| − | # {{context|physics}} A [[logarithmic]] [[measure]] of the amount of [[light]] that is [[absorb]]ed when passing through a substance; the capacity of a substance to absorb light of a given wavelength; optical density. | + | # {{context|physics}} A [[logarithmic]] [[measure]] of the amount of [[light]] that is [[absorb]]ed when passing through a substance; the capacity of a substance to absorb light of a given wavelength; optical density.{{defdate|First attested in the mid 20<sup>th</sup> century.}}<ref name=SOED>{{reference-book | last =| first = | authorlink = | coauthors = | editor =Brown, Lesley | others = | title = The Shorter Oxford English Dictionary | origdate = | origyear = 1933| origmonth = | url = | format = | accessdate = | accessyear = | accessmonth = | edition = 5th | date = | year =2003| month = | publisher =Oxford University Press | location =Oxford, UK | language = | id = | doi = | isbn =978-0-19-860575-7 | lccn = | ol = | pages =9| chapter = | chapterurl = | quote =}}</ref> |
