​Kids and Blue Light (Part 3 of 4): Crystalline Lens Clarity and Blue Light

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. 
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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. 

Natural Blue Light Filtration
Clinically, we see this as a yellowing of the lens.   This yellowing serves as a natural blue light filter, protecting the retina. However, this is a decades-long process, with complete protection occurring around age 80.  With the advent of modern surgical procedures, the cataract is typically removed before this time as it begins to become detrimental to vision. 

Children’s Lack of Natural Filtration
While this may sound like a lot of difficult science, there is a simple, important point to remember. The human crystalline lens is clear until about age 40, after which time it starts to yellow from cataract formation.  A clear lens lets visible light, including blue light, pass through unobstructed.  In terms of retinal luminance, meaning how much light including blue light the retina is receiving, children are getting more than adults because of their clear crystalline lenses.

So there we have it, the three factors affecting retinal luminance making children more vulnerable to blue light: short arms, large pupils, and clear crystalline lenses.​

Figure 2:  From birth until about 40 years of age, the crystalline lens is clear and visible light > 400 nm is incident on the retina.  Starting around age 40, the lens starts to yellow, although this varies by individual. With age the transmittance of the shortest wavelengths diminishes.  For example, by age 50, only >410 nm light may be reaching the retina, and by age 65, only > 450nm light may be reaching the retina.  This loss of transmittance for the blue light wavelengths (400-500nm) is generally complete by age 80, with only >500nm light reaching the retina.  

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.  ​

read part 4 of the series

Reference: Roberts, J.E. (2011) Photobiology of the Human Lens. Original research article, Fordham University, Department of Natural Sciences, New York, NY. 

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​About the author: Dr. Gary Morgan has been in private practice for 25 years in Arizona, with an emphasis on the care of patients at risk of, or with AMD. An advocate for innovation, he serves in a technical advisory capacity to ophthalmic industry enterprises focusing on spectacle lenses, nutraceuticals, and telemedicine that are intent on lessening the effects of AMD and blue light.