boutyhunter, you have to understand a little more about how human vision works. Frequency is not the only determining factor in perception, the difference between the two points is. In the case of a motion images, film uses as low as 24 frames per second, but even in a high action movie the difference between each frame is very small. Same thing with a lightbulb, a common incandescent bulb flickers at twice the line frequency, this flicker is very subtle though, and isn't noticeable to the human eye. An LED fed AC will only be on for half of the AC cycle, so it dims on, then off for 1 half cycle and full off for the other, depending on what the person is looking at or doing this can be very pronounced. As far as what the maximum frequency and modulation depth a person will notice is HIGHLY dependent on the individual. I personally notice flicker most in my peripheral vision, because that's primarily what it's for, to detect movement. I've had CRT monitors with a full screen refresh rate of 75+hz and in my peripheral vision I can still see the screen flicker when my head is moving.