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I purchased a 10 pack of glasses for viewing the upcoming eclipse. They show to be ISO 12312-2:2015 certified. How can this be tested and verified?
The American Astronomical Society (AAS) answers this question on their How to Tell If Your Eclipse Glasses or Handheld Solar Viewers Are Safe page.
Basically they say that since anyone can claim ISO 12312-2 compliance even if they haven't been certified, you should make sure you obtained your eclipse glasses from a vendor whose ISO 12312-2 compliance has been verified by the AAS Solar Eclipse Task Force:
We used to say that you should look for evidence that they comply with the ISO 12312-2 international safety standard for filters for direct viewing of the Sun. But now the marketplace is being flooded by counterfeit eclipse glasses that are labeled as if they're ISO-compliant when in fact they are not. So now we suggest that you make sure you get (or got) your eclipse viewers from one of the suppliers listed on our Reputable Vendors of Solar Filters & Viewers page.
They say that checking eclipse glasses to see if they meet the ISO 12312-2 standard would require a spectrophotometer, which most people do not have access to:
Unfortunately, you can't check whether a filter meets the ISO standard yourself - doing so requires a specialized and expensive piece of laboratory equipment called a spectrophotometer that shines intense UV, visible, and IR light through the filter and measures how much gets through at each wavelength.
So they have assembled a task force to put together a list of manufacturers and vendors of eclipse glasses that they have verified to be ISO 12312-2 certified (their list is longer than NASA's list of 5 manufacturers):
The AAS Solar Eclipse Task Force has been working diligently to compile a list of such vendors, now posted on our Reputable Vendors of Solar Filters & Viewers page. We've checked manufacturers' ISO paperwork to make sure it's complete and that it comes from a recognized, accredited testing facility, and we've personally examined manufacturers' products. We've asked manufacturers to identify their authorized resellers, and we've asked dealers to identify the source of the products they're selling. Only when everything checks out do we add a vendor to our listing. If we don't list a supplier, that doesn't mean their products are unsafe - only that we have no knowledge of them or that we haven't convinced ourselves they are safe.
They also give advice on how to tell if your eclipse glasses are not safe to use:
How can you tell if your solar viewer is not safe? The only thing you can see through a safe solar filter from a reputable vendor is the Sun itself. If you can see ordinary household lights through your eclipse glasses or handheld viewer, it's no good. Safe solar filters produce a view of the Sun that is comfortably bright (like the full Moon), in focus, and surrounded by black sky. If you glance at the Sun through your solar filter and find it uncomfortably bright, out of focus, and surrounded by a murky haze, it's no good.
I purchased 100 pair of glasses from Walmart online from AllStar Gadgets to use at school with students. It says they are certified. How can you be sure?
AAS's Advice for Safe Solar-Eclipse Viewing
By: American Astronomical Society August 1, 2017 2
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In response to alarming reports of potentially unsafe eclipse viewers flooding the market as the coast-to-coast solar eclipse of August 21st draws near, the American Astronomical Society (AAS) has revised some of its safety advice to the public.
How can you tell if your “eclipse glasses” or handheld solar viewers are safe? It is no longer sufficient to look for the logo of the International Organization for Standardization (ISO) and a label indicating that the product meets the ISO 12312-2 international safety standard for filters for direct viewing of the Sun’s bright face. Why not? Because it now appears that some companies are printing the ISO logo and certification label on fake eclipse glasses and handheld solar viewers made with materials that do not block enough of the Sun’s ultraviolet, visible, and infrared radiation to make them truly safe. Some sellers are even displaying fake test results on their websites to support their bogus claim of compliance with the ISO safety standard.
Evan Zucker (left) Paul Deans / TravelQuest International (right)
Given this unfortunate situation, the only way you can be sure your solar viewer is safe is to verify that it comes from a reputable manufacturer or one of their authorized dealers. The AAS Solar Eclipse Task Force has been working diligently to compile a list of such vendors, now posted on its Reputable Vendors of Solar Filters & Viewers page. Task-force members have checked manufacturers’ ISO paperwork to make sure it is complete and that it comes from an accredited testing facility, and they’ve asked manufacturers to identify their authorized resellers and dealers to identify the source of the products they’re selling. Only when everything checks out does the AAS add a vendor to its listing.
“If we don’t list a supplier, that doesn’t mean their products are unsafe,” says AAS Press Officer and task-force representative Rick Fienberg. “It just means that we have no knowledge of them or that we haven’t convinced ourselves they’re safe.”
How can you tell if your solar viewer is NOT safe? The only thing you can see through a safe solar filter from a reputable vendor is the Sun itself. If you can see ordinary household lights through your eclipse glasses or handheld viewer, it’s no good. Safe solar filters produce a view of the Sun that is comfortably bright (like the full Moon), in focus, and surrounded by black sky. If you glance at the Sun through your solar filter and find it uncomfortably bright, out of focus, and surrounded by a murky haze, it’s no good. You should contact the seller and demand a refund or credit for return of the product, then obtain a replacement from one of the sources listed on the AAS’s reputable-vendors page.
What if you received eclipse glasses or a handheld solar viewer from a relative, friend, neighbor, or acquaintance? If that person is an amateur or professional astronomer -- and astronomers have been handing out eclipse viewers like Halloween candy lately -- they’re almost certainly ISO-compliant, because astronomers get their solar filters from sources they know and trust (in other words, from the ones listed on the AAS’s reputable-vendors page). Ditto for professional astronomical organizations, including college and university physics and astronomy departments, and amateur-astronomy clubs.
If you bought or were given eclipse viewers at a science museum or planetarium, or at an astronomy trade show, again you’re almost certainly in possession of ISO-compliant filters. As long as you can trace your filters to a reputable vendor or other reliable source, and as long as they have the ISO logo and a statement attesting to their ISO 12312-2 compliance, you should have nothing to worry about. What you absolutely should NOT do is search for eclipse glasses on the internet and buy whatever pops up in the ads or search results. Check the AAS list of reputable vendors and buy from one of them.
Rick Fienberg / TravelQuest International / Wilderness Travel
The AAS continues to emphasize that it is perfectly safe to look directly at the Sun during the brief total phase of the solar eclipse (“totality”), when the Moon entirely blocks the Sun’s bright face. On August 21st this will occur only within a roughly 70-mile-wide path spanning the country from Oregon to South Carolina, and only for up to 2 minutes 40 seconds. Before and after totality, or throughout the entire eclipse if you’re outside the path (in which case you’ll see only a partial eclipse, which is nowhere near as exciting or magnificent as a total one), the only safe way to look directly at the Sun is through special-purpose solar filters. These are commonly sold as paper- or plastic-framed eclipse glasses or cardboard solar viewers that you hold in your hand. Ordinary sunglasses, even very dark ones, are not safe for looking directly at the Sun they transmit many thousands of times too much sunlight.
* Always inspect your solar filter before use if scratched, punctured, torn, or otherwise damaged, discard it. Read and follow any instructions printed on or packaged with the filter.
* Always supervise children using solar filters.
* If you normally wear eyeglasses, keep them on. Put your eclipse glasses on over them, or hold your handheld viewer in front of them.
* Stand still and cover your eyes with your eclipse glasses or solar viewer before looking up at the bright Sun. After looking at the Sun, turn away and remove your filter -- do not remove it while looking at the Sun.
* Do not look at the uneclipsed or partially eclipsed Sun through an unfiltered camera, telescope, binoculars, or other optical device.
* Similarly, do not look at the Sun through a camera, a telescope, binoculars, or any other optical device while using your eclipse glasses or handheld solar viewer -- the concentrated solar rays could damage the filter and enter your eye(s), causing serious injury.
* Seek expert advice from an astronomer before using a solar filter with a camera, telescope, binoculars, or any other optical device note that solar filters must be attached to the front of any telescope, binoculars, camera lens, or other optics.
* If you are inside the path of totality, remove your solar filter only when the Moon completely covers the Sun’s bright face and it suddenly gets quite dark. Experience totality, then, as soon as the bright Sun begins to reappear, replace your solar viewer to look at the remaining partial phases.
* Outside the path of totality, you must always use a safe solar filter to view the Sun directly.
Some eclipse glasses and solar viewers are printed with warnings stating that you shouldn’t look through them for more than 3 minutes at a time and that you should discard them if they are more than 3 years old. Such warnings are outdated and do not apply to eclipse viewers compliant with the ISO 12312-2 international safety standard, which was adopted in 2015. If your eclipse glasses or viewers are relatively new and are ISO 12312-2 compliant, you may look at the uneclipsed or partially eclipsed Sun through them for as long as you wish. Furthermore, if the filters aren’t scratched, punctured, or torn, you may reuse them indefinitely.
What about welding filters? The only ones that are safe for direct viewing of the Sun with your eyes are those of Shade 12, 13, or 14. These are much darker than the filters used for most kinds of welding. If you have an old welder’s helmet around the house and are thinking of using it to view the Sun, make sure you know the filter’s shade number. If it’s less than 12 (and it probably is), don’t even think about using it to look at the Sun. Many people find the Sun too bright even in a Shade 12 filter, and some find the Sun too dim in a Shade 14 filter -- but Shade 13 filters are uncommon and can be hard to find. The AAS’s Reputable Vendors of Solar Filters & Viewers page doesn’t list any suppliers of welder’s filters, only suppliers of special-purpose filters made for viewing the Sun.
An alternative method for safe viewing of the partially eclipsed Sun is indirectly via pinhole projection. For example, cross the outstretched, slightly open fingers of one hand over the outstretched, slightly open fingers of the other, creating a waffle pattern. With your back to the Sun, look at your hands’ shadow on the ground. The little spaces between your fingers will project a grid of small images on the ground, showing the Sun as a crescent during the partial phases of the eclipse. Or just look at the shadow of a leafy tree during the partial eclipse you’ll see the ground dappled with crescent Suns projected by the tiny spaces between the leaves.
This article originally appeared as a press release from the American Astronomical Society.
Sky & Telescope is an authorized seller of eclipse glasses!
For more eclipse resources, visit our Total Solar Eclipse 2017 page or check out our FREE eBook on eclipse photography!
Small but Stellar
Madras, Ore. It is a largely agricultural community of 6,000 people in central Oregon that is attracting a huge amount of attention from eclipse chasers, given its reliably clear summer weather and position in the center of the path of totality. Hotels in Madras have been sold out for years. The Days Inn, in nearby Bend, Ore., was advertising standard rooms at $1,600 a night as of May 1. But officials in Madras say there are still plenty of campsites, and visitors will have a chance to take in live music, check out a superb aviation museum, hike at Smith Rock State Park, and sample brews at dozens of brew pubs in Bend.Image
Hopkinsville, Ky. A farm just outside Hopkinsville has the distinction of being considered the “point of greatest eclipse,” which means that from the center of the earth, the moon makes its closest path across the sun there, giving viewers the best view of the solar corona, which some say looks like a bright diamond ring. Brooke Jung, who is coordinating Hopkinsville’s eclipse marketing efforts, says the eclipse is the most exciting thing to happen to the town — which they are now branding “Eclipseville” — since the same date (Aug. 21) in 1955, when a dozen “little green men” allegedly landed nearby in a spaceship.
In addition to their annual Little Green Men Days festival, which commemorates the extraterrestrial occurrence that supposedly inspired the 1982 film “E.T. the Extra-Terrestrial,” eclipse chasers can drink moonshine at two bourbon distilleries, attend a Comic-Con event, and take in the Friday night bluegrass jam at the nearby Rosine Barn. Ms. Jung says that the county has closed schools for the day, ordered 100,000 pairs of solar eclipse glasses ($1 each), and has requested 85 members of the National Guard to direct traffic.
“It’s like us and the other towns in the center of the path of totality are preparing to host a Super Bowl,” she said.
How to test direct sun viewing glasses - Astronomy
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Public viewing set for rare transit of Mercury across the sun
The Astronomy Club of Asheville wants people to see a somewhat rare planetary event, when Mercury, 52 million miles away and a little bit larger than our own moon, will pass between the sun and the Earth on Monday, May 9. The club will host a special viewing event at UNC Asheville, using special equipment.
“This is not a wow event, but it’s a unique event, and observing it will lead to greater understanding with what’s going on in our own solar system,” said Dominic Lesnar, president of the astronomy club.
The transit occurs from 7:13 a.m. to 2:42 p.m., and visitors will be able to see the event through a handful of telescopes and binoculars fitted with special filters that allow viewing of the sun’s atmosphere.
Never look at the sun directly without protection, it will damage your eyesight.
Lesnar says the event can be thought of as a very partial eclipse of the sun. “[Mercury] will be shadowing about 1/160 of the sun’s light, so it’s pretty innocuous. You’ll see a beautifully circular shape travel across the sun’s surface, and some sun spots may actually appear to be larger than Mercury,” Lesnar said.
The next transit of Mercury visible in the United States will be in 2019, and after that, 2049.
“This only happens about 13 times per century worldwide, so it’s pretty rare,” Lesnar said. The astronomy club will be answering questions and providing information about our solar system and beyond. “Looking up and being able to observe events like this are great opportunities for education,” he said.
Observers are welcome to drop in any time between 7 a.m. and 3 p.m., on the top level of the Kimmel Arena parking deck on the UNC Asheville campus, where the viewing will be hosted. Visitors may park in the visitor parking spaces on the lower level of the deck, or in lots P11, P12, or any other visitor lots on campus. The Astronomy Club of Asheville will offer solar shades for a donation of $2. No preregistration is required for the free viewing.
The viewing of the transit of Mercury is weather-dependent. A go/no-go determination for the event will be made early in the morning May 9, and the status will be posted at lookoutobservatory.unca.edu and astroasheville.org.
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Astronomy… In the Park… During the Day
For the last week of our Depot Days programs in the park, I wanted to do some astronomy related things. This gave me an excellent opportunity to distribute Moore Foundation eclipse glasses to those in attendance, with instructions for proper use. But these programs took place at 10:30 am, outside. So, how to address the eclipse and other astronomy topics with a large group of elementary-age and younger kids, with the distractions of the playground and frequently passing trains? On Tuesday, it was sunny and perfect weather for outdoor events. Thursday was gloomy and threatening rain, but we still had a full house. The eclipse viewing glasses were a huge draw.
First, as usual, we started with some stories. We shared the books If You Decide to Go To the Moon, by Faith McNulty, and Interstellar Cinderella, by Deborah Underwood. We had lots of solar system, moon, and sun books available to read on-site as well.
Some of the sunny day activities included mapping out the relative distances of the planets in our solar system. Since the park has a large open space, this worked very well. Volunteers were selected to represent the sun and each of the planets, as well as some asteroids, random comets, and other celestial bodies. I have a tape measure which gives accurate relative distances, and I had each of the kids chosen to play the planets simply stand and hold their respective spots on the tape while we measured it out in the field. The tape measure case looks like the sun, and the solar system bodies line out to Neptune at the end of the tape. It can be purchased, among other places, through Amazon.com here: https://www.amazon.com/Educational-Insights-Solar-System-Mapping/dp/B0074A45KU
We talked about how an eclipse works, making use of a poster image of the sun and black construction paper cut in a circle the right size to cover all of the sun’s image except the corona. We talked about what the corona is, and why it’s visible during the total solar eclipse. We also talked about how the relatively tiny moon can cover the huge sun. To help understand the relative size/distances of the sun, moon, and Earth, I had everyone hold up a thumb at arm’s length from their faces, squint, and look at me past their thumbs. I asked whether they could see any of me, all of me, or just part of my head past their thumbs. We then repeated this exercise looking at a much closer neighbor. We repeated again with thumbs closer to eyes, and discussed the differences. It was actually fun for both the kids and the adults!
Since it was sunny on Tuesday, we used that day to create sundial clocks using paper plates, printed clock faces, and drinking straws. It was a relatively simple craft, but they had a lot of fun trying to figure out how to line their sundials up properly to get an accurate time reading. We also played a game of Solar System Bingo, using gummy snacks as bingo markers. I had a bunch of wrist bands and a few pins left from the NASA Orion test launch in December of 2014, and I handed those out as Bingo prizes. Anyone who didn’t want to make a sundial was welcome to play bingo instead… a few played bingo and then hurried back to create sundials also.
Additionally, we created Constellation Key Chains, utilizing circles of cardboard with holes punched in them to represent five of the best-known northern hemisphere constellations. Students were encouraged to draw the lines between the “stars” to help visualize the constellations. When held in the sunlight, the constellation is created by the shadows and light on a sidewalk or other surface. A flashlight can accomplish the same effect inside. At night, they can be used to help identify some of the constellations in the sky.
On rainy Thursday, we created sun corona art with black construction paper and chalk. I asked the kids to draw a large circle on their paper, then color around the outside of the circle only, representing the sun’s corona visible around the eclipsed sun.
Using the Yard Stick Eclipse demonstration from the Night Sky Network, we also added a flashlight to talk about how lunar and solar eclipses differ. https://nightsky.jpl.nasa.gov/download-view.cfm?Doc_ID=327
I had taken the library’s telescope with me to the park on Tuesday, but decided against getting it out at the end of the program due to the very large number of small kids, the fact that I didn’t have a solar filter with me, and the general rambunctiousness of the crowd. At the very end, when there were only a few older kids remaining, I set up the telescope under the shelter roof to allow them to look at the telescope itself and learn about how it works. We then looked at leaves on trees that are across the rail tracks and quite a long way away from the park to talk about how the magnification and mirrors work.
Finally, we handed out eclipse viewing glasses at the end of the programs both on Tuesday and Thursday and gave instructions for proper viewing in our 93% coverage location. Some who attended the program did so solely to obtain the glasses, but I had made it clear at the beginning of the program that glasses would only be handed out at the end of the program. I think we successfully covered a number of astronomical topics, and we shared viewing safety information and glasses for the eclipse as well, proving that you can do outdoor, day-time astronomy programs without the use of a telescope!
'A pity if people didn’t know': The astronomy enthusiast bringing rare solar eclipse to the heartlands
SINGAPORE: For the past three months, Mdm Gerardyn Brittos has been hard at work preparing for a rare annular solar eclipse on Boxing Day.
The 46-year-old homemaker and her husband have been working with the Residents Committees to plan and organise a solar eclipse viewing for 300 people in Ang Mo Kio.
More recently, they have also been preparing 300 solar glasses by hand, to be distributed to the public on the day itself.
READ: How to catch the rare solar eclipse in Singapore on Boxing Day
This involves ordering the paper glasses, folding each of them by hand, and testing each pair under sunlight to ensure that it is in good condition, said Mdm Brittos, who also runs local astronomy Facebook page Stargazing Singapore.
And that’s not all. Mdm Brittos will also be bringing her own eight-inch telescope - one of the bigger types - which will be attached to a DSLR camera. People will be able to see the solar eclipse from the camera’s screen on that day.
“Our aim (of organising this event) is for the heartlanders, who can’t afford to travel out (to see the eclipse),” said Mdm Brittos, who plans to hold the event at the Kebun Baru Spring Amphitheatre.
It is also the first time she is organising a solar eclipse viewing after founding Stargazing Singapore in 2014.
But her interest in astronomy dates back far longer than that. She recalled once, more than twenty years ago, when her colleague brought his telescope to their office and invited everyone to have a look through its lens.
READ: Super blue blood moon - Thousands turn out across Singapore to catch special lunar eclipse
Mdm Brittos gazed into the telescope, and for the first time with her own eyes, saw Jupiter.
“(At the time) I couldn’t believe I could actually see a planet in a telescope,” she said.
She added that she immediately took leave the next day to buy an entry level telescope from the Science Centre, priced at S$600.
Mdm Brittos will be bringing her own telescope for the public to view the eclipse as well. (Photo: Gerardyn Brittos)
Now, armed with more advanced equipment, Mdm Brittos hopes to share her interest in astronomy with the public.
“It’s really something rare, and it’s in the afternoon, so it would be good if people can come out and take a look … It would be a pity if people didn’t know about it,” she said.
ANNULAR SOLAR ECLIPSE
While annular solar eclipses happen once every one to two years around the world, they are a “once in a lifetime” event in Singapore, said the Science Centre.
This is because there will only be three annular solar eclipses visible from Singapore in the 400 years between 1700 and 2100. During the same period, there will be 143 other partial solar eclipses visible from Singapore.
The next one will be a partial eclipse on Jun 21, 2020, while the next annular solar eclipse will be on Feb 28, 2063.
For the uninitiated, an eclipse occurs when an astronomical body such as the moon or a planet moves into the shadow of another such body.
According to the National Aeronautics and Space Administration (NASA), there are three types of solar eclipses: a total solar eclipse, a partial solar eclipse and an annular solar eclipse.
What characterises the annular solar eclipse is the “ring of fire” around the moon when it moves into the Sun’s centre. This is because the moon is at its farthest point from Earth, too small to completely cover the Sun, which leaves the outer edges of the Sun visible.
"What makes them rare is the fact that they are only visible from within a narrow path across the Earth, making it difficult to get to a location to see one," said a Science Centre spokesperson.
File photo of an annular solar eclipse in 2005. (Photo: Wikimedia Commons / sancho_panza)
This year, the moon will cover about 94 per cent of the Sun, which will make it as dark as twilight, added the spokesperson.
A timeline provided by the Science Centre indicated that the eclipse will happen in stages, starting with a partial eclipse at 11.27am, an annular eclipse at 1.22pm and the maximum eclipse at 1.24pm. Immediately after, the annular eclipse will end, with the partial eclipse ending almost two hours later at 3.18pm.
The Science Centre added that viewers in the northern part of Singapore will only get to see a partial solar eclipse, instead of the full annular solar eclipse.
Other than Singapore, residents in parts of Saudi Arabia, India, Sri Lanka, Indonesia and Guam will be able to see the annular solar eclipse, said the Science Centre.
It also reminded viewers that protection is required to watch the solar eclipse, as the Sun could cause permanent damage to the eyes, such as blindness.
Some safe methods to view the solar eclipse include using ISO-certified safe solar glasses, pinhole projections or solar viewers to watch the eclipse.
The use of sunglasses, tinted glasses, photographic filters, or even looking at the reflection in a bowl of water or mirror is considered unsafe.
For those interested in viewing the eclipse, there are several places to go.
From 11am on Dec 26, the Science Centre will have their Eclipse Viewing Event, with about 2,000 people expected to attend. There will be an admission fee, and solar glasses will be available for purchase at the Curiosity Shop.
Local astronomy groups will also be holding their own separate viewings, with most of them providing glasses and telescopes, at the following locations:
How Corrective Lenses Work
One of the most common sights almost anywhere in the world is -- eyeglasses! Since we depend so much on the lenses inside those frames to improve our view of the world, you might wonder just what goes into creating them.
In this article, we will talk about how the eye focuses, how a lens works, how to read a prescription, and finally, how the lens is made, including the steps involved in grinding and shaping plastic lens blanks to fit an individual's prescription and frame.
Let's start with some vision basics.
Try this interactive activity from Discovery Channel -- test your sight and learn how vision works at the same time. Other interactive segments let you explore your body's systems and see how they help you move smoothly through your daily life.
On the back of your eye is a complex layer of cells known as the retina. The retina reacts to light and conveys that information to the brain. The brain, in turn, translates all that activity into an image. Because the eye is a sphere, the surface of the retina is curved.
When you look at something, three things must happen:
- The image must be reduced in size to fit onto the retina.
- The scattered light must come together -- that is, it must focus -- at the surface of the retina.
- The image must be curved to match the curve of the retina.
To do all that, the eye has a lens between the retina and the pupil (the "peep hole" in the center of your eye that allows light into the back of the eye) and a transparent covering, or cornea (the front window). The lens, which would be classified a "plus" lens because it is thickest toward the center, and the cornea work together to focus the image onto the retina. (For more information on how the eye functions, see How Vision Works.)
- Aberrations: ghost images, halos, waves or rainbows caused by imperfections in the curve or lens surface
- Index of refraction: a ratio used to compare refractive power
- Plus lens (+): a lens that is thickest at the center moves the focal point forward
- Minus lens (-): a lens that is thinnest at the center moves the focal point backward
- Focal point: a spot in space where refracted light meets may be actual (plus lens) or assumed (minus lens)
- Pupillary center: the point on a lens directly in front of the pupil
- Astigmatism: a condition caused by a distortion in the cornea that creates an additional lens power
Sometimes, for different reasons, the eye doesn't focus quite right:
- The surfaces of the lens or cornea may not be smooth, causing an aberration that results in a streak of distortion called astigmatism.
- The lens may not be able to change its curve to properly match the image (called accommodation).
- The cornea may not be shaped properly, resulting in blurred vision.
Most vision problems occur when the eye cannot focus the image onto the retina. Here are a few of the most common problems:
- Myopia (nearsightedness) occurs when a distant object looks blurred because the image comes into focus before it reaches the retina. Myopia can be corrected with a minus lens, which moves the focus farther back.
- Hyperopia (farsightedness) occurs when a close object looks blurred because the image doesn't come into focus before it gets to the retina. Hyperopia, which can also occur as we age, can be corrected with a plus lens. Bifocal lenses, which have a small plus segment, can help a farsighted person read or do close work, such as sewing.
- Astigmatism is caused by a distortion that results in a second focal point. It can be corrected with a cylinder curve.
In addition, lenses can be made to correct for double vision when the eyes do not work together ("crossed eyes"). The lenses do this by moving the image to match the wayward eye.
Corrective lenses, then, are prescribed to correct for aberrations, to adjust the focal point onto the retina or to compensate for other abnormalities. You can read more about vision problems in How Refractive Vision Problems Work.
The best way to understand the behavior of light through a curved lens is to relate it to a prism. A prism is thicker at one end, and light passing through it is bent (refracted) toward the thickest portion. See the diagram below.
A lens can be thought of as two rounded prisms joined together. Light passing through the lens is always bent toward the thickest part of the prisms. To make a minus lens (above on the left), the thickest part, the base, of the prisms is on the outer edges and the thinnest part, the apex, is in the middle. This spreads the light away from the center of the lens and moves the focal point forward. The stronger the lens, the farther the focal point is from the lens.
To make a plus lens (above on the right), the thickest part of the lens is in the middle and the thinnest part on the outer edges. The light is bent toward the center and the focal point moves back. The stronger the lens, the closer the focal point is to the lens.
Placing the correct type and power of lens in front of the eye will adjust the focal point to compensate for the eye's inability to focus the image on the retina.
Determining Lens Strength
The strength of a lens is determined by the lens material and the angle of the curve that is ground into the lens. Lens strength is expressed as diopters (D), which indicates how much the light is bent. The higher the diopter, the stronger the lens. Also, a plus (+) or minus (-) sign before the diopter strength indicates the type of lens.
Plus and minus lenses can be combined, with the total lens type being the algebraic sum of the two. For example, a +2.00D lens added to a -5.00D lens yields:
Two basic lens shapes are commonly used in optometry: spherical and cylindrical.
- A spherical lens looks like a basketball cut in half. The curve is the same all over the surface of the lens.
- A cylindrical lens looks like a pipe cut lengthwise. The direction of a cylinder curve's spine (axis) defines its orientation. It will only bend light along that axis. Cylinder curves are commonly used to correct astigmatism, as the axis can be made to match the axis of the aberration on the cornea.
- Compound lens: a lens having both a spherical and a cylindrical component
- Cylindrical curve: a curve that radiates along a straight line, like a pipe cut lengthwise
- Diopter (D): the refractive power of a lens the higher the number, the stronger the lens
- Refraction: the bending of light
- Spherical curve: a curve that is the same in all directions, like a basketball cut in half
To make a lens, the first thing you need is a lens blank. Blanks are made in factories and shipped to individual labs to be made into eyeglasses. The raw lens material is poured into molds that form discs about 4 inches in diameter and between 1 and 1 1/2 inches thick. The bottom of the mold forms a spherical curve on the front face. A small segment with a stronger curve may be placed in the mold to form the segment for bifocals or progressive lenses.
How to Read the Prescription
Most prescriptions have four parts:
- The base (spherical) strength and type (plus or minus)
- The cylinder strength and type
- The cylinder axis orientation (in degrees with 90 degree vertical an "x" means "at")
- The strength of bifocal segment ("plus" indicating "in addition") and type
A short form prescription from the optometrist or ophthalmologist might read:
2.25 -1.50 x 127 plus +2.00
- A +2.25D spherical base curve (plus lens)
- A -1.50D cylinder at 127 degrees (a minus cylinder lens is added to the base curve)
- An additional bifocal segment of +2.00D
Total power of the lens with the cylinder is +2.25 + (-1.50) = +0.75D. At the segment, the power is (+0.75) + (+2.00) = +2.75D. And in case you've ever wondered, OD means right eye and OS, left eye.
- Base curve: a simple spherical curve the primary lens curve
- Lens blank: basic spherical lens the lab grinds the back of the blank to match the prescription
- Optical center: a spot on a spherical lens where light enters at a 90-degree angle to the lens plane
- Segment: the portion of a lens added for reading (bifocal or trifocal) it may be added separately to the lens blank or formed as a blended curve onto the base
Overview: How the Lens is Made
In the lab the patient's full prescription gives these exact details:
- The total power (in diopters) the finished lens must have.
- The strength and size of the segment (if needed).
- The power and orientation of any cylinder curves.
- Details such as the location of the optical center and any induced prism that may be needed.
The lab technician selects a lens blank that has the correct segment (called an add) and a base curve that is close to the prescribed power. Then to make the power match the prescription exactly, another curve is ground on the back of the lens blank.
- In most labs the equipment is designed to grind minus curves, so a strong, plus lens blank is usually selected.
- If the base curve is too strong, then a minus curve is ground in the back of the lens, which reduces the total power of the lens.
For example, a very common lens blank is +6.00 diopters. If the prescription calls for a total of +2.00 diopters, a -4.00 diopter curve is ground on the back: (+6.00D) + (-4.00D) = +2.00D. (See the illustration below.) If it is needed, the cylinder curve is also ground at the same time.
If the prescription calls for a minus lens, the +6.00 diopter lens blank can still be used. To create a lens with the strength of -2.00 diopters, a -8.00 diopter curve is ground on the back: (+6.00D) + (-8.00D) = -2.00D.
Steps 1 through 3
Corrective lenses can be made with glass or plastic, but nowadays, plastic is the most common. While several different types of plastic are used in making lenses, all of them follow the same general manufacturing procedures. Most of the steps outlined also apply to glass, although a few important differences are noted at the end.
A lab, even an automated one, follows 12 steps to make prescription lenses:
Step 1: The technician chooses a lens blank of the desired material with the proper base curve and, if needed, add power.
Step 2: If the prescription calls for a cylinder, a line is marked on the front of the lens to define 180 degrees, and then another line is drawn that matches the axis of the second curve. If there is a segment, the segment edge is used as the 180 degree line. Often the optical center of the lens is made slightly above the segment edge, and the line is marked the appropriate distance. (Note: When there is no segment or induced prism, the lens may be left unmarked and the cylinder axis determined after the lens is ground.)
Step 3: Since the front of the lens will be left as is, it is covered by a special tape to protect it.
- Generator: a compound surface grinder used to grind curves in the surface of the lens
- Induced prism: a technique that moves the optical center away from the pupillary center
Steps 4 through 6
Step 4: Depending on the type of equipment, the lens must be prepared to fit onto the generator, which is commonly a compound surface grinder capable of grinding two curves at once.
A chuck receiver (called a block) is placed on the front of the lens over the protective tape. If there is a cylinder curve, the lens is oriented so the cylinder axis matches the cylinder sweep axis of the generator.
The center of the block will become the optical center of the lens. Depending on the equipment, the lens may be held in place by special adhesive pads, with a special alloy that "glues" the lens to the block or with plastic.
Step 5: The lens is inserted in the generator.
The lens might need other processing besides the compound curves produced by the generator, so the lens may also be tilted in the chuck. This tilt will offset the optical center (called induced prism) often used to allow thinner lenses or to accommodate special requirements of the prescription.
Step 6: The curves are set on the machine and the lens is generated (ground). This step may either be fully automated or operated by hand, where the operator manually sweeps the quill (grinding wheel) across the lens, gradually advancing the lens until the desired lens thickness is achieved. Lens thickness is determined by curve type (plus or minus), lens material (some plastics are tougher and may be ground thinner), or other considerations (safety glasses, for instance, are made thicker than lenses for everyday use). If the lens gets too hot during the operation it may warp or tear, so it is cooled by water, which also washes away the cut material (called scarf).
Steps 7 through 9
Step 7: The lens is taken off the generator and placed in a special sanding machine (called a cylinder machine) to remove any marks left by the generator. To do this, sandpaper is glued to a block with reversed, matching curves (a +2.00 base/+2.50 cylinder, for example, to match -2.00/-2.50 generated curves), and the lens and block are rubbed together. Meanwhile the lenses are kept cool and cleaned with water.
Following the sanding operation, the lenses are polished on an identical machine, except that felt polishing pads washed with polishing compound are used instead of sandpaper and water. When this step is completed, the lens is optically clear without visible scratches.
Step 8: The block is removed from the lens, and the lens is washed and inspected. Sometimes special coatings may be applied to the lens. At this point the lens blank has had additional curves ground in the back of the lens and it has been polished. However, the large diameter blank still has to be sized and shaped to fit into the frame selected by the patient. Several methods are used, depending on the equipment, but they are all based on the following description.
Step 9: The lens blank is shaped on a linear lathe (called an edger) using either a ceramic or diamond grinding wheel or stainless steel blades. The lens must again be prepared to accept a chuck, but since only the edge is being cut, a much gentler system is used. A small chuck receiver is placed where the geometrical center of the finished lens will be, and the lens is then oriented on the 180 axis. Usually, only an adhesive pad is needed to hold the receiver on the lens. The lens is chucked in the edger and held in place by a pressure pad that presses on the opposite side of the lens (like holding a very large coin between your thumb and forefinger at its center).
More on Sleep
For those who can’t—or won’t—unplug in the evening, several companies offer blue blockers, which are glasses that filter out the wavelengths in the blue part of the spectrum. A small 2014 study of Swiss teen boys in the Journal of Adolescent Health found that those who wore blue blockers while using a computer in the few hours before bedtime were significantly more sleepy than the boys who wore clear lenses.
That could be because the glasses “are trying to suppress the intensity of the tremendous amount of blue light emitted from computer screens,” Czeisler says. “The screen you see glowing would actually look like a floodlight” if the human eye were capable of perceiving those wavelengths of blue-enriched light, he says.
Still, he cautions that for blue blockers to offer substantial benefit, they need to block almost all blue light. In addition, he says more research needs to be done to prove that people who wear blue blockers actually fall asleep faster.
Big Things Coming in 2023 and 2024!
On August 21, 2017, millions of Americans witnessed the moon passing between the Earth and Sun, in a total eclipse. If you were not one of the lucky people to witness this awe-inspiring event, then you will want to make sure you are prepared to view future eclipses! In addition to a big annular solar eclipse in 2023, there will be a 2024 total solar eclipse that will cross Mexico, the central and eastern United States, and portions of Canada. It’s never too early to prepare for these rare events!