BOROFLOAT® 33 & Functional Coatings: A Union of Inspiration & Quality

The performance requirements for optical filters and mirrors are highly dependent on the material’s ability to reflect, absorb, enhance or modify incoming light. This can be accomplished with bulk optical glass materials or through coatings that are applied to a pristine glass substrate. Coatings usually allow significantly more freedom for customized light management design options which can be developed even further when a flat glass material with outstanding optical, thermal, mechanical and chemical properties is used. BOROFLOAT® borosilicate glass is such a unique substrate and has hence become the material of choice for robust high-temperature resistant dichroic filters, hot and cold mirrors.

BOROFLOAT® – The sum of its properties is what makes it unique for functional coatings

  • Exceptionally high transparency
  • Outstanding thermal resistance
  • Excellent mechanical strength
  • Broad range of sizes and thicknesses

BOROFLOAT® glass with functional coatings offers exceptionally high transparency

BOROFLOAT® glass - the industrial float glass with the lowest level of iron and other absorbing impurities in the market - offers exceptional light transmission. High UV transparency down to 300 nm, greater than 92 % light transmittance in the visible and near IR wavelength range, outstanding clarity, low auto-fluorescence and low solarization are specific characteristics of BOROFLOAT® glass substrates used in many optical fields.
Optical data
Abbe number
(ve = (ne – 1) / (nF‘ – nC‘))
Refraction index (nd587.6 nm)) 1.47140
Dispersion (nF – nC) 71.4 x 10-4
Stress-optical coefficient (K) 4.0 x 10-6 mm2 N-1

Average reference values, not guaranteed values.
Optical data
Wavelength λ (nm)
Refraction index n
435.8 1.48015
479.9 1.47676 (nF ’)
546.1 1.47311(ne)
589.3 1.47133
643.8 1.46953 (nC’)
656.3 1.46916

Mean reference values, not guaranteed values.

BOROFLOAT® glass - ideal for hot and cold mirrors due to its outstanding thermal resistance

Hot and cold mirrors either reflect or transmit infrared light and hence require high thermal resistance. The high boron content in BOROFLOAT® glass lowers the thermal expansion coefficient to 3.25 10-6 K-1 resulting in excellent resistance to thermal shock and temperature gradients.
The thermal properties of BOROFLOAT® glass (uncoated substrate) are shown below. Maximum thermal resistance of coated filters and mirrors varies depending on the filter supplier.
Thermal properties
Coefficient of
Linear Thermal Expansion (C.T.E.) α (20 - 300 °C)
3.25 x 10-6 K-1 *
Specific heat capacity
cp (20 - 100 °C)
0.83 kJ/(kg·K)
Thermal conductivity λ (90 °C) 1.2 W/(m·K)

* According to ISO 7991.
Maximum operating temperatures
For short-term usage (< 10 h)
500 °C
For long-term usage (≥ 10 h) 450 °C
Resistance to Thermal Gradients (RTG) and Resistance to Thermal Shock (RTS) must be considered when determining max. operation temperatures.
Resistance to Thermal Shock (RTS)
Glass Thickness
RTS (5 % -Fraktil)
≤ 3.8 mm 175 K
5.0 – 5.5 mm 160 K
6.5 mm 155 K
11 mm 142 K
18 mm 144 K
25 mm 128 K
Panels measuring 20 x 20 cm2 (8 x 8 inches) are heated in an oven with circulating air and afterwards doused in the center with 50 ml of cold water (68 °F). The temperature is controlled via pyrometer. The RTS value is the difference in temperature between the hot panel and the cold water, at which breakage occurs in less than or equal to 5 % of the samples. In order to simulate damage that can occur in practical use, the samples are abraded with 220 grid sandpaper before testing.
Resistance to Thermal Gradients (RTG)
Glass Thickness Tempering RTG
    Tchange * Theat-up *
3.8 mm No 123 K 136 K
6.5 mm No 119 K 132 K
11 mm No 52 K 173 K
18 mm No 31 K 188 K
6 mm Thermal > 300 K  
Edges ground or polished
* Tchange : sudden temperature change
   Theat-up: continuous heat-up
Panels measuring 25 x 25 cm2 (10 x 10 inches 2) are heated in the center of the panel to a defined temperature; the edges are maintained at room temperature. The temperature is increased within one minute to a level that initiates breakage of the test panel. The temperature is controlled via pyrometer. The RTG value is the difference in temperature between the hot center of the panel and the cool panel edge, at which breakage occurs in less than or equal to 5 % of the samples. In order to simulate damage that can occur in practical use, the samples are abraded with 220 grid sandpaper before testing.

BOROFLOAT® glass - process robustness guaranteed

Mechanical strength and robustness during manufacturing are essential for high quality substrates for functional coatings. BOROFLOAT® glass has a very strong microstructure with a relatively low amount of non-bridging oxygen resulting in better scratch resistance, excellent abrasion resistance and lower darkening behavior during high intensity radiation exposure than other flat glass types.
Mechanical properties
Density ρ (25 °C)
2.23 g/cm3
Young’s Modulus Ε
(according to DIN 13316)
64 kN/mm2
Poisson’s Ratio μ
(according to DIN 13316)
Knoop Hardness 0.1/20 (according to ISO 9385) 480
Bending Strength σ
(according to DIN 52292 T 1)
25 MPa
Impact resistance
The impact resistance of BOROFLOAT® 33 depends on the way it is fitted, thesize and thickness of the panel, the type of impact involed, presence of drill holes and their arrangement as well as other parameters.

Reference values, not guaranteed values.
Critical forces
Material Mean value FC [mN] Stadev. * [mN]
BOROFLOAT® 33 363.8 4.3
Other borosilicate glass 271.2 1.9
Soda-lime flat glass 214.4 4.6

Summary of critical forces in Scanning-Scratch-Test.
*Standard deviation

The right size and thickness for any application

Forms supplied

BOROFLOAT® 33 is available in the following standard thicknesses and tolerances:


BOROFLOAT® 33 is available in the following standard sizes:
Standard thicknesses
0.70 ± 0.05
1.10 ± 0.05
1.75 ± 0.05
2.00 ± 0.05
2.25 ± 0.05
2.75 ± 0.10
3.30 ± 0.20
3.80 ± 0.20
5.00 ± 0.20
5.50 ± 0.20
6.50 ± 0.20
7.50  ± 0.30
9.00  ± 0.30
11.00  ± 0.30
13.00   ± 0.30
15.00   ± 0.40
16.00  ± 0.50
19.00   ± 0.50
21.00 ± 0.70
25.40 ± 1.00

Panel thickness is continuously measured during production using laser thickness measuring equipment. Further thicknesses and tolerances are available on request.
Standard sizes
Size Thickness
1,150 x 850 mm 0.7 – 25.4 mm
1,700 x 1,300 mm 16.0 – 21.0 mm
2,300 x 1,700 mm 0.7 – 15.0 mm