The human lens already lets most infrared through. https://en.wikipedia.org/wiki/File:Human_crystalline_lens_OD.svg https://en.wikipedia.org/wiki/File:Human_cr

The human lens already lets most infrared through. https://en.wikipedia.org/wiki/File:Human_crystalline_lens_OD.svg https://en.wikipedia.org/wiki/File:Human_crystalline_lens_OD... Infrared is everything to the right of 700 on the x-axis, and ultraviolet is left of 400. Our lens already lets through about the same proportion of infrared as most of the rest of visible light. Presumably that means that our retina is just insensitive to it.