- How it works
- Frame rate
- Field of view
- Focal length
- Colour palette
How it works
A thermal camera detects invisible infrared radiation (or heat radiation) and converts it into a visual image. Even the smallest heat differences (even just 0.01°C) can be detected, with the rule: "the warmer something is, the more visible it becomes". Since our environment continuously radiates heat (even cold-blooded animals!), a thermal camera works even in complete darkness without any source of light; in contrast to night vision goggles. The way the thermal radiation is visualised at each pixel depends on the colour palette (e.g. 'White hot' or 'Ironbow'). The sharpness and refinement of the image depend on the resolution (in pixels) and Frame rate (in frames per second, hertz). The higher these values are, the better the quality of the image.
Features of a thermal imaging camera:
Lens: focuses the infrared radiation (i.e. heat radiation) from the environment onto the heat sensor.
Heat sensor: scans the incoming thermal radiation and creates a thermogram; a detailed temperature pattern. This creates electrical impulses which are transmitted to the processing unit. The type of heat sensor determines the image resolution.
Processing unit: a printed circuit board that processes the electrical impulses and converts them into information that is sent to the display.
Display: converts the incoming information into a visual image. The intensity and combination of the electrical impulses, coming from the elements in the environment, create the final image.
The heat sensor has a certain resolution (80x60p to 1280x1024p), where each pixel captures the heat and translates it into a colour. These pixels are converted into a certain image which the camera displays, i.e. the image resolution. The higher the resolution, the more detailes in the image. Image resolution is relatively low compared to images of visible light (think of a normal camera). Thermal cameras actually deal with light of larger wavelengths, so each detector element (pixel) must be larger.
Please note that this refers to the resolution of the heat sensor and not the display. For marketing purposes, some manufacturers promote the high resolution of the display, while the heat sensor has a low resolution. You should therefore have a close look at this.
‘Frame rate’ (frames per second)
In addition to the resolution of the heat sensor, the refresh rate of the image is also decisive for the image quality of the thermal imaging camera. This so-called frame rate (expressed in hertz, Hz) determines the frequency with which the images are displayed and therefore how smooth the images appear. A frame rate of 9Hz does this 9 times per second and a frame rate of 60Hz does this 60 times per second. The best thermal imaging cameras have a frame rate of 60Hz. A faster frame rate is especially important for applications where objects/animals move quickly (think of flying bats or fast moving cars).
Field of View (FOV)
The field of view is the visible extent of a scene captured by the image sensor. It is expressed in degrees, consisting of a horizontal and vertical number (e.g. 15.4° x 11.6°). The horizontal number defines the angle that spans the width of the focal length and the vertical number defines the angle that spans the height of the focal length. As the focal length (expressed in mm) increases, the angle of view - and thus the field of view - becomes smaller and the magnification increases. The wider the field of view, the easier it is to catch sight of an animal or other object. This can be very useful when observing moving animals, such as bats. A narrower field of view can be very useful when viewing an animal or object in more detail, especially at a longer distance.
Focal length (in mm)
The focal length of a thermal imaging camera determines the magnification and range of your vision. This value is expressed in mm. The greater the focal length, the greater the magnification and range. You can often find these values in the title of the camera model (e.g. Pulsar Helion 2 XP50). A higher magnification can be useful for observations at long distances. However, this is less effective when you approach an animal or object up close, as you might be zoomed in too far. It is therefore good to think carefully about the main use of the thermal camera.
Many thermal imaging cameras have a built-in digital zoom function (not to be confused with optical zoom), with which you zoom in on the displayed image. This certainly has its advantages, but be aware that the quality of the image deteriorates. You are, as it were, cropping the original image. With a higher image resolution you retain sufficient image quality when using zoom.
The way in which the detected heat is shown in the display is determined by colour palettes. The type of colour palettes differs per thermal camera. Below we describe a number of common colour palettes:
- White hot
This is a very versatile and therefore the most commonly used colour palette. White hot shows warmer objects in white and cooler objects in black. These greyscale palettes provide simplicity for scenes with a wide temperature range and generate images with detailed and realistic details.
- Black hot
This is the reverse version of White Hot, with warmer objects in black and cooler objects in white. Black Hot displays heat in a clear, lifelike image and is a favourite with law enforcement officers and hunters.
Ironbow is a universal colour palette that quickly and easily identifies body heat and the smallest heat differences. The use of colour highlights heat distribution and subtle details. Warm objects are shown in lighter, warm colours, while colder objects are shown in darker, cooler colours.
Every user views and interprets the colour palettes differently, so everyone has a personal preference. The simplicity of 'White hot' may not provide enough detail for some, while the shifting colours of 'Ironbow' may be unnecessarily distracting for others. The most important thing is that, as a user, you use the colour palettes that you feel most comfortable with and works most efficiently for you. It is therefore good to gain the some hands-on experience with the various colour palettes so you can interpret a scene efficiently and confidently and make a thoughtful decision when it really counts.