“Calibration of light forces in optical tweezers,” Appl. Opt. 34(6), 977–982 (1995)
This equation can be found at this book:
J. Happel and H. Brenner, Low Reynolds Number Hydrodynamics, 2nd ed. Nordhoff, Groningen, 1973
J. Happel and H. Brenner, Low Reynolds Number Hydrodynamics, 2nd ed. Nordhoff, Groningen, 1973