Thermal imaging cameras work in the same way as normal cameras, but rather than creating an image based on visible light, they are sensitive to infrared frequencies. Visible light ranges from blue light with wavelengths starting at around 400nm, to red with wavelengths of up to 780nm. Infrared has wavelengths beyond visible red light, from 780nm to as much as 1mm. Infrared is classified as IR-A (780nm-1.4µm), IR-B (1.4-3µm) and IR-C, also known as far-infrared (3µm-1mm).
Faults in electrical equipment, and often also in mechanical machines, result in excess heat. Thermal imaging can therefore detect a wide range of faults, whether due to faulty electrical connections leading to resistance heating, mechanical damage leading to frictional heating, or a failed component showing as a cold spot. Defects in buildings that affect energy efficiency, such as thermal bridging and drafts, can also be detected.
Thermal imaging cameras have been used for power line inspection since the 1960s. Chris Jorgensen from Mason County Public Utility of Washington state, USA, explains how this works: “We go out and scan our large scale distribution equipment – our substations, our reclosers, our regulators. As electricity flows through the line, anything that resists the flow of electricity creates heat, and over time heat breaks down that connection and it burns up. The thermal imaging camera sees that heat.
“We can see it before it fails, which allows us to make repairs before a costly outage, or having a great number of people without power… we find dozens of potential failures a year. Anywhere the equipment is bolted together, a fuse, cut-out device… a jumper wire where a jumper connecting one wire to another wire isn’t making a good connection, hot tap devices where we’re tapping from one line to another line fail, those porcelain insulators, they get moisture inside them and start to track – that tracking causes heat. When we change out a piece of equipment that we know was going to fail, we can bypass it, change out that equipment and make repairs, and they’ll never see their lights blink.”
Thermal imaging is now increasingly being deployed by drones for even more efficient powerline inspection. Mingxing Mei of the Zhaotong Power Transmission Control Centre, Yunnan Province, China, says: “Our power lines and pylons are mainly set on the top of mountains… In the past, we could only inspect one or two pylons a day, but now drones have increased our efficiency about ten times.”
Thermal inspection of solar photovoltaic (PV) installations can detect a wide range of power losses and faults. Cells break down over time and inverters can malfunction. Power loses manifest as hotspots which can be easily identified in thermal images. Minor faults often lead to overheating which causes secondary damage and eventual failure of a cell or even an entire panel.
HOW DO THEY WORK?
A body that is in thermal equilibrium with its environment emits radiation. The intensity and frequency of the radiation emitted depends on the emissivity of the object and its temperature. Emissivity is defined as the ratio of the energy radiated from a material’s surface, to that radiated from a perfect emitter – a black body – at the same temperature, wavelength and viewing conditions. It is therefore a dimensionless number between zero for a perfectly reflecting mirror, and one for a perfect black body. The emissivity of a surface depends on the material and the surface properties: a polished metal surface will have a low emissivity, while a rough and oxidised metal surface will have a high emissivity. Emissivity also varies with the temperature of the object. Perhaps surprisingly, many materials that are transparent to visible light, such as glass and water, have very high emissivity in the infrared.
At very high temperatures, much of the radiation produced by a black body is in the visible spectrum. For bodies at temperatures below 500°C, almost all the radiation is in the infrared – typically IR-A and IR-B. Above 500°C, significant quantities of visible light are also radiated, resulting in objects glowing ‘red hot’ or even ‘white hot’, although most of the radiation remains in the infrared until objects reach 5,000°C. The sun has a surface temperature of 5,505°C. Sunlight is therefore well distributed across a broad spectrum of visible light, and even into the ultraviolet, although over half of the sun’s radiation is still in the infrared.
By measuring the intensity and frequency of infrared radiation, a thermal imaging camera can determine the temperature of black bodies, regardless of how far away they are. It is important to understand that the temperature recorded assumes that objects behave as perfect black bodies.
Although analogue photography can capture images in the infrared using photographic film, all modern thermal imaging cameras use digital sensors, similar to those in ordinary digital cameras. When imaging objects with emissivity significantly lower than one, the temperatures recorded by thermal imaging may differ significantly from actual surface temperature. Since shiny surfaces emit less radiation, they will appear colder than they actually are.
BOX: Calibration tips
The emissivity of surfaces can be calibrated by measuring the actual surface temperature with a contact thermometer or thermocouple, and adjusting the emissivity setting in the thermal imaging camera until it reads the correct temperature.
Alternatively, for temperatures below 260°C, a piece of masking tape can be placed on the surface. The masking tape has an emissivity of 0.95 and will quickly reach thermal equilibrium with the surface it is attached to. This allows the thermal imaging camera to determine the true surface temperature, using an emissivity set at 0.95, without a separate contact thermometer. The emissivity can be adjusted until the surrounding surface reads the same temperature. Similarly, it’s possible to coat an area of the surface with a matt black paint, which typically has an emissivity of 0.98.
However, it’s not always important to determine the actual surface temperature. If you are looking for hot spots across a surface which has a fairly uniform emissivity, it is not important to know the actual emissivity or temperatures: it may be easy to identify issues without needing to calibrate for emissivity.
Environmental conditions can strongly affect thermal images. For example, when inspecting a building for heat loss, there must be a large enough difference between internal and external temperatures.
Similarly, when inspecting solar panels, there must be enough solar radiation for the cells and components to produce sufficient heat. Jon Moraglia, founder of drone operator The Drone Life, says: “Ideally you want to have clear skies and do the inspection when the sun is at its peak – around 11am to 3 or 4pm is best. If there are too many clouds, there’s not enough solar energy hitting the panel and it will be a lot harder to distinguish anomalies with the thermal camera.
“A handy device is a solar irradiance meter. For a thermal inspection of a PV system, you need to know how much sun energy is hitting the panels. This is usually measured in W/m2, and for an infrared inspection of solar farm within IEC standards, you need a minimum level of irradiance of 600W/m2.”