SCHOTT Technical Glasses

27
Internal transmittance ––>
0.99
0.98
0.96
0.90
0.70
0.50
0.30
0.10
0.01
1E-04
200 300 400 500 600 700 800 900 1000 1100 1200
Wavelength in nm ––>
UG5
UG11
UG1
Fig. 25. UV bandpass filter for a glass thickness of 1 mm
5.4 Color of glass
Color is a sensation perceived by the human eye when
observing an illuminated filter glass or incandescent object.
The color of glass is a function of its spectral transmission
and the spectral energy distribution of the illuminating light
source. Color stimulus is measurable and is defined by three
numerical values (X, Y, Z) in accordance with color metric
conventions set forth by the CIE (see publication CIE N°
15.2 (1986)). The first value is its brightness (standard tristimulus
value) Y and the other two values define the color
locus. There are two possibilities to define the color locus
F (see Figure 26): Either the chromaticity coordinates x and
y, or the dominant wavelength λ d (at S) and the excitation
purity Pe = DF : DS.
5.5 Stress birefringence
Owing to its very structure, glass is an isotropic material.
Mechanical stress causes anisotropy which manifests itself
as stress-induced birefringence. A ray of light impinging
on glass will be resolved into two components vibrating in
planes perpendicular to each other and having different
phase velocities. After passing through a plate of thickness d
which is subjected to a principal stress difference Δσ, an
optical path difference Δs exists between the two compo­
nents. This path difference can either be estimated by
means of the birefringence colors or measured with compensators:
Δs = K ∙ d ∙ Δσ [nm].
K is the stress-optical coefficient of the glass (determination
according to DIN 52314).
K = Δs
d
1
Δσ [MPa-1 ]
Many glasses have stress-optical coefficients of about
3 x 10 –6 MPa –1 , and borosilicate glasses of up to 4 x 10 –6 MPa –1 .
High-lead content glasses can have values down to nil or
even negative.
5
Stress-optical measurements permit the determination of
S: Point at which the extension DF intersects the spectrum locus at λ d
permanent stress in glass (state of annealing) as well as of
reactive stress caused as a reaction to exterior forces. Stressoptical
measurements for the evaluation of glass seals with
other glasses, metals, or ceramics are of particular importance.
These offer a sensitive method of deter mining thermal
expansion and contraction differences.
1.0
y 0.9
520
0.8
510 540
0.7
560
0.6
0.5
500
S
F 580
0.4
600
0.3
490
D
620
700
0.2
480
0.1
0
400
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
x
Fig. 26. The color of optical filter glasses according to the definition
of CIE 1931
D: Color locus of the radiation source, for example D65