When we think of daytime or photopic vision and color vision, we think of visible light, wavelengths from 400 to 700 nanometers in the electromagnetic spectrum that stimulate our three types of cone photoreceptors (S, M, L) in the retina. The three cone photoreceptor cells convert visible light into electrochemical signals conveyed to the brain via the optic nerve, where visible light is perceived as color images. Conventional wisdom is that ultraviolet light is not part of the “visible spectrum.” Therefore, it is invisible, and for most adults over age 30, this is the case. But according to research at the University of Georgia, children and young adults can see ultraviolet down to wavelengths of about 315 nm under some conditions, as can people with aphakia (missing a crystalline lens) or those with intraocular implants that transmit ultraviolet. In these cases, ultraviolet wavelengths to about 315 nanometers can reach the retina and produce a visual stimulus. In the study published in PLOS One, co-authors Billy R. Hammond and Lisa Renzi-Hammond show that 100 percent of the participants, all young adults, could detect an isolated UV peak at 315 nm. Forty-two subjects were tested (M = 19 ± 1.3 years). Absolute thresholds to UV radiation were collected (λmax = 315 nm, 305–325). Individual variation was large (over a factor of 30). Males were more sensitive to UV than the females (p<0.05). The study shows that our photoreceptors have secondary absorbance that extends into UV, and our lenses transmit at least some UV radiation to which the retina responds. (journals.plos.org/plosone/article?id=10.1371/journal.pone.0199940)
Lisa Renzi-Hammond, PhD, states: “The team didn’t set out to rewrite the rules on visible light. However, knowing that the eye can detect UV presents previously unknown consequences. From a health perspective, there’s a risk to the retina. If you can see the light, it’s getting back to your retina and in a way that could potentially be damaging. Now that we know this fundamental fact about vision, there’s a lot more work to do to understand what this is contributing to us and what the potential harms are.” Dr. Renzi-Hammond is the director of UGA’s Institute of Gerontology and the co-director of the Cognitive Aging Research and Education (CARE) Center.
Curiously, ultraviolet light seems to produce a color sensation, according to research by Dr. Stark, who is aphakic in one eye, and Professor Karel Tan. They conclude that ultraviolet looks like whitish blue or whitish violet depending on UV wavelength. When ocular media transmit UV wavelengths, and the energy level is sufficient, all three of our cone photoreceptor cell visual pigments (opsins) absorb UV equally, producing the sensation of white. The blue/violet hue is attributed to our short wavelength sensitive blue cone photoreceptors, which are more sensitive to ultraviolet than the green or red cones.
• Deborah Kotob
Pro to Pro Director