BK7 is a high quality optical glass that is used whenever the additional benefits of fused silica are not required. Since BK7 performs well in all chemical tests, and no additional or special handling is required, costs of manufacturing are reduced. It is a relatively hard material with extremely low bubble and inclusion content, while providing excellent transmittance throughout the visible and near infrared spectra and down to 350 nm in the ultraviolet.
For all standard optical glass components, or when optical glass is generically requested for custom elements, Glass Dynamics uses normal high quality BK7 optical glass.
OPTIMUM TRANSMISSION RANGE 350nm  2.0µm
Index of Refraction 


Wavelength, nm 
Fused Silica / Quartz 
BK7 
Sapphire 
213.86 
1.53427 
 
 
230.21 
1.52005 
 
 
239.94 
1.51337 
 
 
265.20 
1.50000 
 
1.83360 
280.35 
1.49403 
 
1.82427 
302.15 
1.48719 
 
1.81351 
334.15 
1.47976 
 
1.80184 
346.62 
1.47746 
 
1.79815 
365.02 
1.4752 
1.53626 
1.79358 
404.66 
1.46962 
1.53024 
1.78582 
435.84 
1.46669 
1.52669 
1.78120 
546.706 
1.4008 
1.51872 
1.77078 
706.52 
1.45516 
1.51289 
1.76303 
852.11 
1.45247 
1.50981 
1.75885 
1013.98 
1.45025 
1.50731 
1.75547 
1529.52 
1.4427 
1.50094 
1.74660 
1970.09 
1.43853 
1.49500 
1.73833 
2325.42 
1.43293 
1.48929 
1.73055 
2325.42 
1.43293 
1.48929 
1.73055 
3243.90 
1.41315 
 
1.70437 
3507.00 
1.40566 
 
1.69504 
4954.00 
 
 
1.62665 
5577.00 
 
 
1.58638 
Mechanical Properties of Optical Materials 


Thermal 
Units 
Fused Quartz 
Fused Silica 
BK7 
Pyrex® 
Sapphire 

Softening Point 
C° 
1180 
1120 
550 
565 

Annealing Point 
C° 
1180 
1120 
550 
565 

Strain Point 
C° 
1075 
1025 
624 
515 

Maximum Temperature (Continuous) 
C° 
1000 
950 
350 
230 
1100 

Maximum Temperature (Momentary) 
C° 
1300 
1200 
490 

Thermal Conductivity 
@ 20° C 
Cal cm cm^{2}sec° C 
0.0033 
0.0033 
0.0026 
0.0027 
0.0065 

@ 200° C 
0.0037 
0.0037 

@ 950° C 
0.0064 
0.0064 

Specific Heat 
0° 100°C 
cal g °C 
0.18 
0.18 
0.21 
0.2 

0° 900°C 
0.25 
0.25 

Coefficient of Thermal Expansion 
50° 0° C 
cm cm °C 
2.7 x 10^{7} 
2.7 x 10^{7} 

50° 300° C 
5.9 x 10^{7} 
5.9 x 10^{7} 
71 x 10^{7} 
33 x 10^{7} 

50° 900° C 
4.8 x 10^{7} 
4.8 x 10^{7} 
90.3 x 10^{7} 
Strength/Hardness 


Moh Hardness 
5.5  6.5 
5.5  6.5 
5 
9 

Microhardness 
kg/cm^{2} 
9.4 x 10^{4} 
9.4 x 10^{4} 

Knoop Hardness (100g load) 
kg/cm^{2} 
605 
615 
520 
481 
1525  2000 
Density 
g/cm^{3} 
2.203 
2.203 

Specific Gravity 
2.203 
2.201 
2.53 
2.23 
3.98 

Compressive Strength 
kg/cm^{2} 
1.13 x 10^{4} 
1.13 x 10^{4} 
2.04 x 10^{4} 

Tensile Strength 
kg/cm^{2} 
500 
500 
282 
7000 

Torsional Strength 
kg/cm^{2} 
300 
300 
1.5 x 10^{6} 

Bending Strenght 
kg/cm^{2} 
680 
680 

Young's Modulus 
kg/cm^{2} 
7.45 x 10^{5} 
7.45 x 10^{5} 
8.63 x 10^{5} 
7 x 10^{6} 
79 x 10^{5} 
Shear Modulus 
kg/cm^{2} 
3.2 x 10^{5} 
3.2 x 10^{5} 

Bulk Modulus 
kg/cm^{2} 
3.76 x 10^{5} 
3.76 x 10^{5} 

Poisson's Ration 
0.17 
0.17 
0.208 
0.2 
0.02 
Electrical 


Dielectric Strength 
@ 20° C 
kv/cm 
250  400 
250  400 
480 

@ 500° C 
150  200 
150  200 

Dielectric Constant 
(0MHz) 
3.7 
3.7 
5.1 
8.6  10.6 

Dielectric Loss Tangent 
1 MHz 
1 x 10^{4} 
1 x 10^{4} 
8.6 x 10.6 

10^{2 }MHz 
1 x 10^{4} 
1 x 10^{4} 

3 x 10^{4 }MHz 
4 x 10^{4} 
4 x 10^{4} 

Resistivity 
@ 20° C 
Ω/cm 
1 x 10^{18} 
1 x 10^{18} 

@ 600° C 
8 x 10^{7} 
8 x 10^{7} 
1 x 10^{11} 

@ 1000° C 
1 x 10^{6} 
1 x 10^{6} 
1 x 10^{6} 
1 x 10^{11} 
Key Notes:
Striae and Homogeneity 


Material Code 
Type 
Striae/MIL  G 174A 
Index Variation Δn(x106) 
S1  UVA 
Fused Silica 
A 
6 
S1  UVB 
Fused Silica 
A 
10 
A1 
Fused Quartz or Fused Silica 
B 
10 
G1 
Fused Quartz or Fused Silica 
C  D 
20 
I2  IR 
Fused Quartz 
A 
4 
BK7 
Optical Glass 
A 
10 
Bubbles & Inclusions 


Up to 1.0" diameter 
From 1.0" to 2.0" Diameter 
From 2.0" to 3.0" Diameter 

Material 
Maximum Diameter 
Maximum Quantity 
Maximum Diameter 
Maximum Quantity 
Maximum Diameter 
Maximum Quantity 
(Inches) 
Per Pc. 
(Inches) 
Per Pc. 
(Inches) 
Per Pc. 

S1  UVA 
0.006 
0 
0.008 
1 
0.01 
1 
S1  UVB 
0.008 
1 
0.015 
1 
0.015 
1 
A1 
0.01 
1 
0.015 
1 
0.02 
2 
G1 
0.03 
3 
0.04 
3 
0.06 
3 
I2  IR 
0.01 
3 
0.015 
3 
0.02 
3 
BK7 
0.006 
0 
0.006 
0 
0.01 
1 
Key Notes: