SCHOTT Technical Glasses

25
The transmittance τ d at perpendicular incidence decreases
correspondingly to values between 93.1 % and 90.6 %.
The reflection r d of a parallel glass plate can be reduced
(and its total transmittance τ d can be increased) by using
so-called anti-reflective (AR) coatings. In its simplest form,
a transparent layer on each surface with n layer = √ — n glass and
a thickness of λ/4/n layer will reduce the reflection for perpendicular
incidence and wavelength λ completely. For other
wavelengths close to λ, the reflection is reduced but not
canceled. With more complex multy-layer AR coatings,
reflections in the visible range can be reduced to less than
1 % per surface.
The internal transmittance τ i of an absorbing glass is a function
of the thickness d:
τ i = e –αd , where α is the spectral absorption coefficient.
The relation between spectral transmittance τ and spectral
internal transmittance τ i is:
τ(λ) =
( 2n(λ)
n(λ) 2 + 1) τ i(λ)
The dependence of the factor
( 2n(λ)
n(λ) 2 + 1)
on wavelength is usually small, thus, a constant
reflection factor P
5.3 Transmittance
Normally, glass is transparent to visible light and certain regions
of UV and IR radiation. Losses in transmittance occur
due to internal absorption and reflection, as indicated above.
The so-called internal transmittance τ i can be modified
by using coloring agents (oxides of transition elements or
colloids) or fine particles in the glass, which have different
refractive indices (light scattering).
P d =
2n d
n d
2
+ 1
is sufficient for use in most cases.
The best UV transmission is achieved with pure fused
silica (UV cut-off for 1 mm thickness is in the region
of 160 – 180 nm); particularly good UV-transmitting multicomponent
glasses have cut-offs of up to 220 nm wavelength
(Figure 21); normal technical glasses (Figure 22)
already absorb considerably at 300 nm.
5
100
8337B
100
90
80
8405
80
70
60
60
50
1 mm
8 mm
40
40
Transmission in % ––>
20
0
200 250 300 350 400
Wavelength in nm ––>
Fig. 21. UV transmission of highly UV-transparent technical glass types
8337B and 8405 at 0.5 mm glass thickness
Transmission in % ––>
30
2 mm
20
10
0
200 500 1000 2000 5000 10000
Wavelength in nm ––>
Fig. 22. Transmission of DURAN ® 8330 for thicknesses 1, 2 and 8 mm