The duration the LED is on in each pulse (in microseceonds)
This is the maximum value. Regardless of the value specified here, the
LED on time cannot be longer than the inter-frame interval. Furthermore,
it may also be limited by Self::max_overall_duty_cycle.
This is a safety measure to reduce risk of destroying LEDs. When
overdriving LEDs, it may be that they should only be on for a fraction
of the time, e.g. 10% of the time. Setting this value to anything less
than 1.0 will limit the LED on time to a fraction of total time. The
Self::nth_frame field is taken into account within this computation.