18 which is made of a material selected from those having low heat diffusivity and low reflectivity e g asbestos sensor 12 is completed by stacking the square cut pieces 16a e of the sheet polystyrene by passing the shank portion of bolt 20 through center hole 21 of each of the polystyrene pieces 16a e and through a predrilled hole in backing plate 18 and then affixing nut 22 to the end of the shank portion of bolt 20 as it extends through backing board 18 in this embodiment the square cut pieces 16a e are approximately one inch on each side sensor 14 includes sheet polystyrene pieces 16f g that are rectangularly cut each having three holes drilled therethrough these pieces 16f j are approximately three quarters of an inch by two inches one of the holes is approximately one quarter inch in diameter and is drilled in the approximate center of the individual polystyrene rectangular pieces 16f j the other two smaller holes are drilled near the corners of the individual pieces 16f j along one of the longest sides the pieces are then stacked one on top of the other and a masking board 24 is then placed directly above the top 16f to form sensor 14 and to restrict the exposure of pieces 16f j to.