To see how limited central vision is, hold the back of your hand at arm's length and hold this magazine next to it with your other hand. If you look directly at the back of your hand, you won't be able to read the magazine. You can't read it because the concentration of nerves in the retina drops dramatically as we move away from the center. Within 10 degrees of this central focus division, your best acuity drops to 20/100 or worse. This isn't really a problem, but it means we must look directly at something to get our crispest, clearest view of it.
The central nerves, called cones, also see color, and to function properly they must have enough light. Humans only see three colors - red, green, and blue. When the brain sees a yellow object, it collects three different impressions from the color reporters and understand that the object is yellow.
Cones only see light in wavelengths from red to violet. Wavelengths shorter than violet, called ultraviolet, don't register. Likewise, wavelengths longer than red, called infrared, also don't register. Red to violet is a relatively good mix because these wavelengths are the only ones that penetrate the eye's cornea and get to the retina.
The cornea appears to be clear in the visible wavelengths, but it's opaque when seen through infrared. It doesn't matter that the sensors can't see infrared light because the light can't get to them. The cones also don't work well in low light levels.
Light levels below the equivalent of a full moon aren't enough to stimulate these sensors. Only the peripheral sensors, called rods, can detect such low levels. Rods are very sensitive but not very precise. So at night, you can see quite well - except in the center five degrees of your vision - and can detect motion. What colors are best seen at night? Most of us prefer either red or green for our console lights, but you should be able to see either color almost equally well. Green might have a slight edge, but it's not significant.
The eye also gives the brain important input for coordination. More important, other sensors in the body help "gyrostabilize" your eyes. To prove this importance, try this test. Look at this page and rapidly turn your head from side to side. You'll have no difficulty focusing on and reading these words because motion sensors in your ears and neck stabilize your vision. If you move this page up and down at anything more than a slow pace, you won't be able to read these words because your eyes cannot sense the information, send it to the brain, and have the input returned to the eyes fast enough to track the target.
One other function of the eye is to send some information directly to a small gland (called the "pineal") located between the brain's two hemispheres. The information from the eyes helps the pineal gland tell the brain's biological clock when it is day and when it's night. This important cue helps set our biological rhythm. Bright light tells our body clock that it is early in the day. Darkness tells the pineal gland it's time to put the body to rest. Looking at the equivalent of bright morning sunlight any time you want your body to think it's morning will help reset your biological clock. This is one of our best tools for combating "jet lag."
Getting in Focus
For you to see the best image, light must not only reach the retina, it must be focused. The eye has two lenses that focus light on the retina. The first and most powerful is the cornea. This highly curved lens has a focusing power of 50 diopters, which means it can focus incoming rays of light at a distance of 1/50th of a meter. The cornea is a fixed lens, meaning the body can't adjust it to achieve a fine focus. The eye's focusing mechanism, which is behind the cornea, is the eye's second lens. Unlike a telescope or binoculars, the lens doesn't focus by moving forwards or backwards. Instead, muscles attached to it change its shape, making it thicker or thinner, to alter how much power it has, usually between five and 15 diopters.
Under the best circumstances, the two lenses focus light rays precisely on the retina. In some cases, the eye is too long, and the focal point falls in front of the retina, a condition called near-sightedness, or myopia. If the eye is too short, the focal plane of the light rays is behind the retina, a condition called far-sightedness or hyperopia. A third condition, astigmatism, occurs when the eye focuses light at different angles. That is, light rays approaching in a vertical axis don't fall at the same focus as light rays approaching from a horizontal axis.
Contact lenses or glasses correct near- and far-sightedness easily, and they correct astigmatism in most cases. About 20 years ago doctors began to correct vision surgically. Actually, such surgical procedures had been tried many years before that, but it wasn't done successfully until the 1970s.
In one surgical method, which can correct near-sightedness, doctors perform a radial keratotomy (RK), a star-shaped series of incisions on the cornea. These cuts flatten the cornea and decrease its power so it focuses light appropriately farther back in the eye, on the retina. The U.S. military, which gives a lot of advanced pilot training, took a dim view of RK. Among the reasons it cited were a permanent weakness of the cornea, problems with glare caused by scars, and vision fluctuating with time of day.
The military's concerns have proved to be valid over the last 15 years of long-term study of RK patients, but the procedure has been extremely popular, and thousands of aviators have had it done. The FAA will waive a pilot to fly some six weeks to six months after having RK surgery. The waiver requires a statement that the pilot doesn't experience any severe fluctuations in vision, has no problems with glare, or other complications.
Within the last few years, surgeons have developed a new procedure that is eclipsing RK and will probably render it obsolete. The new technique - excimer laser photoablation or photoreactive keratectomy (PK) - uses an excimer laser to burn off (ablate) areas of the cornea, which reshapes its curve to change its focusing power. The procedure takes about 15 minutes.
Another new technique is LASIK, which is a slight variation of PK. The surgeon peels back the outermost layer of the cornea like a flap. Then he ablates the underlying cornea to the new shape with the laser and replaces the flap.
PK and LASIK have important advantages over RK. The laser procedures correct high degrees of myopia better and more than RK. PK and LASIK can also correct hyperopia (far-sightedness), something RK doesn't do well. Lastly, the laser surgery has faster healing time and fewer complications. The changes PK and LASIK make are so subtle that a typical examining physician can't tell that the surgery was even performed, but a physician can see RK scars by shining a flashlight 90 degrees from the direction the person is looking.
As in the case of RK, the FAA will approve a pilot to fly some six weeks after PK and LASIK surgery, once the FAA is satisfied that the full healing process has taken place. In the military's view, laser surgery is not acceptable. If you're aiming for a military flying career and thinking about having eye surgery, you should consider your actions carefully, especially if you need just a slight correction.
Although refractive surgical procedures can enable someone to fly without wearing contacts or glasses, pilots no longer need uncorrected vision that's 20/100 or better to earn a first-class medical certificate. The new vision requirements - corrected to 20/20 - relegate RK, PK, and LASIK to "cosmetic procedures." This means insurance may not pay the bill for refractive surgery, which typically runs $2,000 to $4,000 per eye.
The eye's last focusing problem affects all of us. As we approach the age of 40, the lens grows increasingly stiffer, which means it can no longer deform enough to focus on things close to our face. To see clearly, we have to hold books and charts farther away from us. Even if we can see 20/20 at distance, we might need "reading glasses" to read the instrument panel clearly.
At present no surgical correction exists for this problem - it just gets worse as we age. Some pilots try wearing one corrective lens for the right eye and a different corrective lens for the left eye to correct one eye for distant vision and the other for near vision. This is a big, bad mistake.
In effect, this makes us "one-eyed," giving us monocular vision, because the brain suppresses the out-of-focus image. Not only is this practice unsafe, it is very much against federal aviation regulations. Don't mess with Mother Nature or the FAA.
Dr. Al Parmet is a retired U.S. Air Force flight surgeon and a current FAA senior aviation medical examiner. A graduate of the U.S. Air Force Academy and the University of Kansas Medical School, his military career included tours as a practitioner and professor of aerospace medicine. With more than 1,500 hours in aircraft ranging from the UH-1 Huey helicopter to the F-15 Eagle, Dr. Parmet built and flies a Monerai sailplane and has been awarded a soaring "C" and silver altitude badge.
20/20
For an average human, "20/20" denotes the best vision. Some people can see as well as 20/10, but this depends on how close the nerves are packed in the central portion of the retina. Whether corrected or uncorrected, 20/20 really should be the minimum vision for any pilot.
A Dutch ophthalmologist, Herman Snellen, defined "20/20" more than 100 years ago. He decided the average person should be able to see an angular measurement of three arc seconds at 20 feet. An E composed of vertical and horizontal bars about 1/8 inch long represents approximately three arc seconds when viewed from a distance of 20 feet. A person who can only see an E with one inch bars has vision eight times worse than 20/20. He sees at 20 feet what a "normal" person sees at 160 feet, so he has 20/160 vision.
Until recently, professional pilots needed 20/20 vision. This requirement stemmed from World War I, when pilots had to wear goggles to keep the castor oil that sprayed out of their engines from getting into their eyes. Goggles then didn't fit over glasses, so pilots who had to wear spectacles couldn't fly.
In 1996, the FAA and its federal air surgeon realized professional pilots rarely fly airplanes that spew castor oil. So it amended the medical certification requirements in Federal Aviation Regulation Part 67 in August 1996. Uncorrected visual acuity is no longer required for any medical certificate. Under the new regs, only the best corrected or uncorrected central visual acuity is important.
A third-class medical requires corrected or uncorrected vision of at least 20/40 in each eye. Second- and first-class medicals require corrected or uncorrected 20/20 vision. Now, pilots who can correct poor distant vision to 20/20 or better with contact lenses or glasses can get a first-class medical without needing a waiver (or statement of demonstrated ability - SODA).
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