There are several things that have to be considered when calculating the amount of light exiting the eye: First of all the wavelength ot the light used for measurement, from which we can derive the fundus reflectance. In addition, the exiting wavefront will have a larger diameter than the incident beam which affects the irradiance on the Shack Hartmann sensor. According to Delori & Pflibsen the spectral reflectance at 800 nm should be around 12%.

Delori, Pflibsen K.P. Spectral reflectance of the human ocular fundus. Appl Opt. 1989;28(6):1061-1077.

Thank you very much Timo.

You are right in your comments on the dependence of the reflectance on the wavelength, I am aware of that.

The objective of my question is to know the power at the microlens array from the entering power at the cornea and the reflectiance of the retina and the backwards absorption of the intraocular media.

Most of the papers tell us the entering power, but I couldn't finfd any with the power at the microlens array of the Hartmann-Shack

Thank you very much again.

Well from my experience using commercial H-S sensors, 12% is overoptimistic, I do not think that in a real setup more than 1,5% can be expected.

I will check again and write you more details.