Aims. Lightning mapper sensors on board weather satellites can be successfully used to observe fireballs. These sensors use a very narrow spectral band at 777nm, which is only a small fraction of the total fireball radiation. In this spectral band, the oxygen O I-1 triplet is dominant for fast meteors and the Planck continuum can prevail in slow meteors. It is possible to estimate the meteor brightness in the visible spectral range from this narrowband radiation, but it is vital to first study the dependence of this radiation on the meteor velocity. Methods. We used observations from the well-established European Fireball Network with newly developed digital spectral cameras that allowed us to study the oxygen triplet in meteor spectra and its relation to the meteor velocity and altitude. In addition, we studied strong magnesium and sodium lines. Results. We developed a method for calibration of fireball observation reported by Geostationary Lightning Mapper (GLM) sensors on board the Geostationary Operational Environmental Satellite (GOES) weather satellites. We confirm that in slow meteors, the radiation of the Planck continuum dominates, but for faster meteors, a correction on velocity is needed. We observe that the altitude where the oxygen line was recorded can also affect the radiation at 777 nm. In addition, determining whether or not the meteor showed a bright flare could also lead to a similar effect. Thus, the meteor brightness estimate may be impacted by these characteristics. We derived simple corrections on the altitude and on the meteor brightness that helped to improve the overall precision of the magnitude estimate of our sample. This allowed us to estimate the magnitude of meteors observed by GLM with an accuracy of ~ 1 in magnitude. The Na/Mg line intensity ratio was found to be constant for velocities above 25 km/s and increasing toward lower velocities.