This Article is about:
- Spatial resolution (IFOV) explained
- IR resolution, Field of View and Spatial Resolution (IFOV)
- Example calculation with a TiS75+
Spatial resolution (IFOV) explained
The spatial resolution of a thermal camera is defined by a parameter called Instantaneous Field of View (IFOV). This parameter indicates the field of view of one pixel of the infrared thermal imager in milliradians.
By knowing the IFOV value and the distance to the object you`d like to measure, you can always calculate what is your actual pixel size at a given distance, how many pixels will cover the object and consequently make a conclusion about the reliability of the measurements delivered by the camera.
The smaller the IFOV value of your camera, the smaller the details you can see with it.
The smaller the IFOV value of your camera, the smaller objects which temperature you can measure accurately.
As a rule of thumb, for temperature measurements accuracy in the field to be close to the camera specifications achieved during laboratory accuracy test, the object which temperature you measure, should be covered by the thermal image area of at least 3x3 pixels.
Usually, when you have smaller area of the object, the readings will start to decline significantly from the actual temperature. With the area of 3x3 pixels the readings will be close to the accuracy specified for the thermal camera.
If you reduce your distance to the object, more pixels will cover it and your measurement will be more accurate.
If you increase your distance to the object, less pixels will cover it and your measurement will be less accurate.
IR resolution, Field of View and Spatial Resolution (IFOV)
Spatial resolution or Instantaneous Field of View is actually defined by two other parameters: your camera Field of View, or the lens used, and your infrared image size.
For example, if your camera has field of view of 42°x30°, and your infrared image size or infrared image resolution is 384x288 pixels, like in case with the TiS75+, then you get the value of IFOV as follows:
IFOVh = 42° / 384 pixels = 0.1094°
Small angular values are convenient to express in milliradians, as they are also more suitable for simple calculations and simple quick evaluations.
0.1094° = 0.1094° x (3.1415926 radian / 180°) = 0.00191 radian = 1.91 milliradian
As a consequence, the bigger the infrared resolution of your thermal camera at a given field of view, the better your thermal camera spatial resolution or IFOV.
But at the same time, the wider the field of view of your thermal camera at a given infrared resolution, the worse your thermal camera spatial resolution or IFOV.
Example calculation with a TiS75+
To understand the importance of spatial resolution and how it works, the best is to provide an example.
The TiS75+ has a spatial resolution of 1.91 mRad.
Suppose, that the object you`d like to measure has the size of 1cm² and you have a distance of one meter to the object.
At the distance of one meter, every pixel of your TiS75+ camera has a size of 1.91mm².
This means that about 25 pixels of your camera will cover the object of 1cm² what leads to 25 measuring points on that object. This far better than the minimum requirements, so in such situation at least your camera spatial resolution is adequate for correct measurements.