title_body: Could light transmittance be used to timely determine extraction?
upvoted_answer: Refractometers are widely used to measure the opaqueness of the coffee during brewing to timely determine the extraction time.
  
  Spectrophotometers are used to measure color instead of opaqueness. They are mostly useful to determine the roast degree.
  
  So, why not?
  
  However, in my opinion, perfect is something defined by personal preference when considered with coffee.
  
  If you want to find the perfect roast degree, you may choose your specific beans first, specific brewing method, etc. Then choose the specific roast degree. After that, a spectrophotometer may help you to find that exact point each time you repeat the roasting process. Similar to that, during brewing, a refractometer may help you to find that previously decided specific point each time you repeat that brewing process.
  
  In summary, there is no guidance you can hear as the perfect point that you can scientifically lean on. But you may hear some advice based on the coffee community's common experience. Generally, if you are not into industrial production, you don't generally need nerdy equipment to follow such advise.