About the Fluke 100 CLKT, a radiation window for indoor and outdoor applications The Fluke 100 CLKT is a radiation window designed for indoor and outdoor applications. This device allows infrared rays to easily pass through a transparent window, allowing for temperature measurements in environments where direct access to the surface is not possible due to weather or safety conditions. These features make this window an essential tool for monitoring and measuring temperature in various industries, including electrical and mechanical industries.