A viewport for a combustion zone includes an inner casing having a flange extending into the combustion zone. A first layer of ceramic fiber insulation is exterior to the inner casing. A middle casing includes two panes of quartz glass separated by traverse plates. A second layer of ceramic fiber insulation is exterior to the middle casing. An outer casing is positioned to the exterior of the second layer of insulation. The middle casing includes L-shaped ledges having third and fourth layers of ceramic insulation. The panes of glass are each positioned between layers of insulation and the insulation positioned in the L-shaped ledges. The viewport is modular enabling the separation of the first layer of insulation, the middle casing, the second layer of insulation and the outer casing from the combustion zone for maintenance or repair. The viewport minimizes heat migration to the outer casing during use.