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We highlight a physical effect that is often not considered that impacts thecalculation of model spectra of planets at secondary eclipse, affecting bothemission and reflection spectra. The radius of the emitting surface of theplanet is not merely one value measured from a transit light curve, but isitself a function of wavelength, yet it is not directly measurable. At highprecision, a similar effect is well-known in transit "transmissionspectroscopy" but this related effect also impacts emission and reflection. Asis well-appreciated, the photospheric radius can vary across $\sim$4-8atmospheric scale heights, depending on atmospheric opacity and spectralresolution. This effect leads to a decreased weighting in model calculations atwavelengths where atmospheric opacity is low, and one sees more deeply into theatmosphere, to a smaller radius. The overall effect serves to mute emissionspectra features for atmospheres with no thermal inversion but to enhancefeatures for atmospheres with a thermal inversion. While this effect can beignored for current \emph{Hubble} observations, it can lead towavelength-dependent 10-20\% changes in planet-to-star flux ratios in theinfrared at $R\sim~200-1000$ (readily achievable for JWST) for low-gravity hotJupiters, although values of 5\% are more typical for the population. Theeffect is mostly controlled by the ratio of the atmospheric scale height to theplanet radius, and can be important at any planetary temperature. Of knownplanets, the effect is largest for the cool "super-puffs" at very low surfacegravity, where it can alter calculated flux ratios by over 100\%. We discusscomplexities of including this photospheric radius effect in 1D and 3Datmosphere models.

Exploring A Photospheric Radius Correction to Model Secondary Eclipse Spectra for Transiting Exoplanets