FLIR ThermaCAM P20 Operator's Manual page 94

16.6 – Other parameters
Atmospheric temperature – i.e. the temperature of the atmosphere between
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the camera and the target
External optics temperature – i.e. the temperature of any external lenses or
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windows used in front of the camera
External optics transmission – i.e. the transmission of any external lenses or
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windows used in front of the camera
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Publ. No. 1 557 536 Rev. a35 – ENGLISH (EN) – January 20, 2004