Thank you.
Maybe we have a misunderstanding. What I want is "Growth speed vs. light intensity". LED current, PWM or others are just means of achieving the desired light intensity.
Thanks again for your suggestion. But I don't quite understand,
With average light output fixed what happens when you change the PWM duty?
"change the PWM duty"? You are talking about PWM frequency? If it is, someone else has already done it, and there is a difference between the frequencies. 