A key characteristic of a spectroradiometer is the instrumental spectral response function (ISRF) that is determined during spectral characterization and calibration. The response shape of the ISRF is commonly assumed to be Gaussian, though this is known to not always be the best description. We show that in the context of laboratory calibration, the largest source of uncertainty lies in the ISRF assumption. We perform the spectral calibration of laboratory measurements obtained with four analytical spectral device field spectroradiometers using several different ISRF "modes" to investigate their respective fitting performance, and examine the impact of choosing an ISRF that differs from a Gaussian when calibrating a MODTRAN6 spectrum. Finally, we conduct the uncertainty analysis of our calibration by propagating uncertainty via a Monte Carlo method.