A thermal imaging camera on a drone turns it into a powerful tool, which can be used in many sectors from construction, mining, electrical, surveillance, firefighting, search and rescue.
Thermal drones which use vision imaging cameras have so many positive uses by detecting heat coming from almost all objects and materials turning them into images and video.
Hereafter, you will find information on how thermal imaging works, how to interpret thermal images, heat maps and color scales.
You will also read about similar but different technologies such as illuminated infrared cameras and night vision cameras. At the end we take a brief look at the various sectors, which use thermal imaging cameras on drones.
Along with thermal cameras, drones can carry many other types of sensors and cameras covering many sectors. We are hearing about new uses for drones every week. Drones certainly are making a big impact in our everyday lives without us even knowing it.
About Thermal Imaging Camera Technology
How Does Thermal Imaging Work Answered?
Heat vision cameras or thermal imaging cameras are in fact really heat detecting sensors. These thermal cameras are also known by various names including:
- Thermal Camera
- Heat Vision Camera
- Thermal Vision Camera
- Thermal Imaging Sensor
- Thermal Imaging Camera
- Temperature Sensor Camera
- Thermal Heat Vision Sensor
- Heat Signature Camera
- Heat Sensor Camera
Thermal vision cameras make pictures or video from heat, not visible light. Heat (infrared
thermal radiation) and light are both parts of the electromagnetic spectrum. However, a camera,
which can detect visible light will not see thermal radiation and vice versa.
Thermal cameras detect more than just heat. Heat vision cameras detect the tiny differences in heat, even as small as 0.01° Celsius. This information is then displayed as various colors on a
display, in thermal software or apps.
Thermal Radiation and Heat Signatures
Everything in our lives give off thermal energy, even ice. The hotter something is, the more
thermal energy it emits. This emitted thermal energy is called a “heat signature.”
The hotter the object, the more it radiates. The Sun obviously radiates off more energy than a hot cup of tea. The temperature also affects the wavelength and frequency of the radiated waves.
Objects at typical room temperatures radiate energy as infrared waves. When you see thermal photographs or videos of the radiation surrounding a person, animal or a hot mug of coffee, the energy radiated from the object is usually a range of wavelengths. This is usually referred to as an emission spectrum.
As the temperature of an object increases, the wavelengths within the spectra of the emitted radiation also decrease. Hotter objects emit shorter wavelength, higher frequency radiation. For example, the coils of an electric toaster are considerably hotter than room temperature and
emit electromagnetic radiation in the visible spectrum. The coils on the toaster glow red and we can feel the heat by putting our hands near the coils providing us with a convenient warning that
the coils are hot.
Thermal radiation can occur through matter or through a region of space that is void of matter (a vacuum). The heat received on Earth from the Sun is the result of electromagnetic waves traveling through the void or vacuum of space between the Earth and the Sun.
Quick Scientific Explanation of Thermal Radiation
Thermal radiation or heat, is the discharge of electromagnetic waves from all matter, which has a temperature greater than absolute zero (−273.15° Celsius). It converts thermal energy into electromagnetic energy.
All objects radiate energy in the form of electromagnetic waves. The rate at which this energy is released is proportional to the Kelvin temperature (T) raised to the fourth power.
Thermal energy consists of the kinetic energy (all moving things) of random movements of atoms and molecules in matter.
These atoms and molecules are composed of charged particles (protons and electrons) and
kinetic interactions among matter particles which result in charge-acceleration and dipoleoscillation.
This results in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of photons, radiating energy (thermal radiation) away from the body through its
surface boundary.
Thermal Sensitivity NETD (mK)
When you look at the specifications of a Thermal Camera, you will often see the technical details for “NETD”. This NETD expression stands for Noise Equivalent Temperature Difference.
It is a measure for how well a thermal imaging detector is able to distinguish between very small differences in thermal radiation in the image. NETD is typically expressed in milli-Kelvin or mK. It is also sometimes referred to as “Thermal Contrast”. Basically it is when the noise is
equivalent to the smallest measurable temperature difference, the detector has reached its limit of its ability to resolve a useful thermal signal. The more noise there is, the higher the NETD value of the detector.
How NETD Is Measured
In order to measure the noise equivalent temperature difference of a detector, the camera must be pointed at a temperature controlled black body. The black body needs to stabilize before starting the measurement. The noise equivalent temperature difference is then measured at a specific temperature. It is not a single snapshot measurement, but rather a temporal measurement of noise.
Infrared Thermography
IR thermography is the method of transforming an infrared image into a radiometric one which
allows temperature values to be read from the image.
How to Interpret Thermal Images
Most thermal cameras produce a video output in which white areas indicate maximum radiated energy and black areas indicate lower radiation. The gray scale image contains the maximum amount of information.
However, in order to ease general interpretation of thermal images and facilitate subsequent presentation, the thermal image can be artificially colorized.
This is achieved by allocating desired colors to blocks of grey levels to produce the familiar
colorized images. This enables easier image interpretation.
Additionally, by choosing the correct colorization palette, the image may be enhanced to show particular energy levels in greater detail.
What Is Emissivity In Thermal Imaging?
To read correct temperatures, another important factor needs to be taken into account. That is a factor known as emissivity.
Emissivity is the efficiency, which an object emits infrared radiation and is highly dependent on properties of the material or object. It is a measure of the efficiency of a surface to emit thermal
energy relative to a perfect black body source. It directly scales the intensity of the thermal emission and all real values are less than 1.0.
The emissivity may be highly dependent on the surface morphology, roughness, oxidation, spectral wavelength, temperature and view angle. A measurement that does not account for the real emissivity of a surface will appear “colder” than it actually is.
For agricultural applications, many organic materials and materials with very rough surfaces have emissivity values approaching 1.0. For other applications, including power line and solar cell inspection, the surface might be a highly polished glass or metal, both of which can have much lower emissivity values.
It is important for the thermal camera to be set to the correct emissivity or incorrect temperatures will be measured.
What Is Reflectivity in Thermal Imaging
Reflectivity is a measure of the ability of a surface to reflect radiation. A camera close to a
surface is sensing both the heat sustained from the surface temperature and the reflected
background environmental temperature. It is very challenging to make temperature measurements of a highly reflective surface because the image is influenced by the background
thermal reflections.
In a UAS application, an unpainted and clean metal roof can appear colder than it actually is because the shiny roof reflects the sky above it. Consider the case of a stainless steel sheet on a rooftop, 0.80 reflectivity and 0.20 emissivity, a radiometric temperature measurement would be highly biased towards the reflected background temperature of the sky.
A clear sky may have a background temperature, which is typically well below 0°C and possibly as low as -20°C. The actual sky background temperature will vary depending on atmospheric conditions and time of day.
Reflective surfaces pose additional challenges in UAS applications. Reflection of the sun in the thermal image will appear as sun glints. Radiometric temperature measurements of the sun glints can be inaccurate by hundreds of degrees.
It is advisable to take a sequence of images of the surface from different angles to reduce the influence of any single sun glint. However, care must be taken to not make measurements at exceedingly oblique angles because reflectivity degrades based on view angle.
Alternatively, a very close range and straight on measurements can result in the camera viewing a reflection of itself and result in inaccurate measurements.
Much like emissivity, the reflectivity of a surface is highly dependent on the surface morphology and roughness.
Since reflectivity (R) is related to emissivity (E) by R = 1-E, the importance of reflectivity can be greatly reduced by making measurements of surfaces with very high emissivity, ideally greater
than 0.90.
For UAS measurements of controlled surfaces, such as a steel tank on a rooftop, high
emissivity/low reflectivity matte-flat black paint can be used to make “measurement patches” that result in highly reproducible measurements.
What Can Heat Vision Cameras Capture?
Thermal energy is radiated off nearly every source on our planet and in our Universe. Heat vision cameras can capture images and differences in heat emitted from the following;
- Living Species – People, animals and vegetation.
- Buildings – Skyscrapers, buildings, factories, houses and tents.
- Machinery – engines, conveyor belts and assembly lines.
- Planes, Boats and Vehicles – all types of automobiles, boats and vehicles.
- Electrical – circuits, power lines, capacitor, coupling capacitor and insulation etc.
- Land, rocks and buoys – These absorb heat from the sun during the day and radiate it off during the night.
- Liquids and gas – these all emit thermal radiation and detected by heat vision cameras.
Because different materials absorb and radiate thermal radiation at different rates, an area which we believe has only one temperature will have many subtly different temperatures. For example, looking at a person through a heat vision camera will show that our bodies have small
temperature differences from one area to another. A heat vision camera detects these temperature differences and translates them into image detail.
Heat Vision Cameras in Dust, Smoke Fog and Rain
Dust and Smoke – The mining industry are big users of thermal cameras. In a mining
environment, if visible light or digital cameras are used for monitoring, they will miss out on any potential defects as mining environments are often very dusty. Thermal can see through dust and smoke due to their infrared wavelength, meaning that it is able to detect any heat energy through most environmental conditions.
Fog and Rain – Although thermal imaging cameras can see in total darkness, through light fog, light rain and snow, the distance they can see is affected by atmospheric conditions.
A thermal imaging camera produces an image based on the differences in thermal radiation that an object emits. The further this infrared signal has to travel from the target to the thermal camera, the more of that signal can be lost along the way.
Fog and rain can severely limit the range of a thermal imaging system due to scattering of light off droplets of water. The higher the density of droplets, the more the infrared signal is
diminished.
To capture images in fog and rain, the higher end thermal vision cameras work best.
Heat Vision Cameras – Easy To Use
While the scientific details of how thermal imaging works is quite complex, the reality is that modern thermal cameras are extremely easy to use. The images are very clear and easy to understand, requiring little training or interpretation.
Note: For mechanical and electrical uses, heat vision cameras should always be operated by qualified engineers who understand the thermal radiation spectrum of the equipment or material being imaged.
Reflected Light Vision Cameras
Thermal imaging cameras only require heat from an object to be able to capture the image or
video of a scene. Very closely related and sometimes confused are night vision and infrared illuminated cameras. Let’s take a quick look at night vision and infrared illuminated cameras.
Thermal Vision Cameras vs Infrared Illuminated Cameras
Infrared illuminated cameras generate their own reflected light by projecting a beam of nearinfrared energy which it’s imager can then see when it bounces back from the object.
This works very well, however infrared illuminated cameras still rely on reflected light to make the image. They have the same limitations as other night vision cameras which depend on reflected light energy, which is generally short in range and has poor contrast.
Thermal Vision Cameras vs Night Vision Cameras / Goggles
Night images or video which are greenish in color come from night vision cameras or
goggles. Night Vision Goggles or NVGs take in small amounts of visible light, magnify it and
project it on to a display.
Cameras made from NVG technology have limitations. If there isn’t enough visible light
available, they can’t see well. The imaging performance of anything, which rely on reflected light is limited by the amount and strength of the light being reflected. In other words, they won’t see anything in total darkness.
Also night vision goggles and other low light cameras are not very useful during twilight
hours. There is too much light for the NVG goggles to work effectively, but not enough light to see with the naked eye.
Thermal cameras aren’t affected by visible light, so they can give you clear pictures even when you are looking into the setting sun. In fact, with a quality heat vision camera you could aim a spotlight at the thermal camera lens and still get a perfect picture.
How Far Can Heat Vision Cameras See?
The distance or range from a thermal vision camera you can see is highly dependent on a number of camera variables such as;
- What is the lens of the thermal vision camera?
- Is the camera equipped with a cooled or uncooled detector?
- What is the sensitivity?
- What is the size of the object?
- What is the temperature of the target and the background?
By answering these questions, you can then choose the correct thermal imaging camera for the work to be completed.
Reference: dronezon.com
