Flame Emission Spectroscopy is an analytical measuring device that relates the intensity of the emitted light relative to to the concentration of the sample.
As I am reading an article about FES, I noticed how the author purposely inserted the different parts of the flame and their activity.
The above image was from an article in Chemistry LibreTexts about FES.
The author said "The Primary Combustion Zone is where the initial decomposition occurs and molecular fragments are observed. The Interzonal Region is the hottest part of the flame and atomic fragment are observed. The Secondary Combustion Zone is cooler overall and a conversion is seen from atoms back to stable molecules and oxides."
I have read the article but it was never mentioned what part of the flame does the transducer or sensor was placed on.