What is a solar eclipse?
A solar eclipse is when the moon blocks any part of the sun.
Can a solar eclipse damage your eyes?
A solar eclipse can damage your eyes if safety measures are not employed. Even though the moon is passing in front of the sun, light from the sun is still shinning. If viewed with the naked eye, the high-intensity visible light triggers chemical reactions within the cells of the unprotected eyes, causing temporary or permanent loss of visual function.
Since the retina does not have pain receptors, improper viewing of the eclipse for a long period of time can also expose the eyes to thermal damage. This occurs when remaining solar radiation that is not absorbed by the eyes’ photoreceptors “cooks” the internal eye tissue, resulting in small blind areas.
How can I protect my eyes while observing a solar eclipse?
For the unaided eye (without the use of cameras, telescopes, binoculars…etc.):
To safely view a solar eclipse with the unaided eye, protective eclipse viewers must be used. These viewers come in different forms. Some are made to wear like glasses, while others are handheld. The viewers have either black polymer or aluminized polyester as their filter material. These filters reduce the intensity of sunlight to a safe level.
Viewers claiming to protect the eyes during solar eclipses should meet the transmittance requirements of ISO 12312-2. If the filter does not meet these requirements do not use them to view the eclipse.
For viewing a solar eclipse with optical devices such as cameras, binoculars, or telescopes:
A protective filter should not be used while observing the eclipse with an optical device (camera, binoculars, telescope…etc.) unless it was made specifically for the device.
In order to be safe, look for metal-coated 1 mil resin film for visual and photographic observation or, filters using metal-coated, optically flat glass for optical devices.
Regardless of the filter or viewer type, make sure they are not damaged. Any rip or tear can allow intense solar rays to penetrate your retina.
An alternative to using filters is indirect viewing, whereby a person is not facing the sun but uses different methods to project the eclipse. Below are tips to achieve indirect viewing:
- Make a pinhole in a card. With your back facing the sun and body facing a screen or white wall (must be a metre away from the wall), raise the card in the direction of the sun. The image of the eclipse will be projected on the wall as a shadow.
- Mount a pair of binoculars or a small telescope on a tripod, facing the sun. On the other side (the viewing side), hang a piece of paper on a wall to see the magnified projection of the eclipse. Make sure no one looks through the unfiltered device.
- Using a small mirror, project the image of the sun onto an adjacent shaded wall or more effectively projected through an open window into a darkened room.
Must filters or alternative methods be used during the entire eclipse?
No. Protective eyewear or indirect viewing does not have to be used throughout the entire solar eclipse. When the sun is completely covered by the moon (i.e. total eclipse) the eclipse is safe to watch with the naked eye. However, whenever the moon is not totally covering the sun (partial eclipse), filters or indirect viewing methods must be used.
Unaccredited vendors and unsafe materials:
Buyers must be careful when purchasing filters. Some vendors print the compliance labels on their products without testing them. As well, others test their products using a third-party tester who is not accredited and qualified to implement the proper testing procedure. If you are about to purchase filters and are unsure of the quality, speak with an optometrist prior to your purchase.
Additionally, individuals should not make their own filters or use the following materials as a substitute:
- “Black” developed colour film
- Exposed film negatives
- Sunglasses (single or multiple pairs)
- Photographic neutral-density filters
- Aluminium foil food wrappers
- Polarizing filters
- Smoked glass
- CDs or DVDs
- “Space blankets”
- Gardening films or similar products
- Filters designed to thread into, or fit over, eyepieces or telescopes
For more information on filters and indirect viewing speak with your local optometrist.