What is the name of the machine used to measure optical density? We used it in a lab but I can't remember what they called it.
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