Thermal Imaging

What you are seeing on the right is a picture of a dog under thermal camera. As you see bright yellow areas are of high temperature and blue areas are of low temperature.
How does one capture a thermal image? there is some interesting science behind everything that goes on the screen.
Anything ( living, non-living ) which has a temperature above 0 Kelvin ( -273.15 deg C ) emits radiation. Radiation is the emission of energy.  Thermal cameras form images by detecting Infrared. Infrared is a part of Electromagnetic spectrum. What we see is Visible light which is also a part of electromagnetic spectrum.
You can see the electromagnetic spectrum below.
Infrared means below red. Infrared is discovered by Sir William Herschel in 1800.














Thermal images are measure of energies emitted, transmitted and reflected by the object.
the sum of  Emitted Energy, Transmitted energy and Reflected energy is always equals to 1.

Any object has emittance less than 1.  Emmisivity can be defined as the ratio of energy radiated from a material surface to that of blackbody under the same conditions (or) simply the measure of radiation emitted by the object. A good emitter is a good absorber of energy. In the below table you can see emissivity of some metals and non-metals.

Reflectance is the amount of light or radiation reflecting off the object.  As you can see low emissivity materials have high reflectance. where as high emissivity materials have low reflectance.
Transmittance is the ratio of light falling and light transmitted through the object.
Now you can understand why the sum of Reflectance,Transmittance and Emittance Energies is equals to one.

Blackbody Radiation : As we have seen above, Emissivity is the ratio of the radiation emitted by the object to that of blackbody at same conditions. Blackbody is something which absorbs all types of Electromagnetic radiation. Blackbody is an ideal body ( cannot be found in nature ). Blackbody emits radiation at thermal equilibrium and it is known as Blackbody radiation.

Kirchoff's Law : when an object is in thermal equilibrium. absoroption = emission. basically you can say, when an object is at the same temperature as that of surroundings. its said to be in thermal equilibrium.
Stefan-Boltzmann Law : relation between tempearture and power radiated. Or simply hot bodies emit more IR energy. Cold bodies emit less IR energy.

Wien's Displacement Law : Wavelength at which maximum energy is emitted decreases with increasing temperature.

Planck Radiation Law : relation between wavelength, temperature and intensity of radiation.

These are the basic principles usually used in thermal imaging. here are some interesting thermal images :