The light source shines on the sample. The monochromator separates what is spectrophotometer pdf wavelengths of light and focuses each of them onto the photodetector sequentially.
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. Many older spectrophotometers must be calibrated by a procedure known as “zeroing. The absorbancy of a reference substance is set as a baseline value, so the absorbancies of all other substances are recorded relative to the initial “zeroed” substance.