Page 140 - Chalcogenide Glasses for Infrared Optics

Characterization of Glass Pr operties 117

FIGURE 4.19 Striae scope photographs of Amtir 5, 8-in plates.

are presented in Table 4.9 for AMI materials along with other widely
used infrared materials for comparison. Also included are approxi-
mate values of ∆N/∆T for the materials. The homogeneity numbers
−6
are given as ∆N/N in units of 10 . Amtir 1 has the lowest value except
for ZnSe which was not a three-dimensional test. One explanation for
the low value for Amtir 1 is that all three constituent atoms, Ge-As-Se,
are on the same row of the periodic table and next to one another.
Thus, since the refractive index primarily reflects atomic mass and
these three atoms are almost the same, compositional variation is not
a factor for this glass. To carry this argument one step further, Amtir 5
is an As-Se glass. Therefore, Amtir 5 should be more homogeneous
than Amtir 1. Using a Wyko Interferometer, AMI evaluates the homo-
geneity of every Amtir 5 plate for use in the Lockheed Martin JSF
Fighter. Averaging the results for five plates, the Wyko results showed
a value for ∆N/N for Amtir 5 of 6.1, lower than the 8 value for Amtir 1.