Determining empirical atmospheric models for the solar chormosphere is difficult since it requires the observation and analysis of spectral lines that are affected by non-local termodynamic equilibrium (NLTE) effects. Here we present Sunspot’s umbra data taken with the 1.5 m GREGOR telescope are analyzed after the effects of the stray-light on the data are removed using a novel deconvolution technique. The full Stokes measurements of a sunspot near disk center in the near-infrared spectral range were obtained with the GRIS instrument installed at the GREGOR telescope. A point-spread-function (PSF) including the effects of the telescope, the Earth’s atmospheric seeing, and the scattered light is constructed using prior Mercury observations with GRIS and the information provided by the adaptative optics system of the GREGOR telescope during the observations. The data are deconvolved from the PSF using a Principal Component Deconvolution method. The data are then analyzed using the NICOLE inversion code which can take into account the NLTE effects in the Si i 1082.9 nm line. The information of the magnetic field in taken into account within the inversion. Results. We found that the Si i 1082.9 nm line appears in emission in sunspot umbra after the effects of the scattered light (stray-light coming from wide angles) is removed. We show how the spectral line shape changes dramatically with the amount of scattered light. Current umbral empirical models are not able to reproduce the detected emission, taking into account NLTE effects. This is, to our knowledge, the first time the Si i 1082.9 nm line is seen in emission in sunspot umbrae.