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There are several ways to do this. Make a light, then animate the brightness. For all frames set set the brightness to zero, but on one frame where you want the flash, make it really bright. Add some Bloom to the camera lens and it will help sell it. I don't use Arnold but I wasn't sure if you had Vray or anything so here is an Arnold file for you to play with. This should give you a start. I'm sure there are other ways to do this but give this a try.
When you play the MP4, right click on it and set it to 'Loop' and you'll get a better idea.
Rob Holmes
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