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We present a study of two spectral lines, Fe I 6173 Angstroms and Ni I 6768Angstroms, that were candidates to be used in the Helioseismic and MagneticImager (HMI) for observing Doppler velocity and the vector magnetic field. Theline profiles were studied using the Mt. Wilson Observatory, the AdvancedStokes Polarimeter and the Kitt Peak McMath telescope and one meter Fouriertransform spectrometer atlas. Both Fe I and Ni I profiles have clean continuaand no blends that threaten instrument performance. The Fe I line is 2% deeper,15% narrower and has a 6% smaller equivalent width than the Ni I line. Thepotential of each spectral line to recover pre-assigned solar conditions istested using a least-squares minimization technique to fit Milne-Eddingtonmodels to tens of thousands of line profiles that have been sampled at fivespectral positions across the line. Overall, the Fe I line has a betterperformance than the Ni I line for vector magnetic field retrieval. We selectedthe Fe I spectral line for use in HMI due to its better performance formagnetic diagnostics while not sacrificing velocity information

Spectral Line Selection for HMI: A Comparison of Fe I 6173 Å and Ni I 6768 Å