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Infrared windows: the importance of field of view

31 October 2011

So, you’re installing infrared windows. How many do you need per panel? And what diameter should they be? In both cases, you need to know the field of view (FOV) of the camera, any lens attachment and the IR window. Chapter five of ‘The Power of Knowing’ is your step-by-step guide.

The two questions most often asked by maintenance professionals getting ready to install infrared (IR) inspection windows on electrical equipment are: "How many windows will I require per panel" and "What diameter window is best?" The answer is "it all depends on Field of View (FOV)." It depends on the FOV of the camera, lens attachment (if used) and IR window.

Measurement Field of View (MFOV)
First, let’s examine the camera and lens specifications. We are primarily concerned with the MFOV (Measurement Field of View), also referred to as the "Spot Size Ratio."

Every camera defines its FOV across a horizontal/vertical axes. The Instantaneous Field of View (IFOV) is the smallest target the camera can "see". Although an IR camera can pick up numerous hot spots, many spots will be too small to measure accurately with a radiometric IR camera (one which displays temperature on the image). The MFOV, or Spot Size Ratio is the smallest target the imager can accurately measure.

The greater the resolution of the camera: the better the IFOV. For example, one top-selling camera uses a 640 x 480 pixel array. It has a MFOV of 500:1, while a 320 x 240 camera has an MFOV of 200:1. In these examples, a one-inch diameter target could be measured from a distance of 500 inches (41.7 feet) by the higher resolution camera; whereas the other camera would have a maximum distance of 200 inches (16.7 feet).

A telescopic lens (typically a 12° or 7° attached to a standard 24° lens) will also improve FOV by a factor of 2X and 3X respectively. Therefore, a 7° lens used on that high resolution camera would allow measurement of a one-inch target from a maximum distance of 125 feet (1":500 x 3 = 1500" / 12").

Think of resolution as the quality of your eyesight. When you go to the stadium to watch your favorite team, the poorer your eyesight, the closer you will need to be to the field to see your favorite player’s jersey number (temperature). Watching the game from high up in the "cheap seats," good eyesight will help, but you may be too far away to see the detail of that jersey number. Good thing you brought those binoculars (the 7° telescopic lens attachment)!

Window Field of View (WFOV)
Typically, IR cameras have a standard FOV of approximately 24°(horizontal) and 20° (vertical). So, it is advisable to do calculations based on a standard lens (since a wide angle may not always be available). Note that the calculation assumes that the FOV begins at the panel cover and extends a distance (d) from the panel cover to the targeted components. The length along that FOV is a distance (D). D is calculated by multiplying the distance (d) by the tangent of half the lens angle, then doubling the result.

Standard calculations assume that FOV starts at a single point, or vertex of the viewing angle. It does not take window size into account. So for a calculated D of 2.8" add an additional two/four inches when using a two or four inch window (yielding a D of approximately 4.8/6.8").

With an 82° FOV lens, a thermographer can view 34.8 inches horizontally inside the panel. However, this calculation assumes the thermographer holds the camera fixed and perpendicular to the window plane. More likely, the thermographer will vary the viewing angle up to 30° from perpendicular in all directions. This effectively increases the FOV area by a factor of three.

Practical FOV Test:
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 for working out what can be seen at set distances with your own camera, lenses, and IR windows:

1. Place a large piece of paper on a flat even surface. Draw a line straight down the length of the paper. Intersect the line in with 6-inch increments, and label the marks from 0 to 36-inches.

2. Place the camera lens at the 0-inches line.

3. Place two heat sources (fingers, warm coffee cup, etc.) at a distance from the camera that is typical of the targets you will be monitoring. For example, if monitoring targets that are typically 18-inches from the switchgear panel, place your heat sources on the 18-inch line.

4. Move one heat source from the center of the FOV, to the left, until it appears just inside the edge of the left-hand (LH) side of the imager display. Mark the paper at this point. Repeat the same procedure for the right-hand (RH) side.

5. Measure the distance between the LH and RH points. The result is the maximum FOV that can be achieved using that camera-and-lens combination, at the defined distance (assuming the camera position is not changed). Make note of the FOV and of the distance from the camera.

6. To calculate the WFOV for different windows sizes used with the camera-and-lens combination above, simply subtract the camera lens diameter from the FOV (noted in step 5); then add the diameter of the IR window that you intend to use. The total is that camera’s Maximum Horizontal WFOV, at the defined distance. Record this measurement.
FOV of a 24 degree lens at 18 inches as measured using the above process = 8 inches.
The camera lens diameter = 1.75 inches, therefore 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
7. Repeat the exercise with your camera turned 90° on its side. The result will be your camera’s Maximum Vertical WFOV. (Remember that your camera sees more in the horizontal plane than the vertical plane.)

8. Record the results in a table. An FOV matrix can be calculated using the above technique and then multiplied by a factor of 3 to give the total WFOV through each type of IRISS IR Window, allowing for roughly a 30° angle of incidence. Keep your matrix with the camera for future reference.

Tip: Most thermographers will take FOV measurements at several distances that are relevant to the various applications they monitor. Also, try moving a target as close to your camera as possible until you can no longer bring the object into focus. This will show you the camera’s "minimum focus distance." i.e. the closest you can get to a target and still be in focus. It is very useful to know your camera’s minimum focus distance, since some cameras can be more than 24 inches, thereby limiting their use in electrical thermography.

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!

Rule of Thumb
Based on countless field tests for WFOV with a wide variety of cameras, most cameras have generated a horizontal WFOV of roughly two to three times (2X to 3X) the distance to the target; and a vertical WFOV of roughly 1.5X to 2X the distance to the target. This is based on cameras with a standard 24° or similar lens, using a maximum angle of incidence of 30°.

Therefore, based on this rule of thumb, a window located 20 inches (51 cm) from the intended targets would allow the thermographer to gather data from points that were separated by 40 to 60 inches (1m to 1.5m) side-to-side and separated by 30 to 40 inches (0.75 to 1m) top-to-bottom.
Note that it is impractical to use a multiplier in excess of 3X due to the difficulty matching the thermal image to locations within the panel. Obstructions in the panel may also make it impractical. As previously mentioned, viewing at too steep an angle can begin to affect other variables such as emissivity. Therefore, IRISS, Inc. recommends a maximum of 3X as a "rule of thumb” multiplier. 

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