A unit is sort of a combination of Time & Intensity.

A photocell monitors how bright the light is. You get more or less real time as the light gets "dimmer" or "brighter".

I spent a lot of time in prepress, where there were many different platemaking and exposure units. I seem to recall them being somewhere near a second, and different machines had different settings, so it wasn't standardized.

But again the time changes with intensity, so you get consistent results.