tokens: in Sect. 3.2 . We did not identify any significant gas contamination from other species in any of our spectra. Second, we selected the lines with intensities above a defined threshold (ten times the noise level). We summarize the results in Table 4 . We find that the number of U I lines differs considerably for different lamps (e.g. in the lamp 0127 we find a factor of 1.8 more lines than in the lamp 0120). The number of U II , Ne I, and Ne II lines is essentially constant. Table 4. Number of confirmed isolated U and Ne lines with S/N higher than ten found in the 12 U–Ne spectra for NIR wavelengths. The last column contains the average of the U line intensity normalized to the average of its line intensity in the 12 spectra ( I U ). We looked for the confirmed lines that can be found in all of the 12 spectra to systematically analyse the difference between the 12 lamps. In total, we detected 222 U I and 104 Ne I lines in the spectra of all lamps. We normalized the 12 spectra to the sum of the Ne lines intensities averaged in each spectrum. Then we divided the line intensity of each U line by the average line intensity of this line in the 12 spectra. Last, we calculated the average of the normalized line intensity of the 222 U I lines (named I U in Table 4 ) for each lamp. In Fig. 8 we plot the normalized line intensity of the common U I lines in the NIR spectral range for seven HCLs (for readability reasons). The dashed lines represent I U for each HCL. From Fig. 8 and Table 4 it can clearly be seen that I U varies significantly between the lamps. Lamps with the lowest U line intensities are HKH0120 and HKH0122. Lamps with the highest U line intensities are HKH0127 and HKH131. The maximum U line intensity difference is 88% (with respect to the minimum value), measured between lamps HKH0127 and HKH0120. Fig. 8. Coloured dots represent the normalized line intensity of 222 U I lines for each HCL in the spectral range from 1000 to 1700 nm. Dashed coloured lines indicate of each lamp (i.e. average of the 222 U I normalized line intensity). For readability reasons we plot only seven of the 12 HCLs. A possible explanation for the observed behaviour of the HCLs could be a different geometry of the lamps or in the light-fibre coupling efficiency, as seen in Huke et al. (2016) . This could affect the gas-metal intensity ratio. Another explanation could be that the metal line intensities are very sensitive to the internal pressure of the buffer gas ( Crosswhite et al. 1955 ). Therefore, even small changes in pressure from lamp to lamp would have an effect on the metal line intensity. We conclude that HCLs can show noticeable differences in number and intensity of metal lines even if they are manufactured in a single batch. 7. U line list We concluded in Sect. 4 that U cathode lamps are more suitable for wavelength calibrators than Th lamps from 500 to 1700 nm. To use HCLs as wavelength calibrators for high precision RV spectrographs, the wavelength of the
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