Black is not always black.
Even if you paint all lighting laboratory surfaces black, there might still be some light bouncing from the walls and the ceiling as the light source rotates during measurement. Some of this straylight may also come into your sensor thus adding false light to your results. Here is some good advice to remove that last bit of straylight.
Good advice to remove straylight.
Check your straylight situation. Mount a powerful omnidirectional light source on the goniometer. Stand behind the sensor and cover your view directly to the light source with your hand. See if any surfaces are very light. This is a sign that light bounces off these walls and toward the sensor.
Cover essential surfaces with Molton curtains. Especially the rear wall behind the goniometer is important as light reflected on this wall will bounce directly back to the sensor. Walls, ceiling, and floor are also important and especially in a narrow room, and Viso recommends using folded Molton fabric whenever possible. Black paint is expensive and often more glossy than expected.
This is how baffles work. The baffle (red indication) prevents the light bouncing off the wall from reaching the sensor.
Design baffle curtains that prevent straylight from walls, ceiling, and floor from bouncing back into your sensor. Again, black Molton curtain is a good material, but a thin wooden board or any blackout curtain would also work. The size of the baffle hole combined with the distance to the sensor should match so that the sensor just sees the whole light source and not much more.
Get a Viso LabDisc – available for both LabSpion and BaseSpion. Viso sensors have a narrow field of view. Thus, LabDisc removes all stray light from side walls, ceiling, and floor. In addition, LabDisc reduces stray light from the rear wall to a minimum.
Read more good lighting laboratory practise in these Viso guidelines.