What Size Infrared (IR) Window Do I Need?

As the manager of the Reliability Maintenance Team, your company’s director of reliability recently assigned you the task of selecting maintenance inspection windows for the facility’s critical electrical assets. Your team has attended several internal meetings learning about the business benefits of adopting maintenance inspection windows, specifically Infrared (IR) Windows. You are relying heavily on your IR Window vendor to help you learn and understand the unique information that must be identified to properly select the correct size of windows.

One question that needs to be answered in selecting the correct size of IR Windows needed to accurately inspect the critical electrical assets in your plant: What dimension of window is best for each asset?

Selecting The Correct Size IR Window

There are three considerations that will determine the answer to this question:

* 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. Therefore, the first consideration is to determine the outside diameter of the infrared camera lens that will be used for the inspection. A Window’s IR optic must be larger than the outside diameter of the infrared camera lens.

* The Infrared Camera’s Field of View (FOV) Calculation: 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. A typical FOV is 22 degrees horizontally and 16 degrees vertically. The calculated values should be used for estimation purposes to determine size of windows needed. Note that a change in lens size in the camera requires the FOV to be recalculated.

* The Window Field of View (WFOV): to calculate the WFOV, the following equation is used:

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

DCT = Distance from Cover to Target

3 = Maximum Viewing Angle Multiplier

Example of Width Calculation

The distance from cover to target is 8 inches and overall target width is 18 inches. 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


In summary, keep in mind the following points:

* The IR Window’s optic must be larger than the outside diameter of the IR camera’s lens

* Every IR camera has a Field of View (FOV) defined in degrees across a horizontal and vertical axis. Consult with the camera manufacturer if this is not listed clearly on the specification sheet for your camera

* The Window Field of View (WFOV) rule of thumb using a standard camera and lens is approximately 2 – 3 times the distance from the window to the target.

* Obstructions inside the cabinet may decrease the actual field of view.

Your IR Window vendor, such as IRISS, will provide the expertise in helping your team select the appropriate sized windows for the target objects to be inspected. They have both tools to help calculate window size as well as the expertise with many different kinds of electrical equipment.

Selecting The Correct Size IR Window

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