A diagram showing the <a href="https://en.wikipedia.org/wiki/etendue" class="extiw" title="w:etendue">etendue</a> for a <a href="https://en.wikipedia.org/wiki/differential_element" class="extiw" title="w:differential element">differential surface element</a> in two dimensions <i>(left)</i> and three dimensions <i>(right)</i>.
The original uploader was <a href="https://en.wikipedia.org/wiki/User:Jcc2011" class="extiw" title="wikipedia:User:Jcc2011">Jcc2011</a> at <a href="https://en.wikipedia.org/wiki/" class="extiw" title="wikipedia:">English Wikipedia</a>..
CC BY-SA 3.0
<span class="int-own-work" lang="">Own work by the original uploader</span>;