5.1 Modulus of Elasticity
Young's modulus,
Modulus of rigidity and Poisson's ratio are determined by measuring
the velocities of the longitudinal and transverse elastic waves in a
well annealed rod of size 100 ~ 150 x 10 x 10 mm at room
temperature. Young's modulus (E), Modulus of rigidity (G) and
Poisson's ratio (s) are calculated using
the following equations. Accuracy is ±1%.
Modulus of rigidity G =
v_{t2}p 

Young's Modulus
E=(9KG)/(3K+G) 
vl : Velocity of
longitudinal waves 
Bulk
Modulus K = v _{t2} p  (4/3)G 
vt : Velocity of
transverse waves n: Density 
Poisson's ratio s=(E/2G)1 
p: Density 
5.2 Knoop Hardness (Hk)
The indentation hardness of optical
glass is determined with the aid of the micro hardness tester. One
face of the specimen with the necessary thickness is polished. The
diamond indentor is formed rhombic so that the vertically opposite
angle from two axes is 172 °30' and 130 ° respectively. The load
time is 15 seconds, the load is 0.98 N. The glass specimen is
indented at 5 places. Knoop hardness can be computed with the
following equation:
Knoop
Hardness =1.45l F/l^{2}
F : Load
(N) : l Length of longer diagonal line (mm)
Table 1 shows how the
glasses are classified according to Knoop hardness. Please note the
Knoop hardness figures have been rounded to the nearest 5 (e.g.
value of 158 is shown as 160.)
Table 1
Group 
1 
2 
3 
4 
5 
6 
7 
Knoop Hardness 
<150 
>150
250< 
>250
350< 
>350
450< 
>> >450
550< 
>
550
650< 

5.3 Abrasion (Aa)
A sample of size 30 x
30 x 10 mm is lapped on a 250 mm diameter cast iron flat, rotating
at 60 rpm. The test piece is located 80 mm from the center of the
flat and is under a 9.8N load. 20 ml of water containing 10 g of
aluminous abrasive as the lapping material, with mean grain size
20µm(#800), is supplied evenly to the test piece for 5 minutes. The
weight loss of the test piece is then measured. The known weight
loss of the standard glass is compared according to the following
equation:
Abrasion = {(Weight
loss of sample / Specific gravity)/( Weight loss of standard sample
/ Specific gravity)} X 100
Glasses showing a
higher value are less resistant to abrasion.
5.4 Photoelastic Constant (ß)
Optical glass is
usually free of strain, but when mechanical or thermal stress is
exerted upon it, glass shows birefringence. Stress F(Pa), optical
path difference d (nm) and thickness of glass d(cm) have the
following relationship:
d = ß d F
In this case,
proportional constant ß is called the photoelastic constant. It is
listed in this catalog at a unit of (nm/cm/10^{5} Pa). The
photoelastic constant is the material constant which will change by
glass type. By using it, optical path difference can be computed
from given stress. Internal stress can also be computed from optical
path difference. 