A curved viewing window assembly for an electrical assembly having a non-flat curved surface such as an isophase bus duct or electrical motor housing, includes a curved lens assembly having a lens section disposed between a curved front grille and a curved rear grille and is transparent to infrared (IR), ultraviolet (UV), or visible light. The curved lens assembly is attached to an aperture disposed through a curved wall of the bus duct at a point proximate to the location in which electrical equipment within the bus duct is to be monitored. Disposed through each at the front and rear grilles is a plurality of viewing apertures permitting the IR, UV, or visible light from the electrical equipment or components to pass therethrough. A curved door is pivotably attached to the front grille to allow selective access to the curved infrared viewing window.