An unused port should be closed, to prevent unwanted light from entering the sphere. Closing it with a diffuse white port plug, however, adds the surface area of that plug to the (diffuse white) effective area of the sphere that is doing the “integrating”. For a calibrated integrating sphere sensor, this change in the behavior of the sphere changes its calibration, and results in incorrect readings. In such applications, a black “Port Cover” should be used.
Power Meters FAQ's
In general, as the divergence angle of the beam entering the integrating sphere increases - and as its diameter increases – the assumptions on which we base the sphere’s performance (infinite reflections inside the sphere walls, perfectly uniform distribution of light inside the sphere, etc.) become less correct. We therefore specify the maximum beam divergence (such as +\- 40 deg), and we also state the maximum possible change in reading caused by change in beam size. For the IS6 for example, we state in the data sheet that the maximum additional uncertainty due to beam size is only +/- 1% for beam divergence < 30 degrees, and +/- 3% for beam divergence > 30 degrees.
To put this in some practical terms: If you measure the power using a beam that is not much bigger than a few mm x a few mm, that has a relatively small divergence angle, and is centered on the sphere’s input port aperture, you can safely ignore this additional uncertainty.