There's been librariesworth written on the 'let there be light' translation of 'fiat lux'; to which I have little to add. But I was thinking, more or less idly, of the particularity. The Latin 'fiat' is the third person singular passive subjunctive of 'facere', to make. We've almost lost the subjunctive in English, a few hangover phrases like 'God save the Queen' notwithstanding. The problem with this is that the idiom 'let there be light' -- the next best thing -- suggests the passive construction. I'm not denying that the 'let' construction is the closest we come to signalling the subjunctive in verbs that no longer inflect subjunctively ('let it be' for instance); but it has the larger problem of elevating the passivity from the mode of the verb to the agent performing the action -- as if God is not actively creating light (facere: to do, to make) so much as holding back a reservoir of already existant light, and finally deciding to let it all come out. But that's not right.
'And God said: brightness be made' doesn't sound quite right, mind.