Thermal images, or thermograms, are actually visual displays of the amount of infrared energy emitted, transmitted, and reflected by an object. Because there are multiple sources of the infrared energy, it is difficult to get an accurate temperature of an object using this method. A thermal imaging camera is capable of performing algorithms to interpret that data and build an image. Although the image shows the viewer an approximation of the temperature at which the object is operating, the camera is actually using multiple sources of data based on the areas surrounding the object to determine that value rather than detecting the actual temperature. This phenomenon may become clearer upon consideration of the formula Incident Energy = Emitted Energy + Transmitted Energy + Reflected Energy where Incident Energy is the energy profile when viewed through a thermal imaging camera. Emitted Energy is generally what is intended to be measured. Transmitted Energy is the energy that passes through the subject from a remote thermal source. Reflected Energy is the amount of energy that reflects off the surface of the object from a remote thermal source.
Emissivity is a term representing a material's ability to emit thermal radiation. Each material has a different emissivity, and it can be difficult to determine the appropriate emissivity for a subject. A material's emissivity can range from a theoretical 0.00 (completely not-emitting) to an equally-theoretical 1.00 (completely emitting); the emissivity often varies with temperature. An example of a substance with low emissivity would be silver, with an emissivity coefficient of .02. An example of a substance with high emissivity would be asphalt, with an emissivity coefficient of .98.
A black body is a theoretical object which will radiate infrared radiation at its contact temperature. If a thermocouple on a black body radiator reads 50 °C, the radiation the black body will give up will also be 50 °C. Therefore a true black body will have an emissivity of 1.
Difference between infrared film and thermography
IR film is sensitive to infrared (IR) radiation in the 250°C to 500°C range, while the range of thermography is approximately -50°C to over 2,000°C. So, for an IR film to work thermographically, it must be over 250°C or be reflecting infrared radiation from something that is at least that hot. (Usually, infrared photographic film is used in conjunction with an IR illuminator, which is a filtered incandescent source or IR diode illuminator, or else with an IR flash (usually a xenon flash that is IR filtered). These correspond with "active" near-IR modes as discussed in the next section.
Night vision infrared devices image in the near-infrared, just beyond the visual spectrum, and can see emitted or reflected near-infrared in complete visual darkness. However, again, these are not usually used for thermography due to the high temperature requirements, but are instead used with active near-IR sources.
Starlight-type night vision devices generally only magnify ambient light.
· It shows a visual picture so temperatures over a large area can be compared
· It is capable of catching moving targets in real time
· It is able to find deteriorating, i.e., higher temperature components prior to their failure
· It can be used to measure or observe in areas inaccessible or hazardous for other methods
· It is a non-destructive test method
· It can be used to find defects in shafts, pipes, and other metal or plastic parts
· It can be used to detect objects in dark areas
Limitations and disadvantages of thermography
· Images can be difficult to interpret accurately when based upon certain objects, specifically objects with erratic temperatures, although this problem is reduced in active thermal imaging[
· Accurate temperature measurements are hindered by differing emissivities and reflections from other surfaces
· Most cameras have ±2% accuracy or worse in measurement of temperature and are not as accurate as contact methods
· Only able to directly detect surface temperatures
· Veterinary Thermal Imaging
· Process control
· Surveillance in security, law enforcement and defence
Thermal imaging cameras convert the energy in the infrared wavelength into a visible light display. All objects above absolute zero emit thermal infrared energy, so thermal cameras can passively see all objects, regardless of ambient light. However, most thermal cameras only see objects warmer than -50°C.
The spectrum and amount of thermal radiation depend strongly on an object's surface temperature. This makes it possible for a thermal imaging camera to display an object's temperature. However, other factors also influence the radiation, which limits the accuracy of this technique. For example, the radiation depends not only on the temperature of the object, but is also a function of the emissivity of the object. Also, radiation originates from the surroundings and is reflected in the object, and the radiation from the object and the reflected radiation will also be influenced by the absorption of the atmosphere.