 - 14 micron) systems can see the emitted energy from objects that are at a temperature of approximately - 35 Celsius or higher. These systems are not dependent on reflection of visible light or high temperatures. As long as there are objects above a temperature of -35 C an infrared thermographic system will see as well in total darkness as it will in full sunlight. The photonic, infrared energy emitted by objects, is focused by a lens onto a specialized detector. The lenses are made of expensive, IR transparent materials, such as Germanium, Silicon, or Zinc Selenide. These lenses are not transparent to visible light. The lenses range in price from $3,000 - $15,000, with some specialty lenses being in excess of $30,000. The detectors in many systems today are un-cooled thermal detectors made from materials such as Vanadium Oxide, and Barium Strontium Titanate. Other detector materials such as Indium Antimonide, and Indium Gallium Arsenide are also in use in a variety of infrared thermographic systems. Most of the newer systems on the market today are Focal Plane Arrays, FPAs. These systems have high resolution compared to the old single element infrared cameras, being comprised of 320 X 240 individual detector elements which produce a display made up of 76,800 picture elements or pixels. The energy that strikes the detector in the camera is then converted to an electrical impulse. This electrical impulse then produces a video type image on a CRT or LCD display screen in black and white or a series of selectable color pallets. In general, the higher the temperature of the object or area in the field of view, the brighter the color on the screen. An infrared thermographic image is representative of the thermal patterns in the scene and has no relation whatsoever to what our eye sees in the visible. Infrared thermographic cameras do not see through glass, plexi-glass or other materials that are transparent in the visible wavelengths. More expensive cameras, $25,000 plus, can also measure this radiated energy and through the use of an onboard computer give calculated temperatures. These temperatures are only valid if the operator is well trained and can input the proper parameter values. This type of thermographic system is called an imaging radiometer or a quantitative infrared system. Infrared thermographic systems that do not calculate temperatures but only display an image are simply called imagers or qualitative devices. Summary: Although both the visible and infrared thermography see electromagnetic radiation, there are very few similarities between the two. The visible sees mainly high temperature, reflected energy, reflected off of most objects in the world. Without a temperature source over 525 C our eyes would see nothing. The image produced by this reflected energy has nothing to do with the temperature of the object that the visible light is reflected off of. On the other hand, infrared thermographic systems are capable of seeing energy emitted by most objects at a temperature above - 35 C. Infrared thermographic systems do see in the dark. Color or visible light usually does not have anything to do with what an infrared thermographic system will "see". The image produced by an infrared thermographic system is generally a representation of the temperature or thermal patterns of the scene being viewed, not its color or other visual qualities. Black and white infrared image of a small dog (Max), taken at night with a long wave infrared thermographic instrument in total darkness:
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