Thermal Imaging Theory

Emissivity

The emissivitiy is the efficiency with which the surface emits thermal radiation. If only half the heat of the body is radiated then logically we have to double the measured value to get the temperature. This emissivitiy value is expressed as a number between 0 and 1, where 1 is equivilent to 100%.

In practice an approximation is normally the best that can be achieved. Thermoworks provides an emissivity table which provides approximations for a range of surfaces.

Skin has an emissivity of 0.98.

http://www.thermoworks.com/emissivity_table.html

Thermal compensation techniques

Non Uniformity Correction

This corrects for the differing sensitivity of different pixels.

http://nuc.die.udec.cl/?page_id=18

Flat Field Correction

Flat field correction is one non uniformity correction technique. The images taken with the shutter closed are designed to be used for flat field correction. This technique should also remove any static errors located under the shutter.

https://en.wikipedia.org/wiki/Flat-field_correction

Thermal Drift or Pixel Drift

As the array heats the behaviour of each pixel of the microbolometer changes in its own special way, both due to its own heat and the reflected radiation from other portions of the element. Ongoing calibration must be peformed to correct for this.