Wouter, I have no numbers here but 'average outdoor lignting' is a trap.
If you put them in the shade (roof or shade above) you can assume
illumination <3000 lux in most cases. There are IEC/ISO standards for
display sizes and contrast factors etc. I will look if I can find
something here, I used to have a book that contains pointers.

Anyway try to look at products from LED makers for special elements for
outdoor panels. And take into account the angle of view. Their data sheets
and application notes are very helpfull.

Peter

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