ave fast response, and are commonly used to measure moving and intermittent targets, targets in a vacuum, and targets that are inaccessible due to hostile environments, geometry limitations, or safety hazards. The cost is relatively high, although in some cases is comparable to contact devices.
Infrared radiation was discovered in 1666 by Sir Isaac Newton, when he separated the electromagnetic energy from sunlight by passing white light through a glass prism that broke up the beam into colors of the rainbow. In 1800, Sir William Herschel took the next step by measuring the relative energy of each color. He also discovered energy beyond the visible. In the early 1900s,Planck, Stefan, Boltzmann, Wien, and Kirchhoff further defined the activity of the electromagnetic spectrum and developed quantitative data and equations to identify IR energy. This research makes it possible to define IR energy using the basic blackbody emittance curves 。The concept of blackbody emittance is the foundation for IR thermometry. There is, however, the term “emissivity” that adds a variable to the basic laws of physics. Emissivity is a measure of the ratio of thermal radiation emitted by a graybody (non-blackbody) to that of a blackbody at the same temperature. The law of conservation of energy states that the coefficient of transmission, reflection, and emission (absorption) of radiation must add up to 1: tl+rl+al= 1 and the emissivity equals absorptivity: El=al Therefore: El=1-tl-rl This emissivity coefficient fits into Planck’s equation as a variable describing the object surface characteristics relative to wavelength. The majority of targets measured are opaque and the emissivity coefficient can be simplified to: El=1-rl Exceptions are materials like glass, plastics, and silicon, but through proper selective spectral filtering it is possible to measure these objects in their opaque IR region. There is typically a lot of confusion regarding emissivity error, but the user need remember only four things: IR sensors are inherently colorblind.If the target is visually reflective, beware-you will measure not only the emitted radiation, as desired, but also reflected radiation. If you can see through it, you need to select IR filtering Nine out of ten pplications do not require absolute temperature measurement. Repeatability and drift-free operation yield close temperature control. If the surface is shiny, there is an emissivity adjustment that can be made either manually or automatically to correct for emissivity error. It is a simple fix for most applications. In cases where emissivity varies and creates processing problems, consider dual- or multiwavelength radiometry to eliminate the emissivity problem.
Design elements
IR thermometers come in a wide variety of configurations pertaining to optics, electronics, technology, size, and protective enclosures. All, however, have a common chain of IR energy in and an electronic signal out. This basic chain consists of collecting optics, lenses, and/or fiber optics, spectral filtering, and a detector as the front end. Dynamic processing comes in many forms, but can be summarized as amplification, thermal stability, linearization, and signal conditioning. Normal window glass is usable at the short wavelength, quartz for the midrange, and germanium
or zinc sulfide for the 8-14 μm range. Fiber optics are available to cover the 0.5-5.0 μm region. From an applications standpoint, the
primary characteristic of the opti
