Sebastian here,

this time I'll show you a useful help to quickly see how and where to paint the OSL (thank you Thomas). So what you need is a smartphone and a screen Flashlight app. In my example I use this (google playstore) one. 

With this flashlight you can set any color you want. From a brassy chimney fire orange or a green lightsaber to a glowing purple plasma gun, everything is infinitely possible here. 

Another big advantage is that you can regulate the intensity with which the flashlight should shine. So you can only show a weak light up to a strong glow of a quasar (something like that) 

All you have to do now is select the color you want and adjust the intensity. Then just hold the flashlight from the direction you want on the mini and you can see what the OSL can look like.

Here some examples...

 

If you only have a small light source and the smartphone screen is too big, then simply cover part of it. This way you get a smaller light section.

Thank you for your interest and reading and I hope that it might help one or the other.

Sebastian

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Check these articles for more OSL information:

Inspiration: OSL
A collection to get inspired.

Painting Object Source Lightning (OSL)

Using Dayglow Pigments for your OSL