What causes this transparent effect with night video on a 5231-ZE?

From the posts I've seen in the LPR forum, capturing a license plate at night usually requires a stupidly fast shutter speed, like 1/2500 or something.
yeah i've been playing with 500,1000,2000. 500 and 1000 seems to work here, but i'm constantly adjusting contrast, brightness, sharpness, iris, noise reduction, focus, etc. whenever i get close thati can actually make it out, it seems another car goes by and it's blurry again. the IR and angle is decent, just an on going battle back and fourth adjusting the above.
 
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