There are two kinds of spectrophotometers: single beam and double beam. A double beam spectrophotometer compares the light intensity between two light paths. One path containing a reference sample, the other holding the test sample. A single beam spectrophotometer measures the relative light intensity of the beam before and after a test sample is inserted. A double beam machine makes comparison readings easier and more stable. But a single beam machine can have measure a wider range of light frequencies. Single beam machines have simple optical systems and are more compact. When the spectrophotometer is built into another device (like microscopes or telescopes) only single beam machines will work.