Hi Eddi,

I use for this purpose Orient Command inside MoI. I learned it from forum some time ago, but I cannot find this thread. On picture below you can find two windows. I've modelled frame (solid) and then glass (surface only), then I've copied this model and rotated by 180°. So now the windows surface is "flipped". In MoI both surfaces looks same, but if you select them and use Transform/Orient command you can see difference in axis orientation. When you use this model in some rendering software for example, you can see same difference as well.

This is very useful trick especial for mesh emitters in renderings sw.

Hope this helps.

Ciao
Mik