Kids and Blue Light (Part 2 of 4): Pupil Size and Blue Light Exposure

​This is part two in a four-part series guest-written by nationally recognized blue light expert, Gary Morgan, OD.

When considering the impact of blue light on children, most of us immediately think of behavioral tendencies that lead to increased exposure.  When we see little Johnny or Suzie immersed in a three-hour game of Minecraft on a tablet eight inches from their eyes, many of us think about the potential consequences of all that screen time on their eyes. 

But there are less obvious physiological factors that come into play when considering blue light exposure, and why children are at increased risk.  

Three key factors play a role in children being at greater risk of blue light exposure: 

• Proximity to the light source
• Pupil size
• Density of the crystalline lens

​In part one of this series, we examined how proximity to the blue light source results in amplified intensity. In part three, we’ll cover crystalline lens density. For today’s topic, we’ll examine how pupil size puts children at an increased risk of blue light exposure.

An Open Doorway to the Retina
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Think of the pupil as the doorway light must pass through to enter the eye and be incident on the retina.  As with any door, the larger the opening, the more that can fit through. So the larger the pupil size, the more light (including blue light) that passes through.  

This increased retinal luminance, can affect two types of photoreceptor cells; intrinsically photosensitive retinal ganglion cells (ipRGCs), which regulate circadian rhythm, and retinal pigment epithelial cells (RPE) for which blue light exposure over time leads to photo-oxidative damage.

Increasing Age, Diminishing Pupil Size

Studies show that pupil size is largest during adolescence, and decreases with age. In fact, the pupil reaches its peak size under dim light conditions between 11-17 years old[1]. And whether they’re doing homework, Snapchatting, or taking in a new e-book, this age group is spends a ton of time viewing digital devices after dark.

In fact, by the time the average American teenager reaches 17 years old, they will have spent 50,000 hours or 1/3 of their life viewing digital screens[2]. That’s a lot of blue light making it to the retina.  

Senile miosis, the decrease in pupil size as we get older, has been attributed to both muscle atrophy, as well as nerve innervation changes along both the parasympathetic (pupillary constriction) and sympathetic (pupillary dilation) pathways[3]. As the following graph shows, the change in pupil size with age is significant[4].

Pupil size in light (photopic) and dark (scotopic) conditions changes significantly with age. From age 20 to 70, photopic size changes by 2.5 mm, and scotopic size changes by 5.0 mm.

​Interestingly, the drop in retinal luminance with age is most prominent in the 400-500nm range as shown in the graph below[5]. While this may be a natural protective measure for older eyes, young eyes are vulnerable.  In fact, due to changes in the crystalline lens and pupil size, a typical 20 year old receives 3x the retinal luminance of a 60 year old[6].

Age-related losses in retinal illumination due to decreasing crystalline lens light transmission and pupil area. The percentage of loss per decade is reasonably uniform and most prominent at shorter violet (400–440 nm) and blue (440–500 nm) wavelengths​. Image: International Journal of Medical Science

And let’s not forget our children are getting an amplified dose of blue light intensity from viewing devices closer up.  So young eyes are receiving more blue light on the retina at a significantly greater intensity.

The question that remains to be answered is whether or not that increased exposure will lead to retinal damage over time?  

In part three of the series, we’ll discuss the density of a child’s crystalline lens, and how that can put them at greater risk of blue light exposure.

Read Part 3 of the Series

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.

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1. Loewenfeld IE: “Simple, central” anisocoria: A common condition, seldom recognized. Trans Am Acad Ophthalmol, 1977; 82:832–839.
2. Source: http://vspblog.com/blue-light-infographic/
3. Richdale, K. Ocular and Refractive Considerations for the Aging Eye, Contact Lens Spectrum, February 2009
4. Benjamin W, Borish I. Borish's clinical refraction. Philadelphia: W.B. Saunders; 2006.
5. Bonmati-Carrion M et al. Protecting the Melatonin Rhythm through Circadian Healthy Light Exposure. Int. J. Mol. Sci. 2014, 15(12), 23448-23500 
6. Lighting Research Center, Rensselaer Polytechnic Institue, Troy NY.  http://www.lrc.rpi.edu/programs/lightHealth/AARP/healthcare/lightingOlderAdults/agingEye.asp