A method of monitoring combustion properties in an interior of a boiler of the type having walls comprising a plurality of parallel steam tubes separated by a metal membrane. First and second penetrations are provided in the metal membrane between adjacent tubes on opposite sides of the boiler. A beam of light is projected through a pitch optic comprising a pitch collimating lens and a pitch relay lens, both residing outside the boiler interior. The pitch relay lens projects the beam through a penetration into the boiler interior. The beam of light is received with a catch optic substantially identical to the pitch optic residing outside the boiler interior. The strength of the collimated received beam of light is determined. At least one of the pitch collimating lens and the catch collimating lens may then be aligned to maximize the strength of the collimated received beam.