Laboratory refractometers are dedicated lab instruments to measure the correct refraction indexes of different mediums. They're also commonly known as Abbe refractometers and they're usually much larger in size than digital refractometers. You can receive extremely precise results with laboratory refractometers. However, they're super costly and you have no need to invest in these refractometers just to measure the salinity levels of multiple saltwater aquariums & ecosystems.