Optical density machine name
Optical Density is synonymous with absorbance which is based on the principle of absorption of light. The machine that is used to measure absorbance is called a spectrophotometer. Spectrometer does not indicate that light absorbance is being measured. A spectrometer can measure any spectrum; for e.g. Mass Spectrometer measures the spectrum of molecular mass (to be precice mass per charge).

Cell density can be also measured by the level of light scattering; turbidimeters or nephelometers are used for these kind of measurements. 

PS: You should pay more attention in your class :P