I took a look at the study, and I'm confused as to whether they're illuminating a certain cell density, or single cells. They say the max power they used was 9.7µW, and HeLa cells have a diameter of ~20µm (according to google), and therefore an area of about 100πµm^2, giving a max intensity of ~0.0308mW/mm^2 (assuming they're illuminating individual cells). Seeing as the retina has an area of ~1000mm^2, at a power of 0.0308mW/mm^2 there would be over 30mW of monochromatic blue light on your retina at these levels.

This seems awfully high to me. Did I screw up my units somewhere? Am I underestimating the power the eye receives on a normal basis? Did I just not understand the paper? I'm not a microbiologist (or any kind of biologist for that matter) so some help here would be appreciated.