The position of the diffuser's entrance aperture needs to be known for accurate calibration. Because the diffuser has a hemispherical shape the effective entrance aperture is not exactly located at the front surface.
Experiments with the lamp at 1.5 m and 0.5 m show that according to the inverse square law the location of the effective entrance aperture is 1.4 mm (±1mm) behind the front surface of the diffuser.
Info, questions and remarks, contact: Richard McKenzie
