We have been researching subjective evaluation experiments using Visual Analog Scale (VAS). VAS has the advantage of expressing subjectivity more intuitively. In this paper, we report an attempt of the subjective evaluation related to human lightness sense as fundamental research on what kind of color sense human has. We used VAS to obtain answers on where each gray image with different lightness corresponds to on the grayscale. Numeric values obtained by this evaluation experiment were analyzed to confirm the characteristics of the sensitivity of the individual. Moreover, at the same time, it was examined whether the VAS accurately reflected the subjective view of the person by the answer of what percentage of the grayscale was by the numeric value input. In order to conduct experiments smoothly, we developed an app that can perform VAS measurement and numeric value input simultaneously. We presented ten grayscale images to 13 subjects and obtained subjective measurements using this app. It was confirmed that when the lightness of the grayscale image is high, it can be recognized relatively accurately, but when the lightness is low, it tends to feel darker.
