In figure 1 we see the projection of two gobos.
You notice that in the one on the left the part that should be black looks gray, so much so that you can also see the stick placed in front of the gobo. It may appear that “some light” passes through the parts that should be black in the gobo. In reality this is not the case. The phenomenon is in fact due to the so-called “back reflections”, that is to the reflections of the light on the lens as shown in figure 2.
The lens of each projector (to varying degrees depending on the quality of the lens itself or better on the quality or presence of an anti-reflective treatment on the lens), rejects a percentage (about 1 to 4%) of the light towards the gobo itself. invests it. This percentage of light returns to the gobo and is reflected again.
What we see as gray is therefore the light reflected by the gobo itself. The stick can be seen because by not reflecting light it appears blacker.
This phenomenon is not appreciated by those who use gobos, especially for projections in very dark environments, because it shows not only the logo, but also the whole halo of the gobo. Figure 3 shows the same logo engraved on a traditional gobo (on the left) and on a GlossyDark gobo (on the right)
It can be understood that the extent of the phenomenon depends on at least 3 factors:
– The type of image reproduced on the gobo which determines the amount of light incident on the lens
– The quality of the anti-reflective treatment of the lens that determines the amount of light that “bounces” on the gobo
– The reflection coefficient of the gobo coating.
Of these factors, the only one on which it is possible to intervene is clearly the reflection coefficient of the gobo.
Goboservice already from the Frankfurt fair 2009 proposes a type of coating with a very low reflection coefficient. In this way the phenomenon is completely canceled as can be seen in the gobos on the right of figures 1 and 3.
Clearly this type of gobo cannot be mounted in reverse, the black or opaque side must always be facing the lens, never towards the lamp. It is therefore good to specify whether a retroprojection should be done.