1. National Institute of Standards and Technology (NIST) Special Publication 811, Guide for the Use of the International System of Units (SI), 1995.
2. International Organization for Standardization (ISO) 31-1, Quantities and units – Part 1: Space and time, Geneva, Switzerland, 1992.
3. Cathey, Wade T. June. 1973: On the Steradian. Applied Optics 12, 1097.
4. Hawkes, Peter W. The duffieux? 1973: Applied Optics 12, 2537.
5. Rohr, Moritz von. Ernst Abbe. Jena. Verein für thüringische Geschichte u. Altertumskunde. Zeitschrift. N.F. Beiheft 21. 1940.