UV-vis spectrophotometers are the swiss army knives of brewing chemistry because of the sheer amount of compounds it can analyze for and for the cheaper price tag. 
 
Spectrophotometers (“specs” for short) are used to measure the amount of iso-alpha acids in beer, which gives you IBUs. 
 
Specs can also measure, in wort and beer, the amount of free amino nitrogen, thiobarbituric acid reactive substances, polyphenols, beta glucans, turbidity, and a multitude of other compounds. 
 
Specs work by using the Beer-Lambert Law to calculate the concentration of a specific substance by measuring the amount of absorbency across a given distance at a given wavelength.