The human eye is a complex sensing device for visible light.?

the energy of a photon is hv, where v is the frequency, and h is Planck's constant, about 6.63E-34

you don't have the frequency but you have the wavelength, lambda = 4.75E-7

and there is the relationship
c = lambda * nu (wavelength times frequency)

so nu = c/lambda = 3E8 / 4.75E-7 = 6.32E14


so the energy of your blue photon is:
E = hv = 6.63E-34 * 6.32E14 = 4.18E-19 J

So to get to 2E-17 J, you'll need 47.8 photons

but you can't get a bit of a photon, so that'll be 48 photons please


Incidentally, this is massively better than most currently available imaging devices out there.

and, the retina has about 125 megapixels, on an area the size of "APS" sensors in DSLR's, which have about 12megapixels these days (really high-end cameras have more, but their sensors are also bigger)

so it looks like it'll be at least another 8-10 years until image sensors maybe match what our eyes can do... ;-)