How are wavelength accuracy, wavelength reproducibility, wavelength resolution, and bandwidth defined?

Document ID

Document ID TE614

Published Date

Published Date 27/05/2017
Question
How are wavelength accuracy, wavelength reproducibility, wavelength resolution, and bandwidth defined?
Summary
Definition of wavelength accuracy, wavelength reproducibility, wavelength resolution, and bandwidth
Answer
Wavelength accuracy is the ability of the device to operate at the select a wavelength. In other words if I select a center wavelength of 546nm, the real center wavelength emitted through the sample may be as low as 544.5 or as high as 547.5 based on a wavelength accuracy of ± 1.5 nm (The wavelenthg accuracy varies from instrument to instrument.  Check your owner's manual for your specific instrument specifications).

Wavelength reproducibility is the ability of the device to return to the same wavelength; so again, if I select 546nm, (and the device actually went to 546.8nm due to the wavelength error in accuracy), then set the spec to a different wavelength, and then return it to 546 nm, upon return I would expect the wavelength to be set to the original wavelength +/- 0.1nm or between 546.7 and 546.9nm.

Wavelength resolution is the wavelength selection resolution.  All of these first 3 parameters are related to the ability of the device to rotate the diffraction grating to known position (accuracy), a specific position (precision), and with resolve (resolution).

Bandwidth is related to the spectral purity of the light emitted through the sample and is controlled mostly by the exit slit width. In contrast, a laser produces true monochromatic light or light of only one frequency. A monochromator selects a small sampling of a dispersion spectrum which generally contains more than one frequency, or wavelength of light. As per our example above, selecting a wavelength of 546nm, (actually 546.8nm), the emission through the sample would comprise wavelengths 544.3 to 549.3, a bandwidth of 5 nm. The slit width is generally fixed in most spectrometers. Although the bandwidth of a monochromator is mostly controlled by the exit slit width, in a monochromator with a fixed exit slit width, the incident angle of light which is dispersed by the diffraction grating can also contribute to small bandwidth changes as a function of wavelength.