to 
.
This is the wavelength region to which the human eye is sensitive.
There are no precise limits for the spectral range of visible radiation since
they depend upon the amount of radiant power reaching the retina and on the responsivity
of the observer.
Source:
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 439