This study investigates the just-noticeable difference (JND) in luminance below the 0.050 c d / m 2 threshold not covered by the DICOM standard in medical imaging displays. A total of 21 healthy young adults were tested using an ultralow-luminance liquid crystal display at a viewing distance of 46 cm. The optotype featured a 4 cycle/deg rectangular wave-like stripe. The average percentage of correct responses for JND indices between -18 and 0 was 70.8%. Using the extended JND based on the Barten model that matches the current JND definition, we confirmed the JND at a very low luminance of 0.0500 c d / m 2 or less. These findings suggest the feasibility of further refining the gradation differences in medical imaging displays.