[yetihehe]

You should also know that at such frequencies, displaying hardware matters. You can have distortions from refreshing frequency. If you show and hide image in software at 50hz and your display refreshes at 60hz, you eaither have tearing (one part of screen is black, other still has image) or "frame drops" when you use v-blanking synchronization (http://en.wikipedia.org/wiki/Vertical_blanking_interval). Such artifacts ARE visible in case of image/noimage transitions and can alter results of experiment. Your friend should use some 200hz tv for tests. For measuring on/off, he could also mask some part of screen, display a white rectangle there and measure on/off with a photodiode (photoresistor may be too slow) under this mask. Of course just turning LED backlight on and off won't have such problems if displayed image is still static.