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Monday, June 10, 2019

Blue light and your retina

Blue-blockers that don't block blue light? What is the world coming to? New discoveries show why we might need them

T here is a lot of confusion about blue light these days. Is blue light harmful to your retina? Does it interfere with sleep? Are blue-blocker sunglasses worthwhile? In this article I will discuss how new scientific discoveries about the retina may affect your decision.

Light sources

Understanding lamp spectra is important because there are different types of blue-blockers for different purposes. Selecting the wrong one could give you a false sense of protection.

The three main sources of light we're exposed to (fluorescent, LED, and sunlight) all have different characteristics. In the table below, nm = nanometers.

Wavelength  Fluorescent   LED      Sunlight   Hazard 
Short ultraviolet (UV-C, 200–280 nm) None None TraceHigh
Medium ultraviolet (UV-B, 280–315 nm)None None SomeModerate
Near ultraviolet (UV-A, 315–380 nm) Trace None LotsSlight
Blue (420–490 nm) Lots Lots Lots?
Near infrared (780–2500 nm) Trace* None LotsLow

* Infrared is emitted by fluorescents only near the filaments
If viewing on a cell phone, drag table left or right to scroll.

There are special types of lamps that can emit in the UV or infrared. So-called “black light” fluorescents are designed to emit light at 365 nm. Germicidal UV lamps emit mostly at 254 nm. These lamps are made of special glass such as quartz that allows the UV to pass. Common LEDs do not produce UV, though specialized, expensive UV LEDs are commercially available. We won't discuss these. Only small amounts of UV escape from regular fluorescent lamps.

Spectrogram of CF and white LED
Spectrogram of compact fluorescent bulb (top) and white LED (bottom). CF bulbs emit light in sharp bands, while LEDs emit a broader range of light. Note the gap between blue and green. This wavelength, around 480 nm, is the peak wavelength for intrinsically photosensitive retinal ganglion cells (ipRGCs), which control pupillary contraction (see below).

White LEDs tend to become bluer with age as the yellow phosphor wears out.[1]

Spectrogram of sun
Spectrogram of light from the sun. Light from the sun is a continuum. The large dark vertical lines in the near infrared (at right) are caused by absorption by water vapor in Earth's atmosphere. The other vertical dark lines are caused by absorption by elements in the sun (for example, iron has many absorption lines in the green). This spectrum was taken with a modified camera that is sensitive to near-infrared light.

Types of blue blockers

There are two main types of blue blockers: untinted polycarbonate and yellow tinted plastic (usually polycarbonate or acrylic). Ordinary polycarbonate strongly absorbs light below 400 nm, which is to say ultraviolet light. Since most people cannot see light at wavelengths below 400 nm, these untinted lenses appear colorless. They are ineffective at blocking blue light, so they have little value for LED sources such as cell phones, which don't emit UV light.

Blue blockers
Both types of blue blockers block ultraviolet light. The glasses are on a piece of paper containing fluorescent dye and illuminated with a 365 nanometer black-light source.
Blue blocker comparison
Comparison of ability of blue blockers to block display of a blue LED clock. Top = tinted, bottom = untinted

Until recently, LCD monitors and TVs were illuminated by small microfluorescent bulbs along the top of the screen instead of LEDs. These emit traces of UV light, most of which is absorbed in its journey through the LCD polymer and the polarized plastic of the screen. Thus, the only benefit of untinted polycarbonate glasses indoors is to make the wearer look smarter. However, they would be very useful outdoors, as they'd block any harmful ultraviolet light from sunlight.

Since plastic prescription eyeglasses are also made of polycarbonate, the untinted blue-blockers would be useless to anyone wearing regular glasses, indoors or outdoors.

The figure at right shows how both untinted and tinted blue-blockers block UV light. I placed them on a sheet of paper (which contains a fluorescent dye) and illuminated them with a black-light 365 nm UV lamp. Both types produced a dark shadow, which means the UV was blocked.

Babies are able to see light as short as 385 nm. As a person ages, the lens gradually turns yellow, blocking light between 385 and 400 nm as well as a portion of the blue light. Some people speculate that this lack of blue light may contribute to the sleep difficulties often seen in elderly people. So, untinted blue blockers might be a good idea for a baby exposed to the sun, but less so for an older person. Persons without a lens can see UV light as short as 300 nm and would need to take special precautions.

How harmful is blue light?

Any LED intense enough to cause a dazzling effect will harm your retina over time. But blue photons possess higher energy and cause oxidative damage at lower intensities than other colors. It's hypothesized that this toxicity is mediated by production of a lipid pigment called lipofuscin, and that lutein (a vitamin that's available in health food stores) may protect. The most dangerous wavelengths are 410–510 nm. Damage is cumulative over a few hours. It is still controversial as to whether blue light can cause age-related macular degeneration (AMD).

The human eye contains three types of photosensitive cells:

 Cell  Function   Peak wavelength(s) 
Rods Night vision 498
Cones Color vision 419, 530, 561
Photoganglion cellsPupillary contraction 480 nm

If viewing on a cell phone, drag table left or right to scroll.

Rods are 100× more sensitive than cones, and maximally sensitive for blue-green light (498 nm). Although usually called red, green, and blue cones, cones are actually sensitive to violet (380–450 nm), yellow-green (470–590 nm), and yellow (490–630 nm). The red, green, and blue we think of as primary colors are produced by neurons adding and subtracting the signals from our cones. It is thought that 2–3% of women have a fourth cone with a peak sensitivity between the red and green cones.

A newly discovered type of cell, called intrinsically photosensitive retinal ganglion cells or ipRGCs, does not send a signal to the brain but controls pupillary dilation and, with some help from the rod/cone system, our circadian rhythm.[1] These cells use melanopsin instead of rhodopsin or the human opsins we're familiar with. They're maximally sensitive to 480 nm light, which is in the LED gap between green and blue, so a person reading an LED or backlit LCD screen has a wider pupil, and their retina gets exposed to more light, than it would get under an incandescent light or sunlight.

Recent studies suggest that the exposure from LED screens is comparable to the weighted radiance of a blue sky (3.4 to 10.4 W·m−2sr−1, depending on the season). O'Hagan et al. reported that at their maximum brightness, screens from computers, laptops, tablets, and smartphones were all below 0.4% of the International Commission on Non-Ionizing Radiation Protection (ICNIRP) blue light exposure limit, suggesting low hazard.[2]

By contrast, regular LED screw-type light bulbs and blue LEDs in a KVM computer display switch, viewed from 100 mm away, exceeded hazardous levels. So it's recommended not to stare at LED light bulbs or electronic gizmos for long periods.

The authors noted that staring at the blue sky in the UK is not hazardous at all. Blue light may be useful in treating seasonal affective disorder.

Some other studies in rats[3],[4] claim that the blue component of white LEDs causes retinal toxicity even at domestic levels. Untinted polycarbonate glasses would have no benefit here because they don't block the relevant wavelengths. More studies are needed before we can all start suing everybody else for making us buy these LEDs.

Blue light and sleep

Blue light from LEDs may interfere with the circadian rhythm, as we have evolved to achieve maximal wakefulness under a blue sky. Coupled with the relative absence of a pupillary contraction from white LEDs, it would make sense to wear orange-tinted blue-blockers when reading from a phone or computer at night in order to prevent sleep disruption. The disadvantage is if the computer screen contains an image of a person who is already a bit orange, the blue-blockers could make him appear even oranger.

A randomized double-blind crossover placebo-controlled study showed that using cell phones with blue light for 150 minutes reduced sleepiness and melatonin levels compared to cell phones with blue turned off, but the effect was small.[5]

Untinted polycarbonate blue-blockers would be useless at preventing sleep disruption, since they don't block any of the light from LED or backlit LCD screens.

What use is a blue-blocker that doesn't block blue light?

Shortwave UV-C from high-altitude sunlight or arc welders causes painful corneal burns, which are usually temporary. Although UV-C has trouble passing through the cornea it is a risk factor for AMD. Over time it can also cause cataracts. UV-B also causes sunburn, while UV-A causes tanning and promotes formation of vitamin D in the skin.

Any polycarbonate lens will block all three forms of ultraviolet light. But if it doesn't give objects an orange tint, it won't protect you from blue light. That's simple physics.

The least energetic ultraviolet photons (400–410 nm) are about 3 electron volts (eV). Below this energy level, no matter how many photons you get, you can't directly break most chemical bonds (I'm simplifying a bit here). At energies above 4 eV, you can damage biomolecules such as DNA. Even though visible light doesn't cause ionization, it causes photochemical reactions. UV-A and blue light can produce oxygen free radicals[6] which can react with DNA and interfere with biological processes in ways we don't fully understand.

A good solution would be for manufacturers to increase the emission of white LEDs at 480 nm to induce pupillary constriction, as Tosini suggested.[1] Personally, if I didn't wear prescription lenses, I'd wear untinted blue-blockers outdoors. They might not block any blue, but they block ultraviolet, and they'll protect your eyes from flying debris, which is a much bigger danger. And they're a lot more fashionable than those PVC safety goggles.

1. Tosini G, Ferguson I, Tsubota K. (2016). Effects of blue light on the circadian system and eye physiology. Mol Vis. 22, 61–72. Link

2. O'Hagan JB, Khazova M, Price LL (2016). Low-energy light bulbs, computers, tablets and the blue light hazard. Eye (Lond). 30(2), 230–233. doi: 10.1038/eye.2015.261. Link

3. Krigel A, Berdugo M, Picard E, Levy-Boukris R, Jaadane I, Jonet L, Dernigoghossian M, Andrieu-Soler C, Torriglia A, Behar-Cohen F. (2016). Light-induced retinal damage using different light sources, protocols and rat strains reveals LED phototoxicity. Neuroscience 339, 296–307. doi: 10.1016/j.neuroscience.2016.10.015. Abstract

4. Shang YM, Wang GS, Sliney D, Yang CH, Lee LL (2014). White light-emitting diodes (LEDs) at domestic lighting levels and retinal injury in a rat model. Environ Health Perspect. 122(3), 269–276. doi: 10.1289/ehp.1307294. Abstract

5. Heo JY, Kim K, Fava M, Mischoulon D, Papakostas GI, Kim MJ, Kim DJ, Chang KJ, Oh Y, Yu BH, Jeon HJ (2017). Effects of smartphone use with and without blue light at night in healthy adults: A randomized, double-blind, cross-over, placebo-controlled comparison. J Psychiatr Res. 87, 61–70. doi: 10.1016/j.jpsychires.2016.12.010. Abstract

6. Godley BF, Shamsi FA, Liang FQ, Jarrett SG, Davies S, Boulton M (2005). Blue light induces mitochondrial DNA damage and free radical production in epithelial cells. J Biol Chem. 280(22), 21061–21066. Link

jun 10 2019, 4:41 pm. last edited jun 14 2019, 5:17 am

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