Abstract: A property such as moisture content in a sheet of material such as paper that both absorbs and strongly scatters infrared radiation is measured by passing the sheet through the central region of a substantially spherical cavity formed between a pair of concave mirror-surfaced, specularly reflecting bodies that are spaced apart to form a pass gap of sufficient width to allow substantially free movement of the sheet therethrough while substantially enclosing a first portion of the sheet on both sides. A spot of infrared radiation is projected onto one side of the sheet in the center of the cavity to illuminate a second portion of the sheet included within and substantially smaller than the enclosed first portion. Thus infrared radiation scattered or transmitted by the second portion is reflected from the mirror-surfaced bodies and directed back to the second portion substantially within the area of the sheet defined by the spot.