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Field of View (FOV) Calculation Tip

Some thermographers find it easier to let the camera show them what it can see rather that completing a number of calculations, the following procedure is a quick method of working out what you can see at set distances with your own equipment, lenses, etc…

1. Place a piece of paper on a flat even surface and mark out a line with increments of 2 inches up to approx 24 inches.

2. Place the camera lens at the 0 datum line.

3. using a heat source (finger, warm coffee cup, etc…) move the object in from the LH side outside the visual frame until it can be seen and mark the paper, repeat the same procedure for the RH side and mark the paper. (see fig 1)

 

 

 

 

4. Measure the distance between the 2 points and this will give you what you can see using the camera and lens at a set distance.
a. Note: once you have worked out the FOV distance that you can see you will need to add on a factor to take into account the IR window that you intend to use, this is done by subtracting the camera lens diameter from the FOV and adding the IR window diameter that you intend to use e.g.

Example:
FOV of a 24 degree lens at 16 inches as measured using the above process = 8 inches. The camera lens = 1.75 inches, thus the FOV of the camera = 6.25 inches.

• Using a 2 inch IR window would give an approx FOV of 8.25 inches
• Using a 3 inch IR window would give an approx FOV of 9.25 inches
• Using a 4 inch IR window would give an approx FOV of 10.25 inches

5. Most thermographers measure a point at 18 inches and then join the points together at the increments marked on the paper and then keep a chart with their camera for easy reference.

6. Also move a target as close to your camera as possible until you cannot focus on the object anymore… this will show you your cameras “minimum focus distance” i.e. the closest you can get to a target and still be in focus… a very useful thing to know as some cameras have minimum focus distances of over 24 inches and their use in electrical thermography is extremely restricted.

Note: Although the above technique is not 100 % accurate it gives an extremely good result. Try it for yourself; it is a simple technique that really works well!!

Tip provided by IRISS Inc

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Comment on todays FOV calculation

There is a very simple formula that will work for finding the field of view in any application, not just for use with windows. It is useful because today’s infrared cameras have a rectangular field of view not a square field of view. It is FOV = angle in radians of your lens X distance. To get the angle in radians from your lens take the angle in degrees given by the manufacturer and multiply it by .0175. For example I have a 26 X 19 degree lens and I want to know my field of view at 15 inches My angles are (26 X .0175) = .445 radians by (19 X.0175) = .3325radians. At 15 inches my field of view would be (15 X .445) = 6.675 inches horizontal by (15 X.3325) 4.98 inches vertical.

Comment by Wayne Ruddock
Advanced Infrared Thermography

 

 

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