LED signs have different display characteristics. The larger the number, for example 12mm, the less crisp an image since there is more distance between the LEDs. A 4mm display will give you the best image quality. If you don't plan to use images on your display and only want text, a 12mm or 9mm board would work.

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Below you can see an example from Cirrus. We work with them as their displays are made in the US which allows quick turnaround time.