Balanced development and the reduction of inequality are central objectives of the United Nations Sustainable Development Goals (SDGs). This study explores the use of Nighttime Light (NTL) brightness and the Nighttime Light Development Index (NLDI) as indicators of socioeconomic development in urban centers, focusing on six Indian cities. It examines the correlation between these indices and socioeconomic inequality across affluent neighborhoods, urban slums, downtown areas, and general urban areas in 2015, 2018, and 2021. The resu lts reveal that lighting brightness in affluent areas can be lower than that in bustling downtowns, due to factors such as lower residential density. This challenges the conventional assumption that higher NTL necessarily indicates greater prosperity. This study further confirmed significant developmental disparities between well-lit downtowns and poorly illuminated peripheral slum areas, as reflected by lower NLDI scores. Notably, the results uncover a phenomenon termed “same value but different spectrum” based on a careful examination of NLDI values of urban centers and their corresponding curves. This suggests that NLDI alone may not fully capture the complexity of urban development, and that underlying development trajectories, along with on-the-ground realities, must be further examined. The findings emphasize the importance of applying NLDI for urban internal analyses. In addition, the study highlights the necessity for nuanced urban planning and targeted policy interventions specifically tailored to the unique conditions of different urban areas.