This is part three in a four-part series guest-written by nationally recognized blue light expert, Gary Morgan, OD.
In parts one and two of this series, we discussed two primary factors affecting blue light exposure: proximity of the light source and pupil size. These factors increase retinal luminance in children viewing blue-light emitting devices more than they do adults.
The third factor affecting retinal luminance is the density of the ocular media that light is passing through. In the eye, the only media that significantly changes density is the crystalline lens.
The Role of 3-OHKyn
The crystalline lens gradually changes density over time due to light exposure. The amino acid tryptophan, found naturally occurring in the lens, is converted into a light-absorbing chromophore, 3-hydroxykyurenine (3-OHKyn). Until middle age, 3-OHKyn serves to protect the lens as well as the retina.
When light, including UV light, strikes these 3-OHKyn chromophores, they absorb it and release its energy before it can damage the lens. This absorption also protects the retina from UV light. However visible light, including 400-500nm blue light, passes through the lens and is incident on the retina.
Figure 1: Endogenous tryptophan within the crystalline lens is converted into 3-OHKyn, which absorbs light and releases it’s energy, serving to protect the lens as well as the retina. With age, the enzyme kynurenine aminotransferase starts to convert the 3-OHKyn into the destructive chromophore xanthurenic acid which leads to cataract formation. Likewise, exposure to light over time converts tryptophan into N-formyl kynurenine, also leading to cataract.
3-OHKyn After Age 40
After about age 40, the protective properties diminish as an enzyme (kynurenine aminotransferase) starts to convert 3-OHKyn into a destructive light-sensitive chromophore called xanthurenic acid. Another destructive chromophore, N-formyl kynurenine, also builds up in the lens over time from exposure of tryptophan to light. Thus these two destructive chromophores start to form what we know as an age-related cataract.
In part four of our series, we’ll look at what all of this extra blue light exposure may mean to ocular health of children, and steps that can be taken to protect them.
Reference: Roberts, J.E. (2011) Photobiology of the Human Lens. Original research article, Fordham University, Department of Natural Sciences, New York, NY.