Editing Film speed (section)

====Hurter &amp; Driffield{{anchor|H&D}}====
<!-- This section is linked from inside or outside this article, preferable by "#H&D". Do not rename without keeping in sync. -->
Another early practical system for measuring the sensitivity of an emulsion was that of [[Hurter and Driffield]] (H&D), originally described in 1890, by the Swiss-born [[Ferdinand Hurter]] (1844–1898) and British [[Vero Charles Driffield]] (1848–1915). In their system, speed numbers were inversely proportional to the exposure required. For example, an emulsion rated at 250&nbsp;H&D would require ten times the exposure of an emulsion rated at 2500&nbsp;H&D.<ref name="Sowerby"/>

The methods to determine the sensitivity were later modified in 1925 (in regard to the light source used) and in 1928 (regarding light source, developer and proportional factor)—this later variant was sometimes called "H&D&nbsp;10". The H&D system was officially<ref name="Konovalov_2007"/><!-- TBD: I found documents stating that there was no standard before 1945, so we should find some historic documents stating that H&D (or whatever) was really an *official* standard in the USSR, and not just semi-standard. Also, since GOST 2817-45 was apparently first defined in 1945, why was the GOST standard not introduced before 1951, when its revision GOST 2817-50 was already in place? Runner1616: Until 1951 both standards existed simultaneously in USSR. Any photo book before 1950 operated with H&D (Х и Д) --> accepted as a standard in the former [[Soviet Union]] from 1928 until September 1951, when it was superseded by [[#GOST|GOST]]&nbsp;2817–50.