Can one admire the splendid spectacle of a total eclipse without seriously impairing one's eyesight?
The answer is yes, and our article offers a detailed analysis of the risks associated with exposure to sunlight and how it is best to protect against them.
Eclipse: an infrequent spectacle
There is great anticipation in the United States for the total solar eclipse, scheduled for 8 April 2024, when the 'black sun' will also become visible from Mexico and Canada, but not from Europe.
Solar eclipses are an infrequent astral event, so much so that the last total eclipse visible from Italy was on 4 December 2021.
In fact, the solar eclipse scheduled for 4 April will be visible in partial form throughout the United States, while only 15 states, from Texas to Tennessee, will experience the great thrill of the sun's total dimming.
What not to do
Millions of eyes will, therefore, be raised to the sky, but great care must be taken because it may only take a few minutes of observation without adequate protectionand to leave unwary observers ocular problemswhich can last a lifetime.
Knowing and preventing risks
It is, therefore, necessary to promote knowledge and raise awareness of what risks are involved in observing a solar eclipse and, above all, to inform people about which devices are suitable for protecting their eyes and how they should be used correctly.
According to data from the American Astronomical Society, the sun is about half a million times brighter of the full moon and emits radiation in the infrared spectrumwhich can be seriously harmful to the eyes.
When gazing at the sun under normal conditions, it is instinctive to look away, due to the annoyance caused by the intense brightness, but when the sun is partially obscured, it becomes easier to stare directly at it.
What are the risks
The only safe time to watch aeclipse with the naked eye is with the sun completely obscureda phase that lasts about 2 minutes, very little indeed!
In addition,
in the phase when the moon passes in front of the sun and the light dims, the pupils of the eyes dilate to allow access to more light, increasing the potential for retinal damage, a kind of burn, known as 'actinic retinopathy' o 'solar retinopathy'which irreversibly damages cones and rods, the photosensitive cells that enable vision.
Even watching a subtotal eclipsewith a small slice of the sun visible, can create irreversible damage to eyesight.
Our eye and the sun
Recall that the dioptric media of the eye, our 'natural lenses' are all transparent structures, which allow light to pass through and image to form on the retina.
These structures include the vitreous humour, the cornea, the vitreous body and the crystalline lens. Our dioptric media have convergent power - that is, the ability to converge (or diverge) the direction of the rays of a light beam - 3-5 times more powerful than a magnifying glass.
How can one safely admire a solar eclipse?
Ordinary sunglasses are not a sufficient shield for observing an eclipse, since even the best quality lenses block about 50% of the sun's rays, which leads to a high risk of macular lesions that, moreover, may not be perceived immediately.
Recall that the macula, yellowish in colour, is the central portion of our retina, of which it is the most sensitive to light stimuli, since it consists of a single type of photoreceptor, the rods.
Eye damage
When one stares at the sun without adequate protection, one does not immediately feel pain or a decrease in visual acuity. Only later does one realise that one sees a small black spot in the centre of the visual field, which corresponds to the damage and to themacular oedema, which subsequently evolves into a full-fledged scarresulting in an area of atrophy.
Protection of 99.9% is absolutely necessary, not least because many people use binoculars or a telescope to admire eclipses, which themselves act as amplifiers of light intensity.
Useful tips
Solar eclipse lenses
A safe guard is solar eclipse lenseswhich can also be purchased online, but which must be certified and marked ISO 012312-2, the specific international safety standard for these devices. These lenses can also be mounted on cameras, binoculars and telescopes.
At the last total eclipse in the United States, which was in 2017 and was observed by 215 million people, only 72% of the viewers used eclipse lenses as they were not available to the 100% of observers, as would have been desirable.
Alternatively, NASA suggests a safe method to observe the eclipse indirectly: create a 5 mm hole on a cardboard, held 1 metre above a white background, turning your back to the sun.
Solar viewing projector
It is a technique that can be traced back to the 5th century B.C. and which Leornardo da Vinci also wrote about: 'When images of illuminated objects pass through a small hole in a very dark room... all these objects with their natural shapes and colours can be seen on paper'.
Solar Viewing Projector | Total Solar Eclipse 2017 (nasa.gov)
Sunlight
To better understand the risks involved in observing an eclipse, let us try to understand what sunlight looks like.
Sunlight is not white, but consists of a range of light radiations that form the so-called solar spectrum.
The spectrum consists of visible radiation (coloured rays) perceived by the eye, and non-visible radiation to the right (infrared rays) and left of the spectrum (ultraviolet-UV rays) respectively.
I UV radiation make up a total of approximately 8% of solar radiation, and are subdivided into UV-A for 75%, UV-B for 19% and UV-C for 6%.
- UV-A rays (320-400 nm) tan us and cause photosensitive reactions in many.
- UV-B radiation (290-320 nm) causes sunburn and is implicated in some forms of skin cancer.
- UV-C rays (200-290 nm) are by far the most dangerous and are absorbed by the ozone layer.
The blue light
The harmful effect of radiant energy depends on its wavelength and, therefore, the energy content of the radiation. The shorter wavelengths close to the ultraviolet are much more harmful; in fact, the high-energy region of the visible spectrum (400 to 500 nm) is considerably more dangerous than the low-energy zone (500 to 700 nm). Since the transition occurs at the boundary between the colours green and blue, the phenomenon is commonly called blue light damage.
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The eye and sunlight damage
The eye, due to its anatomical position and specific organic function, is continuously exposed to the stress caused by light. The eye tissues absorb light radiation differently, depending on its wavelength.
The amount of radiation absorbed by the eye also varies with age. In fact, the crystalline lens, which is the main protective factor against low-wavelength radiation, becomes less transparent with age and, therefore, more effective at absorbing radiation.
In children, the amount of harmful light reaching the retina is greater: suffice it to say that at the age of under 10, more than 75% of ultraviolet radiation (UV-A and UV-B) is transmitted to the retina, and at 25, this percentage is still 10%. The situation changes completely from the fifth decade of life onwards, when, as a result of the ageing process of the crystalline lens, the eye has a natural lens that absorbs more potentially harmful radiation.
- Cornea. The cornea acts as the first filter for light radiation and is, therefore, the first structure to be affected by excessive light exposure. Under these conditions, the risk of certain corneo-conjunctival diseases increases.
- Crystalline. With regard to the lens of the eye, research has long shown how UV radiation accumulated over an individual's lifetime contributes significantly to the development of age-related cataracts.
- Retina. In the case of light damage, the retina has a certain resilience, but if the intensity of radiation increases, the damage may become irreversible. The 'point of no return' is the destruction of the cell bodies of the cones and rods, the photoreceptors which, being nerve cells, are unable to regenerate. Retinal diseases for which UV radiation and blue light have been shown to be involved in the induction or progression of retinal damage are: cystoid macular oedema, solar retinopathy, ocular melanomas and, above all, age-related macular degeneration, which is one of the main causes of blindness in Western countries.
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The eye's natural defences
When light is too intense, the eye uses natural protective systems, such as eyelid blinking, pupil changes, the screen function of the crystalline lens and, above all, the ocular pigments of the retina, lutein, zeaxanthin and melaninwhich filter light, protecting nerve cells from phototoxic damage. In particular, lutein is able to filter blue light and thus protect the most delicate parts of the retina from the harmful effects of light radiation. These pigments are not synthesised naturally by the body, but are taken in through diet. When the latter, as is often the case, is not adequate, one can resort to the use of supplements.
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The child's eye
The perfect transparency of the cornea and crystalline lens of the child's eye allows solar radiation to reach the retina more easily. For this and many other reasons, the child's eye is even more vulnerable to sunlight than the adult's and must be protected.
Studies conducted on ocular malignant melanomas It was found that strong sunlight in the early years of life is an important risk factor for the occurrence of this type of eye cancer.
Finally, it cannot be excluded that the age-related macular degeneration may be an expression in later life of damage produced in early life due to excessive UV-B exposure.
The next eclipses
- In Europe : The next total solar eclipse visible from Europe, after the one on 11 August 1999, is scheduled for 12 August 2026 .
The 'black sun' will be visible from Spain, while in western Italy the dimming may reach 90%.
- In Italy there are three dates to mark in the calendar:
- 29 March 2025
- 12 August 2026
- 2 August 2027.
In all three cases, the solar eclipses will be partial, but with coverage of the Sun's disc by the Moon exceeding 90% in 2026 and 2027.
- Stargazers gravitate to Planetarium for solar eclipse talk. EurekAlert! Accessed February 27, 2024. https://www.eurekalert.org/news-releases/1035171
- Experts offer tips on keeping your eyes safe during the April eclipse. spectrumlocalnews.com. Accessed February 22, 2024. https://spectrumlocalnews.com/nys/central-ny/health/2024/01/18/experts-offer-tips-on-keeping-your-eyes-safe-during-the-april-eclipse
- Ramakrishnan A, News TDM. Why you shouldn't look at a solar eclipse without eye protection. phys.org. Accessed February 22, 2024.
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