Measuring Vision
In testing vision, eye doctors employ a
time-honored reference to measure visual acuity. The Snellen chart,
with progressively smaller letters, has become almost universal.
"20/20" means that the person being tested can see the same letters
on the chart, viewed from 20 feet, as a theoretically normal eye can
see from 20 feet. "20/80" means the eye sees at 20 feet what a
normal eye sees at 80 feet. In Europe (where the metric system is
used), 20 feet corresponds to 6 meters, so normal vision is noted as
6/6.
Technically, acuity of "20/20" corresponds to an angular
resolution of 1 degree. Vision of at least 20/40 is required in
California and most states to obtain a drivers' license. Legal
blindness, by contrast, means that a person's best-corrected vision
is 20/400 or worse.
Snellen charts, with black letters on a white background, measure
high contrast visual acuity. They do not measure
ability to distinguish subtle differences in shades of gray. Other
tests are necessary for this purpose.
One increasingly accepted method of measuring
contrast sensitivity requires special charts and
controlled lighting. Shown at left is a one of the Pelli-Robson
testing charts. There are people who have very good Snellen
acuity, yet describe poor quality of vision -- unease at dusk, at
night, and/or in particular lighting conditions. When tested, these
people can be found to have decreased performance on contrast
sensitivity testing. Some people have decreased scores on these
tests after laser corrective surgery.
Contrast sensitivity testing is not widely performed. Our office
(apparently) is one of the only centers in the greater Los Angeles
area that has this capability. Reasonable professionals differ in
their opinions about the best methods of testing, and interpreting
the results of such testing. At BEST LASIK Laser Eye Center, we
perform contrast sensitivity testing, but find the need to do this
only rarely. The U.S. Customs Service currently requires contrast
sensitivity testing for any applicant who has had prior refractive
surgery, and we can of course accommodate such requests.
Your Eyeglass Prescription
Optical prescriptions are written to specify sphere power,
cylinder power, and axis of cylindrical correction (on a protractor
scale, 0 to 180 degrees). By custom, myopia is written as minus (-)
sphere, and hyperopia as plus (+) sphere. Optical power of a lens is
measured in units called diopters. For the science-minded, 1 diopter
is the amount of focusing power needed to bring parallel light rays
into focus 1 meter behind the lens. To simplify, we are leaving out
discussion of multifocal lenses (e.g. bifocals, trifocals, etc.) and
prism in glasses.
The measurement of the prescriptive needs of
the eye is called a refraction . An optical prescription
defines the refractive error of the eyes measured. All forms of
surgery to correct refractive errors are types of refractive
surgery .
If you don't know your prescription but want an estimate, take your
glasses off, hold them up, and look through the lenses one at a
time. If the lens makes things look smaller, it is concave (myopia
correcting). If things through the lens look bigger, it is convex
(farsighted correcting). To see whether the lens has any astigmatism
correction, rotate it while looking through. If things look
particularly compressed or 'squished' in one direction, and that
direction follows the rotation of the glasses, then there is a
cylindrical power to the lens and you have astigmatism.
