| Number | Name | Date | Kind |
|---|---|---|---|
| 2405960 | Land | Aug 1946 | |
| 3576357 | Levy | Apr 1971 | |
| 3884548 | Linder | May 1975 | |
| 4136926 | Sigler | Jan 1979 |
| Number | Date | Country |
|---|---|---|
| 119260 | Sep 1979 | JPX |
| Entry |
|---|
| Optical Engineering, Jul./Aug. 1974, vol. 13, No. 4. |
| Xerox Disclosure Journal, vol. 6, No. 5, Sep.--Oct., 1981. |