Quantitative effects of artificial lighting parameters on rock discontinuity detection using structure-from-motion photogrammetry in tunnel environments

This study quantitatively investigates how artificial lighting and image acquisition parameters control the reliability of Structure-from-Motion (SfM)-based rock discontinuity detection in tunnel environments. Controlled experiments were conducted using 3D-printed rock plane replicas with known CAD-designed geometry and independently orientations. A comprehensive lighting matrix (4500–6500 K; 100–2000 lx) and acquisition design were evaluated using tie-point stability, point-cloud density, spatial resolution, and orientation accuracy. Lighting distance strongly influenced reconstruction quality. Short lighting distances caused severe non-uniformity and hotspotting, with lux ranges exceeding 15,000 lx and uniformity values below 0.2. In contrast, the most uniform conditions within the tested range occurred at 7–8 m, where uniformity exceeded 0.90 and CCT deviation remained below 0.1%. Capture distance was the main factor governing reconstruction performance. Increasing the camera distance from 1.0 m to 2.0 m reduced tie-point counts by 49.6–64.9% (mean 56.6%) and point-cloud density by 65.7–78.3% (mean 75.4%), while mean pixel size increased from 0.247 to 0.468 mm/pix, indicating degraded spatial resolution. Nadir imaging produced 25–35% higher point-cloud density than oblique capture, whereas combined capture improved reconstruction completeness in occluded regions. The most stable results were achieved using a 1.0 m capture distance, 7–8 m lighting distance, 5500 K CCT, and 1000 lx illuminance. The findings establish quantitative, laboratory-derived and preliminarily field-validated operating recommendations that extend conventional tunnel lighting standards toward reliable digital rock mass characterization. The findings establish quantitative, laboratory-derived operating recommendations, with the recommended configuration preliminarily field-tested on one natural granite surface under controlled night-time, low-ambient-light conditions.