On August 21, you'll be able to watch the moon cross in front of the sun, casting a shadow on the earth that will turn day into night along the narrow path of totality. You're probably aware that bright sunlight can hurt your eyes, and you probably wear sunglasses to block UV rays from solar glare on bright days or for winter sports. You may not be aware that staring directly at the sun to watch the progress of the moon poses a completely different hazard. It's not the ultraviolet (UV) radiation that you need to worry about - if you look directly at the sun, it's the intense visible light that will burn your retina and possibly permanently blind you!
Halo Eclipse Spectacles meet the rigorous ISO12312-2 standard and are one of several options for safely viewing the eclipse. Please don't take any chances and spend some time to learn about safe ways to watch the sun. You don't have any nerves in your retina, so you won't feel any pain to remind you that you are causing lasting damage! The full extent of the damage may not even be apparent until the following day, and you might end up with only a 50% chance of recovering 20/20 vision after six months of recovery!
According to Dr. Ralph Chou, the author of the ISO12312-2 global safety standard covering filters and sunglasses for the direct observation of the Sun, a minimum attenuation factor of 160,000 is required for comfortable viewing (i.e., visual transmittance < 0.0006% at solar noon). Even when 99% of the Sun's surface is obscured during the partial phases of a solar eclipse, the remaining crescent Sun is still intense enough to cause a retinal photochemical injury, even though illumination levels are comparable to twilight and it seems safe to look without a protective filter. Viewing the Sun through binoculars, a telescope, or other optical device without proper protective filters can result in immediate thermal retinal injury because of the high irradiance level in the magnified image.
The only time that the Sun can be viewed safely with the naked eye is during the brief total phase of a total eclipse, when the Moon completely covers the bright disk of the Sun. So wear your Halo Spectacles until the moment of totality, and then take them off and bask in the ethereal glow of the Sun's corona ringing the black silhouette of the Moon in a twilight-dark sky, with sunset colors surrounding you on the horizon.
Just remember to put your glasses back on when the moment has passed and the sun shines again around the other side of the moon!