光通量

luminous flux (physics)

light / ray / CL:道[dao4] / bright / only / merely / to use up

to go through / to know well / (suffix) expert / to connect / to communicate / open / to clear / classifier for letters, telegrams, phone calls etc

classifier for an activity, taken in its entirety (tirade of abuse, stint of music playing, bout of drinking etc)

to measure

capacity / quantity / amount / to estimate / abbr. for 量詞|量词[liang4 ci2], classifier (in Chinese grammar) / measure word

flux