Pupillary reflex and full-spectrum lighting: How does it protect the eyes?

Balance is not only a great characteristic in humans but also in electric lighting. There is another case for electric lighting to emit sufficient light in the cyan spectral region during the day. It is absolutely essential during the day to fix and synchronise our circadian rhythm, but also to regulate the size of the pupil, which is related to the negative effects of harmful blue light on the retina of the eye. This has been described in a number of new studies and we will briefly present the practical conclusions here.

Dilated or narrowed pupils?

If there is little light, the pupils dilate, otherwise they constrict, protecting the retina from damage. This is the pupillary reflex, which not only controls the cones and rods, but also the intrinsically photosensitive Retinal Ganglion Cells (ipRGCs). The same cells also mediate the so-called non-image forming biological effects of light on our organism. If the eye detects light with sufficient emission in the cyan spectral region, to which ganglion cells are most sensitive, the pupil is constricted and the amount of light reaching the retina is naturally reduced. [1], [2], [3].

It’s not just the amount of light that makes the difference

It is general knowledge that the greater the light intensity, the more the pupils constrict. However, many studies also focus on the effect of the wavelength of the incoming light on pupil constriction. A 2018 study [4] found that cyan light with a wavelength of approximately 480 nm caused the fastest pupil constriction compared to blue light (437 nm) and red light (627 nm). In an experiment in which subjects were exposed for 5 minutes successively to 9 monochromatic lights of different wavelengths from 420 (blue-violet) to 500 nm (cyan), it was found that the fastest pupil constriction occurred at 480 nm, while the slowest occurred at 430 nm. Maximum pupil constriction was achieved with light stimulation at 470 nm, and the greatest transient and sustained pupil constriction responses were achieved at 460 and 490 nm, also in the cyan region.

Harmful blue light

If the light hitting the retina contains intense radiation in the so-called harmful blue light (HBL) region in the range from 415-455 nm, the risk of retinal cell damage increases, which can also contribute to the development of eye diseases such as age-related macular degeneration (AMD). The blue light in this region most strongly stimulates the production of reactive oxygen species, the so-called oxidative stress, which can damage the mitochondria in Retinal Pigment Epithelium (RPE). [5].

The key is light with a balanced spectral waveform

These findings suggest that the use of electric light emitting light in the blue spectral region from 415-445 nm with a dip in the cyan region increases the risk of damage to ocular structures. Sufficient cyan radiation then leads to a faster and greater pupil constriction, which reduces the amount of blue light reaching the retina and thus the amount of oxidative stress that creates harmful blue light (HBL). The amount of blue light is partially offset by the presence of the red spectral component through photobiomodulation effects [6], [7].

Based on these findings, it can be concluded that a balanced spectrum in the visible light range during the day is not only absolutely crucial for the regulation of circadian rhythms, but also for the health of our eyes.

Mgr. Tereza Ulrichová, Mgr. Martina Kemrová, Spectrasol

References:

[1] L. A. Ostrin, ‘The ip RGC-driven pupil response with light exposure and refractive error in children’, Ophthalmic Physiologic Optic, vol. 38, no. 5, pp. 503-515, Sept. 2018, doi: 10.1111/opo.12583.

[2] D. M. Graham, ‘Melanopsin-expressing, Intrinsically Photosensitive Retinal Ganglion Cells (ipRGCs)’.

[3] A. J. Zele, P. Adhikari, D. Cao and B. Feigl, ‘Melanopsin and Cone Photoreceptor Inputs to the Afferent Pupil Light Response’, Front. Neurol., year 10, p. 529, May 2019, doi: 10.3389/fneur.2019.00529.

[4] M. A. Bonmati-Carrion et al, ‘Effect of Single and Combined Monochromatic Light on the Human Pupillary Light Response’, Front. Neurol., year 9, p. 1019, November 2018, doi: 10.3389/fneur.2018.01019.

[5] M. Marie et al. ‘Light action spectrum on oxidative stress and mitochondrial damage in A2E-loaded retinal pigment epithelium cells’, Cell Death Dis, year 9, No. 3, p. 287, Feb. 2018, doi: 10.1038/s41419-018-0331-5.

[6] A. Françon, F. Behar-Cohen, and A. Torriglia, ‘The blue light hazard and its use on the evaluation of photochemical risk for domestic lighting. An in vivo study’, Environment International, year 184, p. 108471, February 2024, doi: 10.1016/j.envint.2024.108471.

[7] W. Chen, R. Lin, K. Xiao, K. Yuan, Z. Chen, and Y. Huang, ‘Effects of Different Spectrum of LEDs on Retinal Degeneration Through Regulating Endoplasmic Reticulum Stress’, Translational Vision Science & Technology, Year 12, No. 6, p. 16, Jun. 2023, doi: 10.1167/tvst.12.6.16.