Spectral Assessment of LED Heritage Lighting: Visual Quality and Photobiological Impacts Beyond CCT Specifications in Protected Areas

Correlated color temperature (CCT) thresholds are widely used in European lighting regulations to limit the ecological impact of artificial light at night in protected areas, yet CCT does not uniquely determine the underlying spectral power distribution. Prior studies have examined receptor-specific responses to CCT in isolation – human circadian stimulation, sky-glow potential, or insect phototaxis – but none has simultaneously quantified intra-CCT spectral variability across the full range of receptor systems ecologically relevant to a legally protected natural area. This study provides the first receptor-resolved, multi-metric quantification of intra-category spectral variability for heritage façade lighting in such a context. A Neo-Gothic monument adjacent to the Natura 2000-protected Knyszyńska Primeval Forest, Poland, served as a case study. The spectral power distributions of commercial LED sources spanning 2200–6500 K were evaluated against a metal halide reference using 11 metrics encompassing human circadian response, scotopic-to-photopic (S/P) ratio and sky-glow index (SLI) for sky-glow potential, receptor-weighted effectiveness for nocturnal fauna, and phytochrome sensitivity for plant photoreceptors. Warm-white sources (2200–2700 K) consistently produced the lowest ecological impacts across all evaluated metrics; the S/P ratio and SLI were mutually consistent and confirmed that all warm-white CCT categories fall below the metal halide reference in sky-glow potential, in line with CIE PS 003:2025 guidance. The most critical finding is the magnitude of intra-category spectral variability: at the regulatory threshold of 3000 K, differences between sources sharing the same nominal CCT exceeded in several cases the ecological gap between entire adjacent CCT categories. Notably, the phytochrome metrics revealed a non-monotonic CCT dependence – a pattern invisible to CCT specification alone – demonstrating that plant photoreceptor impacts cannot be inferred from color temperature. These results demonstrate that nominal CCT compliance, even at values not exceeding 3000 K, provides only partial ecological assurance; verification of the spectral power distribution is necessary to guarantee meaningful environmental protection in ecologically sensitive heritage lighting applications.