The Reflectance (Transmittance) measurement consists in the integration of all the reflected (transmitted) signal (scattering + specular) into a hemisphere: the reflected (transmitted) light from the sample is integrated into a reflective hemisphere then normalized by the total reflected (transmitted) power of the incident beam.
In practice, for Reflectance the sample is placed on the exit port of an integrating sphere and lighted up by a laser source. Then, the reflected intensity by a Lambertian Spectralon is measured. Knowing the calibrated reference reflectance, reflectance of the sample is calculated.
For Transmittance measurement the sample is placed at the entrance port of an integrating sphere. Then, the transmitted intensity of the incident light without anything at the entrance of the sphere is measured. From this, the transmittance of the sample is calculated.