What is the difference between a photometer and a spectrophotometer?

Answer

The difference is complexity. Either may be used for quantitative or absorbency purposes depending on the situation. Also of note, each may have a single or double beam integrated into it. For clarity:

A spectrometer is a general term for an optical instrument. Spectrometric methods convey a large group of analytical methods based on molecular or atomic spectroscopy. Spectroscopic instruments will typically contain a stable source of radiant energy, a transparent veil for the sample or blank, a component to separate different wavelengths of light, a detector, and a readout/processor.

Photometers are simpler in design compared to spectrophotometers. They are less expensive, more convenient, easier to maintain, and good for field work. Some photometers include: visible photometers, probe-type photometers, and general purpose photometers using adjustable filters.

Spectrophotometers may have a few more mirrors, slits, a grating, and maybe a filter to top if off. Some examples include the popular Spectronic 20, Varian Cary 100, or even miniature fiber-optic spectrometers.