People may risk serious eye damage if they look at the partial phases of the upcoming total solar eclipse without proper protection.

The “only time it’s safe to look directly at the event” is during “totality,” when the sun is blocked in its entirety by the moon, Dr. Ralph Chou, an eclipse watcher and professor emeritus at the School of Optometry & Vision Science at the University of Waterloo in Ontario, told Fox News.

Not everyone will be able to see the total solar eclipse, though many people may be able to see a partial eclipse.

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Chou emphasized that if you’re in an area where only a partial eclipse can be viewed, there’s never a safe time to look straight-on at the sun.

Eclipse blindness

Eclipse blindness “describes the injury that occurs when a person looks at the sun without a protective filter during the eclipse,” Chou said.

If someone watches the sun without protection, light hits the fovea -- a part of the retina which helps humans with fine detail and color vision -- and “overwhelms with the sheer volume of light coming in,” he said. Chou likened the intense amount of light hitting photoreceptors to ocular “force-feeding.”

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“Structures that respond to the light break down,” and they begin to release oxidizing chemicals like peroxide “that will attack what’s left of the cell,” in what’s known as a photochemical injury, Chou said. If enough damage is accumulated, the cell’s function will be impaired and it may eventually die.

There can also be serious cases which result in thermal injuries. When there's so much light in the eye, some of the light is absorbed by a layer of cells behind the retina, Chou explained. If that layer is “hit by a huge volume of light,” pigment there reacts by absorbing the light and turning it into heat, which “can start cooking cells in back of the eye,” he said.

Noticing what’s wrong

People may not immediately figure out something is wrong because there’s a delayed response period of about 12 hours -- and there are no pain receptors in the back of the eye which could indicate there’s a problem, according to Chou.

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For example, Chou said that if someone is out around noon, it’ll take until about midnight for the retina to realize that it’s dying. Someone could wake up in the morning and have trouble looking at their reflection, a newspaper, or a smartphone screen.

Path to recovery

“Most people who have this problem recover their vision to what it was before the injury” in weeks or months, with extreme cases taking about a year, Chou said.

An “unlucky few” will have permanent problems, where they are not entirely blind, but they may have blindness or blurriness with their central vision.

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“You could lose your driver’s license” or be considered legally blind, he said.

Think you’re suffering from eclipse blindness?

Get checked immediately, Chou advises. Though there isn’t much that can be done with treatment, an “eyecare provider will be able to at least give advice on how to deal with it, such as lifestyle changes during recovery.”