Infrared and Thermal Sensors on Drones

Most pilots flying a drone will be using it for stills or video work. It's the most obvious and popular use for aerial camera platforms. But increasingly other types of sensors are coming to the fore, especially infra-red cameras for thermal imaging. The potential number of uses for infrared cameras is vast and includes search and rescue, surveillance,  crop and forest health, electrical gear including solar panels, pipeline inspection, leak detection, and building efficiency. Before we dive in it's worth taking a while to shed some light on the electromagnetic spectrum. [caption id="attachment_2148" align="aligncenter" width="555"] The electromagnetic spectrum[/caption] As you can see, visible light occupies a relatively tiny part of the spectrum, which ranges from gamma rays right the way through to AM radio. The difference between these forms of electromagnetic radiation is frequency and wavelength. The good thing is that some sensors can turn the invisible into visible. Infrared isn't visible to the human eye but you can feel it as radiated heat when it increases in intensity. Once you convert it to a visible image you can differentiate between temperatures by assigning them a different colour. So by revealing areas of higher temperatures for example, a thermal camera attached to a drone can detect poor insulation in a building or areas that need to be repaired. It will reveal overheating sections of electrical equipment in things like switch-gear, substations or pylons and it can do this quickly, safely and from a distance. The cost savings achieved by cutting down on access equipment and shutdown time can be immense. You can also factor in improved safety and the long term savings that can bring. [caption id="attachment_2162" align="aligncenter" width="555"] A rooftop survey using an infrared camera. Credit: RectrixAS[/caption] And there's more. The advantages of non contact temperature measurement or thermometry include being able to take measurements from moving or dangerously hot objects in hazardous environments, very fast response times and no mechanical wear. All this is achieved by a radiometric camera - a camera that can provide accurate temperature information down to individual pixel level. They can be expensive but the information they can provide is invaluable. On the other hand cameras used for night vision and surveillance do not need to be as precise and so most of them are non-thermometric. But, if you think about it, that doesn't matter. If you're looking for a missing person in undergrowth you don't need to know how hot their face is; you just need them to stand out from the background. Night vision cameras amplify low level infrared and convert it to visible light. Sometimes infrared is used to illuminate the subject.

Near Infrared for crop surveys

Near Infra Red (NIR) is closest to the visible region of the spectrum. NIR, or red edge as it's often referred to, is ideally suited to gathering data on vegetation. It's used widely to monitor the health of crops because it can detect the levels of chlorophyll. It compares near infrared levels with red light from the visible band  and then calculates something called NDVI - Normalised Difference Vegetation Index. Vegetation that's actively growing and producing energy from photosynthesis, absorbs most of the red light that hits it but reflects much of the near infrared light. Vegetation that is diseased, stressed  or dead reflects more red light and less near infrared light. Flying over a field with a drone equipped with a near infrared camera can produce dramatic evidence of the different state of crops. Images can show areas that need spraying for disease, require extra nutrients or need the amount of water adjusting. It can also indicate areas that might be of interest to archaeologists or possibly the police. What is a near infra-red camera? Most digital cameras have a filter which blocks infra-red light and stops it reaching the sensor. To make the sensor pick up near infra-red you have to remove that filter and replace it with one that blocks visible red, greed and blue light. [caption id="attachment_2161" align="aligncenter" width="555"] 3 images of the same field - normal, NIR and combined. Credit: Agribotix[/caption]

Getting a thermal camera into the air

So what about attaching a thermal camera to a drone? First of all you'll need to allow for having at least two cameras on board - one normal visible spectrum and one thermal. You'll need a standard sensor to provide a video feed for flying the UAV but you will almost certainly need another one to track down and line up your subject before switching to thermal. And it will help if that camera has the same field of view as the thermal one. Attaching a small first person view (FPV) camera to the same mount or gimbal as the thermal would make sense, provided the FOV was the same. As with a standard, electro-optical camera, the thermal sensor output can be recorded onboard and fed a signal to a monitor on the ground. Assuming that you'd need a pilot and a separate camera operator, then you'd need one or even two FPV cameras for the pilot too. There are simpler configurations that use one of each type of camera side by side. For example Skywatch have incorporated a Forward Looking Infrared (FLIR) into Anthea Technologies' Huginn X1. The operator can switch from one camera to the other. This type of quadcopter is often used by emergency services, humanitarian relief teams and conservation organisations. [caption id="attachment_2172" align="aligncenter" width="555"] The Huginn X1 with infrared and optical cameras side by side[/caption]

Here at Heliguy we're working on ways of using different sensors on our range of multirotors that would normally carry videos or stills cameras. We'll keep you posted an any developments as they happen.