While thermal camera or thermal sensor has many important technical parameters which will determine its accuracy of temperature measurement, there are some optical parameters which one should know to determine its real world usage. Like a normal camera, Thermal Camera also has very important component, lens which will influence the temperature measurement. I will be trying to list and explain those optical parameters one should know or consider while selecting a thermal camera for specific application
1. FOV ( Field Of View) : This is one of the widely known or mentioned optical parameters or specification mentioned in any thermal camera. It can be considered as an window or a solid angle through which electromagnetic radiation will enter into the sensor of the thermal camera and the size of the window is the “field of view” of the camera for that distance. Now, it should be evident that, as its an angle, it will get wider and wider as it goes far from the sensor. This angle of view is dependent on focal length of the lens, longer the focal length, narrower the angle of view is and shorter the focal length, wider the angle of view would be. But the FOV is not dependent fully on the focal length of the lens, but also the sensor or detector size and its pixel pitch as well as its pixel count, both horizontally as well as vertically.
Normally, for a same pixel count, if the pixel pitch is greater, it will have greater FOV than the one with smaller pixel pitch.
Normally, all thermal camera manufacturers provide FOV calculator where one can select the camera model and compute the FOV on selection or focal length or compute the focal length requirement on selection of FOV requirement.
2. Spot size or Pixel Size At Range : This is a very important parameter for any real world, practical applications of any thermal camera of any size and shape. This will greatly impact usefulness of the thermal camera in measuring temperature.
Spot Size or Pixel Size can be defined as the real world size of the measuring object ( or portion of the measuring object) one pixel of the sensor would represent in real word measurement unit, in millimetre. For example, if a thermal camera of resolution 384X288Pixels with pixel pitch of 17micron, uses a 13mm lens, then the spot size or one pixel will represent a real-world size of 5.7mm at a distance of 4 meters from the thermal camera. It means that, a Thermal Camera with 384X288Pixels resolution and 17mm Pixel Pitch will be able to measure temperature of object whose minimum size is 5.7mm if the object being measured is at the distance of 4 meters from the Thermal Camera. Any object which us smaller than 5.7mm will not be measured accurately as its evident from above. Though, in practical world, I think, anything less than 11mm object will not be measured or represented well by that thermal camera.
To explain it more, size of human face is say 120mm ( W) X 160mm ( H). In order to measure this skin temperature of human face, if one uses a thermal camera with 384X288Pixels with 17micron pixel pitch and 13mm lens, if the person stands at a distance of 93 meters, one pixel would represent the whole face of that person at a lucky moment! And if the person stands at 10 meters from the thermal camera, his face would be represented by some 11X16Pixels matrix! So, in order to measure skin temperature on face of a person, it would be advisable to stand within 10 meters of the camera in its FOV, then only a reasonably satisfactory accuracy of temperature measurement can be expected!
I have tried to explain FOV and SPOT Size in simple language and examples, but I am not sure if I have been helpful enough! I will try to write better next time!
With above two optical parameters, one should be able to select a right thermal camera for their applications. The right resolution and pixel pitch, right lens. In my next post, I will try to explain NETD in a thermal sensor which greatly effect resolution, image quality of the thermal camera!
I welcome any suggestion to improve this post in the comment section!