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Specs of Gravity

Specifics of Gravity

Definition of Specific Gravity

Specific gravity is a measure of the density of a mineral or in this case optical lens material. It is derived from the density of the lens material divided by the density of water. Since water's density equals 1 gram per cubic centimeter (at specific conditions), then a mineral's specific gravity (SG) would also correspond to a mineral's density as expressed in grams per cubic centimeter (g/cm3). Since a glass lens has a specific gravity of 2.54, then it is nearly three times as dense as water as well as Trivex.

Various Materials by Wieght

The following chart shows the index of refraction along with the specific gravity

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Index of Refraction + Specific Gravity

Trivex (1.53) - 1.11
Hoya (1.56) - 1.20
Polycarbonate (1.59) - 1.21
Sola (1.53) - 1.21
Plastic (1.50) - 1.32
Essilor (1.60) - 1.34
Nikon (1.60)
Signet (1.60) - 1.34
Sola (1.60) - 1.34
Varilux (1.60) - 1.34
Pentax (1.66) - 1.35
Essilor (1.67) - 1.35
Hoya (1.67) - 1.35
Nikon (1.67) - 1.35
Kodak (1.67) - 1.35
Varilux (1.67) - 1.35
Essilor (1.56) - 1.36
Nikon (1.56) - 1.36
Varilux (1.56) - 1.36
Seiko (1.67) - 1.36
Optima (1.66) - 1.36
Hoya (1.70) - 1.41
Essilor (1.74) - 1.46
Crown Glass (1.52) - 2.54
Glass (1.60) - 2.60
Glass (1.70) - 3.20
Glass (1.80) - 3.66

Tensile Strength

Hoya Vision Care's Phoenix lens, made from Trivex lens material from PPG Industries, withstood up to 65 kg of force during this tensile strength test performed at the Hoya booth during Vision Expo. 

For each test, a standard rimless screw was inserted through the two drill holes on opposite ends of the lens. Collets attached to the screws pulled them in opposite directions until either the lens or the screw failed. In some cases, the screw even broke before the Phoenix lens. 

 In contrast, standard CR 39 lenses all failed when 10 to 20 kg of force were applied. The test demonstrates the durability of the Phoenix lens, which makes it ideally suited for rimless and three-piece frames in which the lens is an integral part of the design.
Tensile Strength

Drop Ball Test

Hoya Vision's Phoenix lenses, made with Trivex lens material from PPG Industries, bounced back from the impact of a 1 kilogram (2.2 pound) steel ball dropped from 50 inches in this drop ball test displayed at the Hoya booth during Vision Expo. 

The fragments seen on the floor of the test are from a shattered CR-39 lens, tested in the same manner as the Phoenix lens just moments before this demonstration.
Drop Ball Test

Strain Scope

The lens on the left, a Phoenix lens, made of Trivex material from PPG Industries appears clear when viewed through this strain scope, even around drill holes needed for three-piece frames. 

The polycarbonate lens (right) displays colored striations, indicating chromatic aberration caused by stress, especially around the drill holes.
Strain Scope
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