How Far Can You Measure with a Thermal Camera?

A major advantage of thermal imaging cameras is that they can accurately measure temperatures from larger distances. The distance at which a certain thermal camera can accurately measure a target of a given size and still get an accurate temperature measurement is known as the Distance:Size ratio (D:S ratio). Most thermal imaging cameras can accurately measure the temperature of a target of this size (1 cm) from several meters away. The FLIR TG54 for example, has a D:S ratio of 24:1, meaning it can measure a 1 cm target at 24 cm distance (or 1 inch target at 24 inches distance). The FLIR E8, a thermal camera with a 320 × 240 pixel resolution, has a D:S ratio of about 120:1, meaning it can measure a 1 cm target at a distance of 120 cm. Thermal cameras like the FLIR T865 are capable of accurately measuring temperature from long distances. Many of the more advanced thermal imaging cameras feature interchangeable lenses, which affect the camera’s D:S ratio. Some thermal imaging cameras can be paired with a 6° FOV telephoto lens to perform thermal inspections on targets from long distances. Even if a hotspot is too small or far away to be accurately measured, a thermal camera may still be able to detect it when scanning over an area, giving the operator a chance to move closer and get a more accurate reading.