Officially, Source Four is the name of a family of luminaires from Electronic Theater Controls (ETC), so named because the patented HPL lamp's filament contains four segments--"four sources." Included in this family are the Source Four PAR, Source Four PARNel, Source Four Revolution, Source Four jr, Source Four MultiPAR, and the full-size Source Four ERS, sometimes called a senior to distinguish from a junior. In common usage, "Source Four" refers solely to ETC's ground-breaking Ellipsoidal Reflector Spotlight. Released in 1992, it redefined the ERS and has become the industry standard. They are so widely used that some people now refer to all ERS instruments as "Source Fours". Over three million units have been sold since 1992. http://www.etcconnect.com/products.fixtures.aspx The Source Four ERS: Cat# - Name: - Beam Angle/Field Angle: - Old Style Equivalent: 405 - 05° - 6/7 410 - 10° - 8/11 414 - 14° - 14/15 419 - 19° - 15/18 - 6x16 426 - 26° - 18/25 - 6x12 436 - 36° - 27/34 - 6x9 450 - 50° - 36/50 - 4.5x6.5 470 - 70° - 60/70 490 - 90° - 79/87 The beam/field angles are measurements (with a light meter and protractors) from an actual representative fixture, and are according to the latest version of each cut sheet. The numbers vary slightly depending on the date of the cut sheet.