Instrument Aperture
Openings in the wall of an integrating sphere allow for the inclusion or exclusion of the mirror-like reflection from a sample surface. The specular gloss port is positioned at an angle equal and opposite to the illumination beam to capture the light reflected directly from the top layer. Controlling this aperture enables a spectrophotometer to measure either the total appearance or just the diffuse reflectance of the paper.
This choice determines whether the resulting data accounts for surface shine.
Reflectance Mechanism
Reflectance measurements taken with the port closed include both the diffuse light from the pigments and the specular light from the surface. This mode, known as specular component included, is less sensitive to the texture or gloss level of the sheet. Opening the port allows the specular beam to escape into a light trap so only the diffuse light returns to the sensor.
This second mode, specular component excluded, better represents how the human eye perceives colour on high gloss packaging.
Operational Constraint
Calibration of the light trap within the port is necessary to ensure that no stray light reflects back into the sphere. If the trap is dirty or misaligned, the gloss exclusion becomes incomplete and the data loses accuracy. Users must select the appropriate setting based on the finish of the material and the requirements of the colour match.
