Pixel Requirements For Text Legibility?

I met with a client this week that has some specific requirements for a cashiering function. They would like the cameras mounted above the cashiers to capture text on certain documents, not just bill denomination. They specifically asked for the ability to forensically read 9pt font.

Outside of managing expectations around privacy concerns and who would have access to the recorded video, etc, etc; this brings up a good point for me in technical design which is - What number of pixels density per foot equates to text legibility for a certain font size? How exactly do we define legibility (based on assumed 20/20 vision of the human looking at the image of course)?

I don't yet have the measurements of the target table surface and distance from the ceiling to table for this project, but for the sake of establishing a baseline; let's say that the taget surface area is 2' x 2' 6", and the distance from the camera to the target surface is 4' 6".

What resolution and lens is required to achieve legibility of 9pt font at that distance within the entire target area? What about 10 or 12pt font? Any simple calculation we can use to scale up or down based on a different target area and camera distance?

Thanks in advance for the input!

-C

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