BK-7 OPTICAL GLASS

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 through-out 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

BK7 Transmission Range

 

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
1180
1120
550
565
Annealing Point
1180
1120
550
565
Strain Point
1075
1025
624
515
Maximum Temperature (Continuous)
1000
950
350
230
1100
Maximum Temperature (Momentary)
1300
1200
490
Thermal Conductivity
@ 20° C
Cal cm
cm2sec° 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/cm2
9.4 x 104
9.4 x 104
Knoop Hardness (100g load)
kg/cm2
605
615
520
481
1525 - 2000
Density
g/cm3
2.203
2.203
Specific Gravity
2.203
2.201
2.53
2.23
3.98
Compressive Strength
kg/cm2
1.13 x 104
1.13 x 104
2.04 x 104
Tensile Strength
kg/cm2
500
500
282
7000
Torsional Strength
kg/cm2
300
300
1.5 x 106
Bending Strenght
kg/cm2
680
680
Young's Modulus
kg/cm2
7.45 x 105
7.45 x 105
8.63 x 105
7 x 106
79 x 105
Shear Modulus
kg/cm2
3.2 x 105
3.2 x 105
Bulk Modulus
kg/cm2
3.76 x 105
3.76 x 105
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
(0-MHz)
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
102 MHz
1 x 10-4
1 x 10-4
 
3 x 104 MHz
4 x 10-4
4 x 10-4
 
Resistivity
@ 20° C

Ω/cm

1 x 1018
1 x 1018
 
@ 600° C
8 x 107
8 x 107
 
1 x 1011
@ 1000° C
1 x 106
1 x 106
 
1 x 106
1 x 1011

Key Notes:

  • All Properties measured at 20° C unless otherwise specified.
Striae and Homogeneity
Material Code
Type
Striae/MIL - G -174A
Index Variation Δn(x10-6)
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:

  • These specifications are worst case conditions.
  • Materials can be selected and guaranteed to customer requirements.
  • Bubbles less than the maximum diameter count as a fraction of the allowed limit.
  • Bubbles less than 0.004" are regarded as undetectable.
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