Monochromators are used in many optical measuring instruments and in other applications where tunable monochromatic light is wanted. Sometimes the monochromatic light is directed at a sample and the reflected or transmitted light is measured. Sometimes white light is directed at a sample and the monochromator is used to analyze the reflected or transmitted light. Two monochromators are used in many fluorometers; one monochromator is used to select the excitation wavelength and a second monochromator is used to analyze the emitted light.
An automatic scanning spectrometer includes a mechanism to change the wavelength selected by the monochromator and to record the resulting changes in the measured quantity as a function of the wavelength.
If an imaging device replaces the exit slit, the result is the basic configuration of a spectrograph. This configuration allows the simultaneous analysis of the intensities of a wide band of colors. Photographic film or an array of photodetectors can be used, for instance to collect the light. Such an instrument can record a spectral function without mechanical scanning, although there may be tradeoffs in terms of resolution or sensitivity for instance.
An absorption spectrophotometer measures the absorption of light by a sample as a function of wavelength. Sometimes the result is expressed as percent transmission and sometimes it is expressed as the inverse logarithm of the transmission. The Beer–Lambert law relates the absorption of light to the concentration of the light-absorbing material, the optical path length, and an intrinsic property of the material called molar absorptivity. According to this relation the decrease in intensity is exponential in concentration and path length. The decrease is linear in these quantities when the inverse logarithm of transmission is used. The old nomenclature for this value was optical density (OD), current nomenclature is absorbance units (AU). One AU is a tenfold reduction in light intensity. Six AU is a millionfold reduction.
Absorption spectrophotometers often contain a monochromator to supply light to the sample. Some absorption spectrophotometers have automatic spectral analysis capabilities.
Absorption spectrophotometers have many everyday uses in chemistry, biochemistry, and biology. For example, they are used to measure the concentration or change in concentration of many substances that absorb light. Critical characteristics of many biological materials, many enzymes for instance, are measured by starting a chemical reaction that produces a color change that depends on the presence or activity of the material being studied. Optical thermometers have been created by calibrating the change in absorbance of a material against temperature. There are many other examples.
Spectrophotometers are used to measure the specular reflectance of mirrors and the diffuse reflectance of colored objects. They are used to characterize the performance of sunglasses, laser protective glasses, and other optical filters. There are many other examples.
In the UV, visible and near IR, absorbance and reflectance spectrophotometers usually illuminate the sample with monochromatic light. In the corresponding IR instruments, the monochromator is usually used to analyze the light coming from the sample.
Monochromators are also used in optical instruments that measure other phenomena besides simple absorption or reflection, wherever the color of the light is a significant variable. Circular dichroism spectrometers contain a monochromator, for example.
Lasers produce light which is much more monochromatic than the optical monochromators discussed here, but only some lasers are easily tunable, and these lasers are not as simple to use.
Monochromatic light allows for the measurement of the quantum efficiency (QE) of an imaging device (e.g. CCD or CMOS imager). Light from the exit slit is passed either through diffusers or an integrating sphere on to the imaging device while a calibrated detector simultaneously measures the light. Coordination of the imager, calibrated detector, and monochromator allows one to calculate the carriers (electrons or holes) generated for a photon of a given wavelength, QE.