Testing Photometric Techniques for Measuring the Rest-frame UV Spectral Slope against JWST PRISM Spectroscopy

We present a sample of 53 galaxy spectra at z SPEC ∼ 5–12 from the JWST CEERS and RUBIES surveys, combining NIRSpec PRISM spectroscopy with NIRCam photometry. We aim to use these data to establish best practices for measuring the UV spectral slope ( β ) in the era of JWST. We adopt power-law fits to the rest-frame UV continuum from the spectroscopic data as our fiducial, or “true,” β values, and compare them to photometric estimates derived through three methods: (1) photometric power-law fitting, (2) power-law fitting to a spectral energy distribution (SED) model fitted to the photometry, (3) single-color fitting near the Lyman break, and (4) single-color fitting at fixed rest-frame wavelengths. We find that photometric power-law fitting and SED fitting are comparable and most closely recover the spectroscopic slopes, with minimal bias and scatter. Single-color estimates, while commonly used in past studies, show the most significant deviations and are not recommended when more than a single color is available. Our results highlight the limitations and strengths of each approach and provide practical guidance for measuring β from photometry when spectra are unavailable or are of insufficient quality.