This study investigates the effectiveness of narrow-band illumination (NBI) in medical endoscopic imaging, particularly for enhancing the visualization of vascular patterns in the human tongue. The research compares the contrast between vascular patterns and adjacent mucosa using five narrow-band illuminations and three broadband illuminations. The results show that the contrast of capillary patterns is significantly improved with narrow-band illumination at 415 ± 30 nm, which is crucial for early cancer detection. The study also evaluates the appearance of capillary patterns on color images using three sets of filters and selects the NBI filter set (415 ± 30 nm, 445 ± 30 nm, 500 ± 30 nm) for clinical tests. Clinical tests in colonoscopy and esophagoscopy indicate that NBI is useful for visualizing early cancers and small adenomatous polyps, enhancing the accuracy of biopsy and reducing the risk of missed diagnoses. The findings suggest that NBI can be a valuable tool for improving the detection and classification of early gastrointestinal cancers.This study investigates the effectiveness of narrow-band illumination (NBI) in medical endoscopic imaging, particularly for enhancing the visualization of vascular patterns in the human tongue. The research compares the contrast between vascular patterns and adjacent mucosa using five narrow-band illuminations and three broadband illuminations. The results show that the contrast of capillary patterns is significantly improved with narrow-band illumination at 415 ± 30 nm, which is crucial for early cancer detection. The study also evaluates the appearance of capillary patterns on color images using three sets of filters and selects the NBI filter set (415 ± 30 nm, 445 ± 30 nm, 500 ± 30 nm) for clinical tests. Clinical tests in colonoscopy and esophagoscopy indicate that NBI is useful for visualizing early cancers and small adenomatous polyps, enhancing the accuracy of biopsy and reducing the risk of missed diagnoses. The findings suggest that NBI can be a valuable tool for improving the detection and classification of early gastrointestinal cancers.