Editing Light meter (section)

===Actinometers===
[[File:Watkins Standard Exposure Meter.jpg|thumb|Watkins Standard Exposure Meter - a type of actinometer]]
The earliest exposure meters were called ''actinometers'' (not to be confused with the [[Actinometer|scientific instrument with the same name]]), described as early as 1840<ref name=Clerc70>{{cite book |url=https://archive.org/details/photographytheor0000cler |title=Photography: Theory and Practice |last1=Clerc |first1=Louis-Philippe |last2=Mannheim |first2=L.A. |editor1-last=Spencer |editor1-first=D.A. |volume=3: Films {{!}} Subject and Exposure |date=1970 |publisher=Focal Press |edition=Completely revised |isbn=0-240-50706-1 |url-access=registration}}</ref>{{rp|415}} but developed in the late 1800s after commercial [[photographic plate]]s became available with consistent sensitivity. These photographic actinometers used light-sensitive paper; the photographer would measure the time required for the paper to darken to a control value, providing an input to a mechanical calculation of shutter speed and aperture for a given plate number.<ref name=Secret-Exposure/>{{rp|69}} They were popular between approximately 1890 and 1920.<ref name=EP-meters/>