All the targets radiate energy in the infrared spectrum. The hotter the target, the more energy it radiates. Very hot targets radiate in both the visible and infrared spectrum. As the objectives cool, they no longer shine but continue to radiate. The radiation can be felt in a hand placed near the surface of the lens, but the brightness can not be seen because the energy has changed from red to infrared. Infrared detectors can detect infrared radiant energy and produce useful electrical signals proportional to the temperature of the target surfaces. Instruments that use infrared detectors allow a fast and highly sensitive measurement of the temperature of the target surface without contact. Instruments that combine this measurement capability with the ability to scan a target surface area are called infrared thermal cameras. They produce thermal maps or thermograms, where the brightness intensity or the color of any point on the map is representative of the surface temperature of that point. In other words, they extend the measurements of temperature of the points without contact to the thermography without contact.
In using infrared instruments for making non-contact temperature measurements, three sets of characteristics need to be considered:
• Target surface
• Transmitting medium between the target and the instrument
• Measuring instrument
Figure shows how the instrument is aimed at the target and makes the measurement through the medium
Every target surface above absolute zero radiates energy in the infrared. The hotter the target, thermo radiant infrared energy is emitted.
Emissivity is a very important feature of a target surface and must be known to make accurate non-contact temperature measurements. The proper configuration necessary to make the instrument produce the correct temperature reading can be learned experimentally by using samples of the actual target material. This most practical configuration value is called effective emissivity
Although the transmitting medium is usually air, non-contact temperature measurements can be made through a vacuum, gas, or certain solid materials. Figure below shows the necessary components of an infrared radiation thermometer that makes a single point non-contact temperature measurement on the target surface.
The collection of optical elements (ie, infrared lenses, etc.) is necessary to focus the energy radiated from the target to the sensitive surface of an infrared detector. The detector converts this energy into an electrical signal that is representative of the temperature of a point on the target. Adding scan elements between the objective and the detector allows the instrument to scan the target surface and produce a thermogram.
Most built-in infrared thermal imaging cameras incorporate multi-detector focal plane array (FPA) sensors that are scanned electronically and eliminate the need for an opto-mechanical scanning mechanism. When an infrared radiation thermometer points to a target, it collects energy within a collector beam, whose shape is determined by the configuration of the optics and the detector. The cross section of this collector beam is called the instrument’s field of view (FOV) and determines the size of the area (point size) on the target surface measured by the instrument. In the scanning and image instruments this is called field of instantaneous vision (IFOV) and becomes an image element in the thermogram.