To fully understand how a thermal image, or thermogram is produced, some background knowledge of the physics of light is needed.
Light is emitted in waves. The amount of energy in each light wave is related to its wavelength; shorter wavelengths have more energy.
Visible light is made up of a spectrum of colours (those you seen in a rainbow), in this spectrum violet has the most energy and red has the least.
As well as the visible light spectrum there are also UltraViolet (UV) and InfraRed (IR) Spectrums either side of it. These can't be seen with the naked eye.
The IR Spectrum is the part of light that we're interested in when capturing IR images.
Infrared light can be split into three categories;
1. Near IR; closest to visible light, near infrared has wavelengths ranging from 0.7 to 1.3 microns (or 700 to 1300 billionths of a metre);
2. Mid-IR; has wavelengths ranging from 1.3 to 3 microns;
3. Thermal-IR; has wavelengths ranging from 3 to 30 microns.
You're probably very familiar with both near and mid-InfraRed as they're used by a variety of electronic devices, e.g. remote controls. Thermal-IR makes up the largest part of the IR Spectrum and is also known as heat. Thermal-IR is the part of the Spectrum we're interested in for Thermography (Thermal Imaging).
The key difference between Thermal-IR and the other categories is that it's emitted by an object instead of being reflected off it.
Animals emit IR radiation as a result of normal physiological processes. About 60% of all the heat an animal produces passes into the surrounding air as radiant heat, or heat photons. The remaining heat is lost to the environment by evaporation (25%), conduction to objects (3%) and conduction to the air (12%).
An IR camera is used to measure the heat photons emitted from the animal and to convert them into electrical impulses which are then displayed as coloured images on a monitor. This visual image graphically maps the animal’s body temperature and is referred to as a thermogram. The heat detector, or microbolometer within the thermal camera can detect differences in temperature of less than 0.05oC, which is 40 times more sensitive than the human hand. More reading about heat detectors can be found on Wikipedia.
Move the slider across the image of a sheep below to compare what you can see with and without the naked eye. These pictures were taken with the FLIR veterinary thermal imaging camera, which has the ability to simultaneously take and then fuse together visible light and thermal images.
Wool is a fantastic insulator, so heat is not emitted through her fleece, and this area doesn't even register on the thermal scale. The corners of the eye are always the warmest points on any animal as the skin is thinner here, allowing heat to radiate out easily. As the ear is concave, the infrared radiation bounces around and concentrates in this area making it appear warmer.
What are we Looking for
Since there is a high degree of thermal symmetry in the normal body, subtle abnormal temperature asymmetries can be easily identified. A "hot spot" indicates inflammation or increased circulation. Hot spots generally are seen in the skin directly overlying injury. A “cold spot” is a reduction in blood supply usually due to swelling, thrombosis, scar tissue or increased tone in the nervous system.
After the images have been taken by the thermal imaging camera, they are digitally mastered on a PC. This ensures that differences with the thermal scales between images are eradicated, and a true like-for-like comparison is produced. The images can also be enhanced to make viewing and interpretation easier. These remodelled images are then compiled electronically into a Veterinary Thermal Imaging report.
"Artefacts" or anomalies can be found in the image. To the untrained eye these can give a ‘false positive’, so experience and knowledge is a key to producing accurate thermograms for interpretation. For example, if legs have been bandanged, or blisters or liniments have been used, they will show up as areas with increased heat. External environmental factors can also play a part, if the animal has been in the sunlight or lying on one side, or there are draughts, this will again affect the image, therefore a strict preparation protocol must be followed before each consultation and time is spent carefully taking a history and examining the environment.
Take a look at the two pictures below.
On the left an eventer prepares to head out on the cross country course; would his knees worry you?
On the right a mare shows some unusual cold spots on her side; would you want to do further tests?
Both of these are trick questions.
The eventer's knees appear unusually hot, but this is due to the radiation from the sun warming him. The clue is in the lorry ramp which has also faced the direction of the sun for some time and warmed up. It's important to note external factors and their influence on the thermal image.
The mare had just been grooming herself, there is nothing untoward going on. Water blocks the emission of infrared radiation, even a small amount like this saliva can create dramatic looking thermal patterns.