In PWM dimming, the LED is turned off and on very quickly so that the human eye does not detect the transitions, but averages the illumination so that the LED appears to dim. The current for some LED lighting systems is derived directly from a 60Hz AC line instead of a constant current DC power supply. In these applications, some individuals have reported that they see the LEDs flickering at 60Hz though. For this reason many designers have chosen 400Hz for the frequency of their PWM dimming circuits. The PLED device will need to switch on and off with every PWM pulse, and it is important that the PLED device does not interfere with the timing or wave shape of the PWM wave form. PLED devices have been tested to 10kHz, well beyond the range of most PWM systems.