In a few words - UV and HEV light are bad; red and NIR (near infrared) are good.
But there's a little more to it than that -
Photo-Therapy for the Skin. We don't give it much thought, but our skin
is damaged throughout the day; it's also being repaired at the same time. Recent tests
indicate that the repair rate for skin is: retarded while exposed to blue light; does
not change for green light; and actually increases under exposure to red light. In other words,
exposure to red light can be therapeutic.
Vision Therapy - from Sunlight. Is red light and near infrared (NIR) therapeutic to the eye? Is it even possible? The answer to the second question is, "Yes," - because the lens and vitreous humor (the ultra-clear, jelly-like material that fills the cavity of the eye) transmit the red and NIR; that is, the vitreus transmits light between 700 nm and 1400 nm. Furthermore, sunlight has a significant amount of energy between 700 nm and 2300 nm - not enough for the indirect sunlight in the 700nm to 1400 nm range to cause thermal damage to the eye, but enough to potentially offer a therapeutic effect But is there evidence for any therapeutic effect from NIR on vision? The answer again is, "Yes." (see the References below) So red light and near IR from sunlight can reach the retina and offer constant therapy - provided that our eyewear use lenses that do not attenuate this light.
Melanin Provides Phototherapy to the Eyes. Melanin filters the blue light strongly; it filters the green moderately; and it strongly transmits
the red and near infrared (See the Figure below).
Apart from protecting the skin around the eye from wrinkles caused by UV and HEV (high energy visible - violet and blue) light, a melanin lens is therefore therapeutic to the eye and to the skin around the eye, at least relative to other sun lenses that don't have this dual feature of protection and therapy.
Light Therapy Lenses? This is a bold new idea for lenses - let alone a new way to think about the sun - that
Photoprotective Technologies is proposing (US Patents Pending - 2010). But the concept is supported by decades of science. Here is a short summary of the
(1) J. Zuclich, “Veiling glare: the visual consequences of near-UV/blue light induced fluorescence in the
human lens,” Ophthalmic Technologies XV. Edited by Manns, Fabrice; Soederberg, Per G.; Ho, Arthur; Stuck,
Bruce E.; Belkin, Michael. Proceedings of the SPIE, Volume 5688, pp. 440-447 (2005).
(2) S. Zigman, Sunlight and human cataracts, Invest. Opthalmol. Visual Sci., 18(5), pp. 462-467 (1979)
(3) UV-induced skin damage, M. Ichihashi et. al., Toxicology, 189, (1-2) pp, 21-39 (2003)
(4) Sunlight exposure, antioxidants, and age-related macular degeneration. Fletcher AE, Bentham GC, Agnew M, Young IS, Augood C, Chakravarthy U, de Jong PT, Rahu M, Seland J, Soubrane G, Tomazzoli L, Topouzis F, Vingerling JR, Vioque J. Arch Ophthalmol., 126(10):1396-403 (2008)
(5) B. Mahmoud et. al., Effects of Visible Light on the Skin, Photochemistry and Photobiology, 84: 450-462 (2008).
(6) M Denda and S Fuziwara, “Visible Radiation Affects Epidermal Permeability Barrier Recovery: Selective Effects of Red and Blue Light,” Journal of Investigative Dermatology (2008) 128, 1335–1336;
(7) Julio C. Rojas,1 Jung Lee,1 Joseph M. John,1 and F. Gonzalez-Lima1,2,3, Neuroprotective Effects of Near-Infrared Light in an In Vivo Model of Mitochondrial Optic Neuropathy. The Journal of Neuroscience, 28(50):13511-13521,(2008).
(8) JE Roberts, pp. 211-224, Effects of Visible Light and Near Infrared in Humans, in "Biophysiological Effects of Solar Radiation on Human Skin," ed. Paolo Giacomoni Royal Society of Chemistry, Cambridge (2007) .