This paper is a slightly modified version the one presented orally at THERMOSENSE XXVI April 13 - 15 2004 SPIE [5405-46] Session 8: Research & Development. Many industrial applications exist for which, due to the lack of access or for other reasons, thermal images or thermometry measurements cannot be generated. This happens especially when the target is inside an enclosure and cannot be viewed, for safety -- process reasons, except through a window transparent to visible energy only. The traditional materials transparent to visible radiation ~380 nm - ~780 nm are not, in general, transparent to infrared radiation. It is clear there is a more specific exception in the NIR band. This lack of IR transmission in traditional glass/crystals has required the search for other alternatives in order to carry out measurements or generate images with instruments that work in the thermal bands (MWIR - LWIR) of the electromagnetic spectrum. This paper includes infrared windows basics and a study of attenuation with LWIR sensor of two windows two colors transmission (MWIR/LWIR) commonly installed in medium voltage electrical enclosures.