Infrared Training Center

Friday, January 26, 2018

Lens Choice for Looking Through IR Windows

by Gary Orlove
More and more organizations are installing IR windows or sight glasses in their electrical equipment so inspections can be made with an infrared camera.
The advantages are obvious: worker safety is improved as thermographers and electricians don't have to open live electrical cabinets, and inspection times can be reduced.
However, what lens choice is the best to use with your infrared camera to provide adequate resolution, and yet see as much of the electrical equipment as possible?
Here are three images that were taken at a power plant looking at an ISO phase buss. The buss is only about a foot away and is taken looking through an infrared window (crystal type).
25 deg.
45 deg.
90 deg.
Images courtesy - John Fricot, FLIR Systems
As you can plainly see, the 90° lens is clearly the way to go.  It provides enough detail for analysis, yet extends your field of view by a huge amount.

2 comments:

  1. What is the transmissivity of the IR window? Is it posted on the window? How does this affect the accuracy of the temperature measurement?

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  2. The transmission of an IR window is dependent on the material from which it is made, the thickness, any coatings applied to the window, and the wavelength band of the instrument used to look through it.

    That being said, most Calcium Fluoride windows have a transmission between 0.4 and 0.5 (40 to 50 %). We recommend to our students to measure the transmission with their own infrared camera to obtain more accurate values.

    And yes, the transmission of the window can greatly affect the temperature and temperature difference readings taken through it with an infrared camera.

    The link below will take you to a document that shows how to measure the transmission with your infrared camera.

    https://flir.app.box.com/s/9ow0imphc1wv2t5whz84o8pfb43zav0u

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