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Explanation: In Jim Breens dictionary, the two halves of this word come up like this:
測光 - photometry, photometric
機構 - mechanism
but 測光 could also be "photometer" as in 測光ベンチ - photometer bench
So, you could use photometer mechnism, but, if your text is about cameras, you might need another word. Read on.
Google brings up lots of pages with 測光機構 on them, they all seem to be camera-related. However, if you search in English, photometer brings up more physics related pages, "light meter" seems to be used in camera related texts.
Again, lack of context prevents further research, sorry.