exemplified by the January 1950 issue of Popular Photography, as follows: Upon introducing the new inverse of the squares concept in 1939, General Electric initially referred to the new system as "Flash Numbers". Two years later, Flash in Modern Photography (1941) used the term "guide number" on page 47, on the very next page used the term "Flash Number" (title case), and later still used the term "flash number" (lowercase). Terminology was similarly mixed in the United Kingdom for years after the introduction of the guide number system; circa 1954, "Flash Factor", "Flash number" (and sometimes "Guide number") were in use. Glossary See also Exposure (photography) Exposure value Flash comparison Flash (photography) Flash synchronization Flashtube Notes References Further reading Bryan Peterson, Understanding Flash Photography: How to Shoot Great Photographs Using Electronic Flash, (paperback – August 30, 2011), Amphoto Books, External links D. 'n' A. Seaver: Conley cameras Photographic Memorabilia: Flash Photography ~ History & ILFORD Scantips.com: Understanding Flash Guide Numbers, plus guide number Calculator Scantips.com: EV – Exposure Value (with EV table and EV calculator) Sekonic.com: EV/Lux/FootCandle Conversion Chart Flash photography Photographic lighting Light sources