Transillumination as a Clinical Light Detector: A Critical Narrative Review of Optical Principles, Diagnostic Performance and Methodological Constraints

Abstract

Transillumination places a light source on one side of a tissue and observes the light that survives passage to the opposite side, so that internal structure is inferred from patterns of attenuation. The technique is old, inexpensive and technically undemanding, and it has been repeatedly rediscovered as illumination sources, detectors and computational tools have improved. Its clinical fortunes have nonetheless been uneven. Near-infrared transillumination of the dentition is now embedded in commercial diagnostic devices and supported by numerous accuracy studies, whereas optical transillumination of the breast was abandoned after prospective comparison with mammography, and several bedside applications remain supported largely by tradition. This review examines transillumination as a single physical strategy applied across disparate anatomical settings, and asks why performance differs so markedly between them. Literature was identified through structured searching of open scholarly databases and indexes, supplemented by citation searching, and appraised for design adequacy, reference-standard validity, applicability and reproducibility. Three findings dominate the synthesis. Diagnostic reach is set by tissue optics rather than by device sophistication, so the depth, geometry and mineral content of the target largely predetermine what any transillumination system can achieve. Reported accuracy varies less with technology than with the definition of the target condition, which explains the substantial divergence between pooled estimates for dental transillumination derived from different lesion thresholds. Evidence for most non-dental applications remains dominated by small studies, surrogate endpoints and process measures rather than patient-important outcomes. The most defensible conclusion is that transillumination performs well as a triage and localisation aid within a narrow depth range, and poorly as an independent detector of deep or optically heterogeneous disease. Priorities for research include prospective studies using explicit and clinically anchored lesion thresholds, evaluation of downstream treatment decisions rather than image-level agreement, external validation of automated image analysis, and honest reporting of reader dependence.