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History of Eyeglasses
Transcript of History of Eyeglasses
3 Main Varieties
of Optical Glass
1. Crown: 7% Slilica, 14-16% Soda (Sodium Oxide),
11-13% Lime (Calcium Oxide)
2.Flint: 45-65% Lead Oxide, 25-45% Silica, 10% Soda & Potassium Oxide
3. Barium: 25-40% Barium Oxide
1. Refractive Index
2. Abbe Value (Constringence)
3. Specific Gravity
- an amorphous substance made primarily of silica fused at high temperatures with borates or phosphates
History of Eyeglasses
William Victor Y. Sasam, O.D.
Salvino d' Amarti (Marco Polo)
The first glassmaking manual from the library of the Assyrian king Ashurbanipal (669-626 BC) dates back to around 650BC.
It was luxury item and few people could afford it.
The discovery of new technique of ``glass blowing`` around the end of the 1st century and was a revolutionary event in the history of glass making.
Natural Glass Materials
Obsidian: a dark, volcanic glass that forms when lava
Fulgurite: a tube of hard glassy material formed by
lightning striking sand.
Tektite: glassy objects thought to be a result of
meteoritic impact on either earth or the moon.
More Chromatic Abberation
ABBE VALUE (Constringence)
after Ernst Abbe
the degree to which a given lens material will disperse light
Chromatic Aberration is inversely proportional to Abbe Value
Ratio of the weight of a substance to the weight of water with same volume
Desirable Characteristics of Optical Glass
1. Correct Index of Refraction & Chromatic
2. Freedom from color
3. High Degree of Transparency
4. High Degree of Chemical & Physical Stability
None homogeneity will result into the ff:
1. Striae: streaks or lines in the glass caused by uneven mixing (also called veins)
2. Bubble: also called seeds, air-bells or boil
3. Inclusions: stones & crystallites; are undissolved particles
4. Cloudiness: caused by precipitated colloidal material during the cooling period
Polymethyl Methacrylate (PMMA)
Also known as Lucite, Plexiglas & Perspex
Material used in hard contact lenses
Commonly known as Acrylic
Made from Allyl Diglycol Carbonate
Common Ophthalmic Lens
Has high impact resistance
Usually used in safety industrial frames
Absorbs UV up to 380nm even without additional treatment
CR-39 vs. Crown Glass
Strength of Glass
Drop Ball Test: A 5/8'' steel ball weighing approximately 0.56 ounce is dropped from a height of 50 "
*Fracture: the lens cracks, through its entire thickness and across an entire diameter, into two or more pieces, or that lens material visible to the naked eye becomes detached from the ocular surface.
TYPES OF LENSES
Types of Lenses
Characteristics of a Convex Lens
Image: Large (Magnify)
Characteristics of a Concave Lens
Image: Small (Minify)
Characteristics of a Cylinder Lens
Bifocals began when Benjamin Franklin had the lenses of two different spectacles cut in half and wired into a single frame. As technology has advanced, more complex lens configurations were created, with multiple areas of magnification, called segments, cut into the same lens.
Also referred to as a round lens, the magnifying area in the lower part of the lens is a complete circle. The lens can restrict viewing up top, due to the round shape. There is less significant transition between reading and distance vision.
Also known as the "D" style mutli-focal lens, this type of lens has an a magnifying area in the lower part of the lens that resembles a letter D turned on its side, with the flat side up. This is among the easiest of multi-focal lenses to adapt to. It is also an available lens design and can be produced in any lens material. The flat top of the lens provides a definite transition between reading and distance vision.