Science of light (6)
I believe the answer is:
optics
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">science of light</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for optics that I've seen before include "Organs of sight, eyes" , "Spirit measures" , "Science of vision/light" , "Devices to measure out alcohol" , "Science concerned with vision" .)