Design and validation of a compact goniophotometer for luminous flux assessment of endoscopes

Endoscopes are inseparable illumination and imaging tools in technical and medical diagnostics, yet no standardised methodology exists for measuring their photometric parameters. Specifically, the luminous flux assessment of elongated, non-uniform light sources remains insufficiently addressed by existing standards and measurement techniques. This study presents a goniophotometric method dedicated to the quantitative evaluation of endoscopic illumination parameters. A compact two-axis measurement system was developed to enable angular intensity measurements under controlled conditions. Two approaches to luminous flux calculation from discrete angular data were evaluated. Furthermore, the influence of sensor geometry and measurement distance was analysed. The system was calibrated and validated regarding angular accuracy, repeatability, and uncertainty. Experimental validation on selected endoscopic light sources demonstrates stable results with an angular repeatability of 0.6°. Data processing using bivariate polynomial regression achieved R² values up to 0.981. The method addresses a gap in current photometric practice and provides a practical solution for the quality control of non-standard light sources.