Previously, the light source of a UV detector was a mercury lamp. This lamp was employed for a fixed wavelength of 254 nm in detectors because of having a bright line (a wavelength with extremely high energy) at 253.7 nm. Fortunately, many components containing benzene rings can absorb light at this wavelength, which enabled many samples to be analyzed with this fixed wavelength. Hence, the detection wavelength of 254 nm is sometimes used, even now.
However, most current UV detectors employ a D2 lamp as the light source, for which the wavelength can be changed. Usually, components are measured not uniformly at 254 nm, but at each component's maximum absorption wavelength, because high sensitivity is required for the measurement.
Here, a question is given. What is the wavelength of the bright line of a D2 lamp? The answer is 656.1 nm. Energy is scantly observed around this wavelength; only this wavelength has high energy. Using this fact, wavelength is checked for deviations in detectors. The L-2000 series detectors can be controlled accurately, because they are equipped with a mercury lamp for wavelength calibration to check the wavelength in the ultraviolet region.