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.

The Magic of IR - Transparency

A video from David Brown of the Infrared Training Center explaining transparency in the world of infrared.

IR Transparent Materials for Windows and Lenses

"Which material (Quartz or Sapphire) has better transparency to IR spectrum? Is there any material that is totally transparent to entire IR spectrum?"

The chart below illustrates a variety of materials and the wavelengths at which they transmit.


 

Courtesy http://www.ispoptics.com

Here are detailed data sheets on quartz and sapphire.

"Which IR (Near IR or Far IR ) carries more thermal energy?"

Photons at shorter wavelengths of IR carry more energy. So all other things being equal, will produce more heating on an absorbing surface.

"While traveling through the IR transparent lens will be there any damage to the lens?"

Any infrared energy absorbed by the lens will produce some heating. Anti-reflective coatings are commonly applied to lens systems to reduce absorption and improve the transparency.

For more information please contact:

ISP Optics Corporation
1 Bridge Street, suite 205
Irvington, NY 10533, USA
Tel: 914-591-3070, 800-909-4207
Fax: 914-591-3715
E-mail: sales@ispoptics.com 
Web Site: http://www.ispoptics.com/