In this study was investigated the range of optimal luminance contrast needed to enhance user physiological comfort and psychological satisfaction while viewing displays. Diverse instances of luminance contrast were collected, of which both ambient luminance and object luminance were measured, and subjective judgment was notes for first-time viewing and after continuous viewing. The result revealed that the optimal luminance contrast is not static. The optimal ratio between ambient luminance and object luminance changes gradually as viewing time increases, and in particular, it converges into a smaller range. The optimal brightness of object luminance in a dark environment needs to be increased, whereas that in bright environments needs to be decreased. Therefore, the duration of viewing should be considered to define optimal luminance contrast, and hence a dynamically adaptive luminance contrast is proper to maintain affective viewing quality of internally lit objects such as smartphone displays and e-books.