I finally, I should say FINALLY !!, found a way that my script works.
I tried with .Join() ... it was without success.
The last idea before I gave up was the right one !
I tried with a parameter "self" in the line "th = Thread(ThreadStart(LED_Calibration(self)))", but it produced each time an error.
I found that the "ThreadStart" function can be changed in "ParameterizedThreadStart" and the line "th.Start()" can be changed in "th.Start(self)".
I add in the function "LED_Calibration" the button changes ... and ... and ... IT WORKS !!
In reality, it worked only at the second try ... only after adding "ParameterizedThreadStart" in the line
"from System.Threading import Thread, ThreadStart, ApartmentState, ParameterizedThreadStart".
Good evening,
Jean-Francois