What does it measure?
A spectrophotometer works by measuring and comparing the intensity of light entering and exiting a sample. Measurements can be expressed as the percentage transmittance (which is the % light exiting compared to entering). If all the light passes through a solution, then T = 100%. This would happen for distilled water where the majority of light would pass straight through
Measurements can also be expressed as the absorbance (A), that is the amount of light taken in or absorbed by the solution. If all light is transmitted (T = 100%), then A = 0. If no light is transmitted, we would say Absorption is infinite.