How Did the Romans Read in Sunlight?

In the heart of Timgad, Algeria, stood the public library of Rogatianus, a Roman building that has fascinated archaeologists and architects for centuries. Recent research used 3D modeling and Radiance software to virtually recreate the lighting environment of its reading hall. The first step compared Vitruvius' architectural rules with findings from previous studies on other ancient libraries. Vitruvius advocated wide openings and layouts that harnessed natural light, but Timgad's climate is brighter than Rome's. Simulations revealed that daylight was not evenly spread: some areas were brightly lit, while others stayed in shade. This suggests ancient readers could choose where to sit based on activity—consulting texts, silent reading, or storing volumes—and perhaps even the time of day. The outcome offers a fresh view of how Roman architecture adapted to local conditions, showing that light was a key factor not only for visibility but also for the functional use of cultural spaces.

The detailed analysis of Timgad's library was carried out using an advanced radiometric simulation pipeline. After laser scanning the site, a polygonal model with sub-centimetric precision was generated, including openings, columns, and interior surfaces. Radiance software, known for its ability to compute spectral light distribution, was configured with historical meteorological data for the region, incorporating the average solar angle for each hour of the day during spring and summer seasons. Illuminance (lux) and Daylight Factor calculations were performed for every point on the analysis grid. Results show Daylight Factor values ranging from 2% to 12%, with peaks in areas closest to the north‑eastern windows. Spectral analysis also revealed a dominance of blue‑yellow light, typical of desert conditions, which could have affected the colour perception of manuscripts. The irregular light distribution suggests a possible functional zoning of spaces: brighter areas for consulting large‑type texts, darker zones for storing scrolls sensitive to UV light. These data support the hypothesis that Roman architects, while following Vitruvian canons, adapted the design to local climatic specifics, optimizing natural light use for various cultural activities.