Smooth Wash diffuser & D40 diffuser front

Hello, 

maybe a dumb question but i'm still curious. With the smooth wash diffuser you have to put the mirroring side towards the front, while the D40 diffusers I use for the colorsource par states that the mirroring side has to go towards the fixture.

I haven't notice a real difference with side you put towards the front except when you look towards the diffuser.

Anyone can shine a light on this?

Kind regards.

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