Spectacles: Parts of Spectacles
Introduction
The history of spectacles is steeped in antiquity. The
earliest evidence dates from 500 B.C. in
The word "spectacle" has been attributed,
circa 1317 A.D., to Salcino de Armato, a Dominican friar and a friend of
Roger Bacon, the famous father of science. Subsequent references to glasses
and spectacles appeared in Chaucer's "The Wives Tales" in 1386. The
only significant advance afterwards in the field of spectacles occurred when
Benjamin Franklin in 1784 designed the first bifocal lens.
Countless names may be mentioned but through the ages from
the day the original glasses were worn, thousands of years ago to the
present, the concept has remained the same—a concave lens for short sight, a
convex lens for long sight and a cylindrical lens for astigmatism, all in the
form of two pieces of polished glass supported by a frame resting on the
nose.
What are spectacles?
Spectacles are optical appliances consisting of lenses
fitted in a frame or a mount with sides extending outwards to the ear.
What is the difference between a
frame and a mount?
A mount is essentially a frame with no rim at the
bottom so that the lenses are supported by holes or slots.
What are the parts of spectacles?
A central "bridge" with "pads"
which rest on the nose; the lenses which are supported by the
"rims" which extend to form the "lug", an extension on
each side fitted with a "joint" to which is connected a
"side" or "temple" which passes over the ears.
What is a lens?
Basically a solid material, glass or plastic or even
synthetic resin combinations. Its only requirement is that it be transparent
and be capable of bending or refracting the light.
How does one know one has a good
"lens" fitted in one's spectacles?
(a) Have them made by a "reputed" dealer. He
has his reputation to maintain.
(b) Have them checked by your doctor.
(c) Look for lens flaws, striations, pits in the glass,
the edge finish and the way it is mounted in the spectacles.
Unfortunately there are many reject lenses that find
their way through unscrupulous operators to "bargain houses".
How does one know if a spectacle
lens is plus, minus or cylindrical (astigmatic)?
A convex lens is thicker in the centre than at the
edge. Looking through it seems to magnify the letters. It is used to correct
long sight and is denoted by a plus (+) sign. Holding up the lens near the
eye and moving it will show distant objects moving in a direction opposite to
the movement of the lens.
A concave lens is thinner in the middle than at the
edges. Therefore objects look smaller when you look through the lens. It is used
for short sight or myopia. It is always denoted by a minus sign (-). Holding
up the lens near the eye and moving it will show that distant objects will
move in the same direction as the movement of the lens.
Astigmatism requires a tonic or cylindrical lens. To
know if a lens is cylindrical look at a straight edge or line, say the edge
of a door and rotate the lens. If the line tilts as you rotate the lens, it
is a cylindrical lens. Cylindrical lenses may be plus or minus and have their
angle denoted by an axis setting.
How do spectacles work?
Spectacles are purely a device to converge or diverge
the rays of light. Much has been made of this simple contraption. It has been
praised to the skies by one faction, while the other "without
glasses" body of opinion has placed all the ills of the world at its
door.
The simple facts are that spectacles refract the light
rays to a sufficient degree to permit the eye to take over and give adequate
vision.
How is the power or number of a
spectacle lens expressed?
The power or number is expressed in terms of dioptres.
It is the most convenient system for ophthalmic purposes. A lens of 1 dioptre
focuses parallel rays of light to its focal point, 1 meter away. A lens of
focal length of half a meter will be twice as strong as that with 1 meter and
its power is 2 dioptres. Thus a 4 dioptre lens has a focal length of 25 cms
and so forth.
What is the optical or
"dioptre" power of the human eye?
The human eye has a power of roughly + 60.00 dioptres.
Considering that average spectacles are hardly ever more than ± 5.0, it shows
what little difference they make to the actual full power of the eye.
What are the normal problems with
vision due to a spectacle lens?
Since a spectacle lens is a far from perfect optical
system, there are inadequacies of light focusing, differences in clarity and
object size (termed collectively as optical aberration). These aberrations
have been reduced in specialized optical units like a microscope, binocular,
telescope and good cameras by a combination of two types of optical lens,
usually of different density material, with specialized anti-reflection
coatings. This is not however possible in spectacles, since it would increase
the size and the weight of the lens and give a poor cosmetic appearance.
The common optical aberrations are:
(1) Spherical aberration
Looking through the center of a lens gives much higher
clarity of vision than from the sides. This is because the light from the
sides does not have an identical plane of focus.
(2) Chromatic aberration
White light passing through a prism tends to split up
into various colors. The edges of lenses, especially in myopic patients, tend
to act like prisms, breaking up light. The result, especially in daylight, is
that objects have fine colored rings along their edges.
(3) Multiple reflections
Reflections occur in spectacle lenses both from the
back and sides. Reflections are normal to lenses but some people find them
extremely bothersome. They an be however reduced by anti-reflection coatings.
(4) Image distortions
These are extremely obvious in large spectacle lenses,
typical of the present-day fashion. The horizontal and vertical lines tend to
be distorted, thus affecting perspective, the lines bulging outwards in a
plus lens and inwards in a minus lens.
(5) Comma
(Literally like a comma used in punctuation). Instead
of a fixed point of focus, the edges of a spectacle lens blur out in a comma
like fashion.
Most of these aberrations are inherent to spectacle
lenses. However by a proper selection of curves and coating these problems
can be reduced to the level where they would be acceptable.
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Diagram of Reading
Glasses Parts
Have you experienced confusion in trying to differentiate between the compositional parts of your reading glasses? For instance, can you locate the bridge versus the top bar, the temples versus the end pieces or nose pads versus pad arms? If you answered no, you are not alone.
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1.
Bridge: The area
that arches up over the nose between the lenses thus supporting the majority
of the glasses weight. There are several different types of bridges:
2.
End pieces: The
portions of the frame front that extend outward from the lenses and connect
to the temples.
3.
Eye Wires/Rims:
Part of the frame front into which the lenses are inserted.
4.
Frame Front: (not
pictured) Outermost front part of the eyeglass frame which holds
the lenses in place and bridges the top of the nose; consists of bridge, end
pieces, rims and lenses.
5.
Hinges: Portion of
the frame that connects the frame front to the temples and allows the temples
to fold inward in a closing motion.
6.
Lenses: Clear
glass, plastic, or polycarbonate eyeglass parts which hold a wearer's
prescription.
7.
Nose Pads: Plastic
pieces which may be attached directly to the frame or pad arms. These help
keep the frame in its proper position on the wearer's face, while providing
comfort and a snug fit.
8.
Pad Arms:
Attachments that hold the nose pads in place; typically allow adjustments so
that they may conform to the wearer's nose.
9.
Rimless Frames/Mountings:
(Not pictured) When
the temples and bridge attach by mountings, or metal fixatives, directly to
the lenses without the use of eye-wires or rims.
10.
Screws: Tiny metal
fasteners found at eyeglass hinges which connect the temples to the frame
front; and on the bridge, which hold the nose pads in place.
11.
Temples:
"Arm" pieces of the frame that extend over and/or behind the ears
to help hold the frame in place. There are several types of temples:
13.
Top bar: A
reinforcing bar that crosses the top of the glasses, between the two lenses,
on some metal frames; popular in aviator style glasses.
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