7.2 Homogeneity
It is sometimes necessary to measure the index variation across a blank. In such cases, OHARA pays special
attention to each process and can supply high homogeneity "Grade Special A" blanks. Grade Special A is our term
for precision annealed high homogeneity (Low 
n)
optical glasses. Our Grade Special A glasses are available (depending on glass type and dimensions) in the
following homogeneity levels:
| Classification | Homogeneity |
| Grade Special A0.5 | ± 0.5 × 10-6 |
| Grade Special A1 | ± 1 × 10-6 |
| Grade Special A2 | ± 2 × 10-6 |
| Grade Special A5 | ± 5 × 10-6 |
| Grade Special A20 | ± 20 × 10-6 |
| Table 11 |
Please note that the Grade Special A number indicates n
in the sixth decimal place. The annealing required must be specified in terms of birefringence (nm/cm). Generally, low
n implies low birefringence from precision
annealing. Using phase measuring interferometers, we measure transmitted wavefront of each test piece. Interferograms
are supplied with each blank when a Grade Special A5 or higher homogeneity level is specified. The more detailed a
nalysis of homogeneity gradients, astigmatisms, and spherical aberrations are an important part of our daily
measurement requirements. We are using state of the art phase measuring interferometers produced by ZYGO USA.
Our interferometers are able to make precision measurements on 9, 12 and 18 inch apertures.