Why is Field of View (FOV) Critical for Sizing Infrared Inspection Windows?

Companies are purchasing new equipment and replacement panels to comply with the new guidance for reducing risk while performing maintenance tasks in NFPA 70E (2018 Edition). You are receiving more requests from consultants and end user customers to install Infrared Inspection Windows into your switchgear. To understand the size of inspection window needed, you are researching Field of View (FOV) capabilities.

There are two FOV parameters that must be determined when choosing the size of infrared inspection window needed in a piece of switchgear:

1. Infrared Camera’s Field of View

The camera’s FOV, in degrees, for any given distance from the object can be calculated using this formula:

Camera FOV = {(tangent ½ viewing angle) x distance} x 2

* Distance is defined as distance from the panel cover to the target to be measured

* Viewing Angle of the Camera is obtained from the manufacturer’s specification

A typical FOV is 22 degrees horizontally and 16 degrees vertically for the average camera lens. The calculated values should be used for estimation purposes to determine size of windows needed. Note that many IR cameras have interchangeable lenses with various viewing angles and a change in lens type in the camera requires the FOV to be recalculated.

A Practical Field of View Test

2. Window Field of View (WFOV)

The following equation is used to calculate the WFOV:

Window Size (W & H) = Target Size (W&H) – {(2 Tangent(CVA/2)) x DCT x 3]

* W = Width

* H = Height

* CVA = Camera Lens Viewing Angle from manufacturer’s specifications

* DCT = Distance from Cover to Target

* 3 = Maximum Viewing Angle Multiplier

Example of Window Width Calculation:

The distance from cover to target is 8 inches and overall target width is 18 inches and the camera’s viewing angle is 22 degrees. What is the minimum window width required?

Ws = 18 – {(2 tan (22/2)) x 8 x3}

Ws = 18 – (0.389 x 8 x 3) = 18 – 9.34 = 8.66 inches

One More Important Parameter For Window Sizing:

There is one other parameter to consider when determining the size of infrared window needed: the outside diameter of the infrared camera lens. To accurately measure temperature, the infrared camera’s lens must “see” the full view of the target objects being measured. Any obstruction, even a partial obstruction, will introduce measurement errors that cannot be compensated by a setting change. A Window’s IR optic must be larger than the outside diameter of the infrared camera lens.


Collecting accurate infrared temperature data is critical in maintenance inspections of energized electrical equipment. The accuracy of the data is dependent upon the infrared camera’s interaction with the infrared inspection window. Any obstruction will cause temperature measurement errors. Three elements are required for proper Window sizing: the window’s infrared optic diameter, the field of view of the camera and the field of view of the window.

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