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

 

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 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	86 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

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