How do you measure relative diffuse reflectance?

Attach a barium sulfate or other white reference plate at position (1) in the integrating sphere shown here. Perform baseline correction. The sample beam shines on the white reference plate.

Remove the white reference plate from position (1) and place the measurement sample in its place. Measure the sample to obtain the diffuse reflectance. Specular reflected light is reflected out of the integrating sphere via the sample beam inlet port and, therefore, is not detected.