Optical space-based lightning sensors including NOAA’s Geostationary Lightning Mapper (GLM) detect lightning though its transient illumination of the surrounding clouds. What space-based optical lightning sensors measure is influenced by the physical attributes of the light source, the location of the source within the cloud scene, and the spatial variations in cloud composition. We focus on the lightning channels that serve as optical sources for GLM groups and flashes in this first part of our thundercloud illumination study. We match Lightning Mapping Array (LMA) sources with GLM groups and flashes during two thunderstorms to examine channel segments that are active during optical emission. We find that in each storm, the LMA sources matched with LMA groups are small (median: 2-3 km) compared to GLM pixels (nominal: 8 km), and preferentially come from high altitudes in the cloud (>8-10 km). The detection advantage for high-altitude sources permits GLM to resolve faint optical pulses near the cloud top that might be missed from lower altitudes. However, the most energetic groups can be detected from all altitudes, and the largest groups largely originate at low altitudes. The relationship between group brightness and illuminated area depends on flash development within the cloud medium, and flash development into different cloud regions can be identified by tracking GLM metrics of cloud illumination over time.