The importance of emissivity in electrical thermography
30 September 2011
Having explored the different lens materials in the second of its series ‘Ten Things To Know About Infrared Windows’, IRISS now turns its attention to emissivity – the relative ability of a material surface to emit energy by radiation. Understanding the importance of properly compensating for the range of different values is critical to the accurate diagnosis of equipment health. And to explain this fully, chapter three delves into the science behind infrared thermography.
A typical factory is full of equipment that requires periodic infrared (IR) inspection. The challenge, as any thermographer knows, is getting an accurate indication of equipment health. Properly compensating for the various emissivity values of all the components one encounters on the factory floor is possibly the most critical factor in performing accurate and meaningful inspections.
Even slight errors in emissivity compensation can lead to significant errors in temperature and ?T (difference in temperature) calculations. Electrical cabinets are a good example, as they may contain materials with emissivity values ranging from
0.07 to 0.95.
The electromagnetic spectrum is a continuum made up of cosmic rays, gamma rays, X-rays, ultraviolet light, visible light, infrared radiation, microwaves and radio waves (in order of increasing wavelength and decreasing power). Infrared is that portion of the spectrum between 0.75µm (microns) and 1000µm in wavelength, starting just beyond what the human eye is capable of seeing.
All objects above absolute zero emit infrared radiation. As an object heats up, the intensity of emitted radiation increases exponentially (Stephan-Boltzmann’s Law) and the peak radiation shifts to shorter and shorter wavelengths (Planck’s Law), moving eventually into the visible spectrum. This is why a hot burner will glow red (“incandescent”) after it achieves roughly 500°C (923°F)
Today’s radiometric IR imagers are capable of “seeing” and calculating the emitted radiation from a target object. There are only three sources of this radiation: it can be reflected from other sources; it can be transmitted through the object from a source behind it; or the radiation can be emitted by the object.
An extension of Kirchhoff’s Law tells us that the sum of the radiation leaving the surface of an object equals one: expressed as Watts Emitted + Watts Transmitted + Watts Reflected = 1 (or e+t+?=1).
Therefore, emitted energy, reflected energy and transmitted energy are the only three possible sources of infrared energy coming from a target object.
A perfect emitter is referred to as a blackbody. A blackbody emits 100% of the energy it absorbs. Since by definition there is no reflection or transmission, a blackbody has an emissivity value of 1. For real-world objects (referred to as “real -bodies”), emissivity is expressed as the ratio of the radiant energy emitted by that object, divided by the energy that a blackbody would emit at that same temperature.
One of the most misunderstood concepts in thermography is the degree to which errors in emissivity settings (and errors in window transmissivity compensation) will affect temperature and ?T (difference in temperature) accuracy. As demonstrated by the Stefan-Boltzmann Law (illustrated), the radiated infrared energy emitted by a target surface is exponentially related to the absolute temperature of that surface.
Therefore, as the temperature increases, radiant energy increases proportionally by the absolute temperature to the 4th power. Incorrect camera settings such as emissivity and infrared window transmission rates will result in errant temperature values. Furthermore, because the relationship is exponential, this error will worsen as the component increases in temperature. Consider the effect on ?T comparisons, which are by their nature a comparison between different temperatures. The resulting calculations are apt to be radically understated, which could easily lead thermographers to misdiagnose the severity of a fault.
For some components, it can be difficult to determine the correct emissivity value. In the case of a highly polished component like a bus bar, the actual emissivity may be so low as to make temperature measurement impractical. It is strongly recommended that thermographers understand the surface of the primary targets. Once identified, those surfaces should be treated with a high-emissivity covering so that all targets have a standardized emissivity.
Thermographers can apply electrical tape, high-temperature paint (such as grill paint), or high-emissivity labels (like the IR-ID labels from IRISS). When all targets have a standard emissivity, refection issues are minimized and measurement errors from reflected ambient energy are greatly reduced. High-emissivity targets of varying shapes can also provide a useful point-of-reference both for the thermographer and the technician making repairs.
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