In the absorption spectroscopy techniques, a carefully controlled light source excites molecules or atoms.

The intensity of light is monitored with a light intensity measuring system of some kind, such as a photoelectric cell. This photocell is referred to as the detector. Measurements are made with and without the sample in the light path to determine the amount of light absorbed by the sample. Different light sources and detectors are used for different wavelengths of light, but the same principles apply in each case.

In modern instruments the detector is frequently a solid state device like a charge coupled device (CCD) similar to the ones found in video cameras.

Absorption spectroscopy is used primarily for translucent samples - either solutions or fused solids like the KBr discs common in IR spectroscopy.

In emission techniques, the sample is excited by some type of energy such as an electric spark. When molecules return to the ground state, light is emitted. The intensity and wavelength of the emitted light is measured. In fluorescence spectroscopy, the sample is excited by ultraviolet light, and the emitted light is measured at lower wavelengths (usually visible wavelengths).

Introduction to spectroscopy