The invention relates to a light integrating cavity device, such as an integrating sphere, for measuring diffuse reflectance of a sample. A light trap is movable within a light scattering cavity of the device for controlling specular reflections during measurements. The light trap may be rotatable around the sample under test inside the cavity so that specular reflections off the sample can be included or excluded from the measurement. The sample may also be placed at the outside against a measurement port, and a measurement instrument is moveable on a rotating arm within or outside of the cavity.