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Not everything is as it seems, especially when dealing with lights, shadows and reflections.
For example, take a look at the following picture which shows the projection of two black-and-white gobos.

 

In the gobo on the left, the part that should be black looks so much gray that even the pencil in front of the gobo can be seen.
You could think that a bit of light goes through the parts that should be black, but this is not the case.
This phenomenon is caused by the so-called “back reflections” i.e. light reflections on the lens (look at the following scheme).

 BACK REFLECTONS

Every lens reflects from 1% to 4% of the hitting light (according to the lens quality and to the presence of an anti-reflection treatment on the lens’ surface). This percentage comes back on the gobo, only to be reflected again.
So what you see as grey is actually the light reflected from the gobo itself. The pencil is visible because it looks darker, since it does not reflect the light.
This phenomenon is not appreciated by the user, especially when projecting in dark environments, because, as well as the logo, also the gobo halo can be seen.
In the following picture, you can see the same logo printed on a traditional gobo (on the left) and on a Refstop gobo (on the right).

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