The U.S. Military defines Quality Level or Grade as different striae levels per Din 3140 and according to MIL-G-174-B. Striae is localized inhomogeneity within the optical material. Test is visual and compared against reference samples. Since thistype of test is nonquantitative, the following list is based on homogeneity measurement of the index changes within a material by interferometric teating.
CATEGORY
nd SPEC
1. Commercial
Not specified (typical application is front surface) mirror
2. Grade B
+/- 1 x 10-4
3. Grade A
+/- 1 x 10-4
4. H-1
+/- 2 x 10-5
5. H-2
+/- 5 x 10-6
6. H-3
+/- 2 x 10-6
7. H-4
+/- 1 x 10-6
8. H-5
+/- 5 x 10-76
Source: A.G.I.
Formula for wave front distortion to nd tolerance:
Total wavefront distortion
=
Surface Dontribution
+
Inhomogeneity Contribution
4
=
2(n-1) ó
+
( nmax) t
Where:
( nmax) is homogeneity is the wavelength of light to be used
t is the geometrical thickness along the optical path
µ is the Peak to Valley departure for flateness of one surface
2(n-1) is the owrst case contribution of both surfaces
Note: A nanogram can be used to determine the specification of inhomogeneity once the limit of the wavefront distortion is determined using the following equation:
( nmax) =
yt
Where:
is the wavelength of light used to interferometer is the fringe deviation (P-V)
t is the sample thickness; i.e., dimension along the light path in the interferometer
nmax) t
y