HPLC detectors which does not allow to change the wavelength of the radiation called fixed-wavelength detectors. They are usually very cheap, in the nowadays you probably wont be able to find that type of the detector on the market.
low-pressure mercury vapor lamp emit very intense light at 253.7 nm. By filtering out all other emitted wavelengths, manufacturers have been able to utilize this 254 nm line to provide stable, highly sensitive detectors capable of measuring subnanogram quantities of any components which contains aromatic ring. The 254 nm was chosen since the most intense line of mercury lamp is 254 nm, and most of UV absorbing compounds have some absorbance at 254 nm.