When an LED is driven with its rated current (typically 20mA for 5mm LED), but the current is pulsed, the light output is directly proportional to the ratio of pulse width (P) to pulse period (T)
Figure 1: Pulsed LED
Incandescent light bulbs are being dimmed routinely by varying the pulse width of the applied 60 Hz AC current. This is possible due to the thermal inertia of the bulb’s filament. LEDs respond to changing current much more rapidly and they require a DC current. A 60 Hz pulse frequency would result in an annoying flicker. Therefore. we need to increase the pulse frequency to a value where the human eye no longer notices flickering.
Very high frequencies are not desirable either since long wiring may radiate interfering energy to sensitive electronic equipment. I selected a frequency of about 1.4 kHz, well above a noticeable flickering and not causing electromagnetic interference.