LED Illumination for Fluorescence–Based Lesion Observation Using a Balanced Beam Diffusion and Concentration Approach

Fluorescence emission–guided blood flow and lymph node location detection are important observation methods in cancer removal surgery, where near–infrared LED illumination is used to induce fluorescence emission. However, conventional LED light sources have narrow beam widths, resulting in a limited excitation area and a restricted field of view (FOV). In this study, we propose a balanced optical illumination module that combines a beam–focusing condenser lens and a beam–diffusing lens to expand the beam width while efficiently redistributing optical energy. When only the LED was used, the beam diameter and central irradiance were 4.0 cm and 1.43 mW/cm2, respectively. With the condenser lens, the beam diameter remained nearly unchanged (3.98 cm), while the central irradiance decreased to 0.91 mW/cm2. When the condenser was combined with the proposed diffuser structure, the beam diameter increased to 14.1 cm, corresponding to an approximately 3.5–fold expansion, while the central irradiance was measured at 0.72 mW/cm2, reflecting the redistribution of optical energy from an initially Gaussian–like irradiance distribution into a wider and more uniform illumination area. This irradiance level exceeds the minimum threshold of 0.6 mW/cm2 required to induce fluorescence emission, as defined for the experimental working distance of 30 cm and LED power of 200 mW. By integrating the irradiance distributions of both the bare LED and the proposed structure over their respective illuminated surfaces, the measured total power is physically consistent with energy conservation, showing an expected transmission loss of 18.8% due to optical absorption and scattering. These results demonstrate that the proposed beam diffusion–concentration approach provides an effective and practical solution for wide–field fluorescence–guided lesion observation during cancer removal surgery.