About ten years later at the University of   Chicago, researchers discovered that the average wavelength of cell tissue in the human body ranged between 600 nanometers and 720 nm; 660 is the mid-point. So in essence, the reason a 660 nm works better than any other single frequency is because it is closer to the resonant frequency of cell tissue. The other reason is that 660 nm absorbs better in hemoglobin.