This article discusses the basic principles of thermal photography. It also talks about the possibilities of using a thermal camera in practice when equipping drones with it.
What is hidden under the concept of “thermal imaging” and how it can help in performing different tasks, what you need to know when using this technology?
The concept of thermal photography
Thanks to the use of thermal cameras, the human eye can see what is impossible or difficult under normal conditions. Invisible radiation, reflection from objects, the camera can capture without reference to the available lighting.
The infrared spectrum should be understood as part of the electromagnetic spectrum. It is a constant or a constant that is always around us. It is not visible, except for a small part in the center of the spectrum, which is called visible light.
A person can only capture light reflected from surfaces. In this case, the wavelength can be different, which ensures the perception of different colors. This does not apply to thermal energy, it can not only be reflected, but also radiated.
Speaking about the efficiency of thermal energy return, they mean radiation. It works differently for different objects. This is affected by the material, the state of the surface.
So, people, animals, rocks, are characterized by a high degree of return. Metals, which are characterized by luster, have a low return. If they are painted or corroded, the situation will change.
For 2 different objects placed next to each other, having a similar temperature, the appearance in the image captured by the thermal camera will be similar.
An example is an image of a roof that is not painted. It reflects the cold sky.
Given this fact, remember the following:
- Thermal energy is a surface phenomenon, and the devices themselves do not capture radiation if there is glass or walls.
- Thermal imaging is affected by the state of the surface, the distance to the object.
- The cameras can measure the difference in the energy that is emitted, but it is not related to temperature.
What do the colors in thermal images say?
To obtain different thermal images, certain color schemes are used. Since infrared and heat cannot be seen by the eye, abstract tones are applied to the display to make the energy visible.
Bright shades or variations of different gray saturation are specific, have a large data set. Knowing the capabilities of what can be displayed with them helps to use them when necessary, even in the field. The user can define and apply information on various objects.
The main options include:
White hot for an image burning in white.
Black and white in this context makes it easier to understand the essence of the content with a large limit relative to the temperature scale. This is convenient if necessary to explore natural landscapes, urban areas.
Black Hot (display hot as black).
This version allows you to display warm objects in black, and cold objects in white. This option is used, in most cases, by law enforcement agencies, hunters.
This palette is universal. It allows you to capture thermal deviations, the temperature of an object to provide information regarding heat distribution and some details. In this case, warm areas are visualized in warm colors, and those that are cold in shades of blue.
- The less popular Sepia variant displays warmth in gold and white, and cold in dark tones. It is generally accepted that this option is more easily perceived by both the human brain and the eyes. It is used in situations requiring long-term video surveillance.
- Rainbow HC. This option is relevant for items with a small difference in temperature, as well as Arctic, which implies gold backlighting of warm and cold together with Ironbow and Rainbow HC.
When choosing a display method, it is impossible to say right away which option is better. Some users found White Hot to not provide the right amount of detail, while others found the multi-color image to be distracting. It is better to use White Hot and Dark Hot when searching for small heated objects, as well as in conditions of high humidity and high temperature. Ironbow can also be attributed to a universal tool that is relevant for studying roofs, solar panels, and electrical equipment. Arctic is suitable for increasing the contrast of colors.
The concept of an isotherm
Isotherm – a type of color palette. It allows you to see the range of a specific color temperature.
An isotherm is used to highlight a specific temperature limit so that a specific object is not missed.
It is not necessary to use only isotherms. An example is a person and an object that is located near him. If they have the same temperature, there will be approximately equal amounts of energy, which will hide the object.
To avoid such failures, you can use Black Hot and White Hot first. Since such a selection may not be sufficient, it is necessary to use not only isothermal images.
When choosing a lens to determine the optimal resolution, do not rely on the fact that it is easy to find the right option.
With a higher resolution, you can get more pixels. This will provide better detail. A small lens will provide a larger field of view, which will be captured by this space. At the same time, this can lead to a decrease in comfort during the search. Thus, it is necessary to ensure the optimal ratio of the captured area by detailing and magnification.
As an example, here is a sample with a 9mm lens, a camera on the left with a resolution of 336 * 256, a field of view of 35° * 27°; right lens 640*480 field of view 69°*56°. The figures below show how the picture changes depending on the distance of the lens.
It can be seen from the figures that when using a resolution of 640 mm, it turns out to perform more coverage. At the same time, it becomes more difficult to make out the content at a distance.
This suggests that it is necessary to find a middle ground between detail, field of view and detection range.
Another example is on the 640 but with a 32° x 26° FOV.
In this example, the FOV is smaller, the observed object is visible further at 640.
The last object was obtained at a resolution of 640, with lenses 9 mm (left), 13 mm (center), 19 mm (right). With these parameters, you can choose the optimal match between the coverage area and the range in which you can distinguish objects at a certain resolution.
What is the viewing range?
When it comes to the use of drones, the main points are the range of vision and the flight altitude.
For a detailed consideration of the issue, you can make the answers separate for the range, so that the object becomes noticeable and the range for measuring temperature.
To determine the distance, the following points must be considered:
- object size;
- the amount of thermal energy;
- conditions in the atmosphere;
- temperature difference of the environment object.
If accurate data is needed, the main thing is to understand that seeing an object is not just to get the temperature.
It is also important to pay attention to such parameters as:
- focus – the focal length of the camera lens;
- range – a value that characterizes the sensitivity of the camera, which should also be taken into account during preparation;
- distance – the distance from the information recording device to the object itself.
It is also important to know the size of the goal that needs to be determined. It is also necessary to draw up a flight plan in such a way that the viewing angle, height, distance of the template are predetermined, otherwise the object will be difficult to find.
It is important for the thermographer to plan the flight route in advance, based on the “measuring field of view” of the video camera.
Dave Lee from FLIR says that MFOV is not 1 pixel. This is a complex of 5 * 5 pixels. To obtain an accurate measurement, the object must fall within this spot or slightly out of it.
He also points out that if the MFOV is larger than the measured object, the data may be distorted, since they will be correlated with the energy of neighboring objects, and not just the measured one.
How does the weather or environment affect a drone with a thermal imager?
When it comes to using a drone with a thermal imager, it should be understood that search tasks and inspections of industrial facilities will differ.
Inspection thermographs, in most cases, are used to observe known areas, objects. They can be significantly smaller than those used by rescuers. The reason is the mission of rescuers, which is to search for a small object in a large area.
It must be remembered that for operations in the direction of safety, the amount of heat energy, as well as the location, plays a big role.
The key lies in understanding the principles of the influence of the surrounding space on the visible. There are also 3 interrelated environmental causes that should not be forgotten:
- daily cycle.
- action of the sun.
- Thermal crossover (crossing).
The first concept should be understood as a cyclical process associated with the beginning of the day at sunrise and ending when it sets. The longer the day is, the sky is cloudless, the more heated the earth and objects, objects on it will be.
As the sun sets, heat is released back into the atmosphere.
A large amount of accumulated energy negatively affects the definition of specific objects on the ground. This applies to solar load.
In the process of observation, one can notice that with an increase in the degree of absorption of radiation, the temperature difference between the desired object and the surroundings becomes smaller.
At the time of a large solar load, it is difficult to distinguish one object from another, which can be clearly seen in the figure. The green arrow shows that a person is standing there, but it is not always possible to determine the object in the resulting image.
At this point, thermal intersection begins, i.e. the visually determined temperature difference between the object of interest and its environment drops to such a limit that the camera is not able to determine them.
There are cases when previously visible objects on a cool background are shown cold on a warm one.
It should be remembered that heat energy and temperature are different concepts and the feeling of warmth by a person will not always be shown by heat.
Another important factor is the weather. In particular, this applies to fog, cloudiness, humidity. They can change the look of the image. This also applies to changing the picture at night. It will be different compared to the one taken during the day.
Humidity will also absorb or dissipate heat energy, reducing the detection distance, blocking the detection of energy from the object to the camera. That is, the greater the humidity, the greater the result. Rain is a prime example of this, as it significantly lowers the temperature of various objects.
To some extent, observation can become more difficult. However, after the rain stops, those objects that radiate heat will be more visible. This can be seen in the image of people on a multi-colored background.
The possibilities of using thermal cameras on a drone
The use of a drone with a thermal imaging camera (Mavic 2 Enterprise Dual, Matrice 200/210 V2 with an XT2 thermal imager) may be relevant in situations where promptness in saving a person is important.
Examples of areas of application are as follows:
- supervising public events;
- search for missing tourists;
- fighting forest fires;
- inspection of industrial facilities.
The use of drones with such technical support increases the chances of a quick and safe solution to the current situation, and also reduces the cost of the necessary rescue measures.
An example is the situation when a drone surveys large areas, collecting important information. Subsequently, thanks to her, it is possible to make effective strategic decisions at a distance from the operation zone.
Police use drones to search for missing fugitives, tourists. Firefighters use them to monitor the speed and extent of the spread of fire in forest areas and how operations are being carried out to eliminate it. The thermal camera also works through smoke.
In terms of technical inspection, thermal camera drones are also important. Taken into account:
- Price: in the ratio of the necessary costs for solving different problems, heat chambers are cost-effective compared to other methods.
- Speed of obtaining the result: the image is obtained quickly in a shorter period of time. This increases the reliability.
Thermal imaging cameras can also be used to inspect buildings. Thanks to this, it is possible to timely determine the leakage of heat, water, problems with electricity, and more. With just one drone, many problems can be solved. At the same time, the time usually spent on certain procedures will be significantly reduced. It is only important to pay attention to the features of the operation of the equipment, the weather, the speed of the device, and other characteristics.