Optimal luminance and chromaticity for viewing mobile displays under low illuminance

Abstract

With the increase in various types of electronic devices, people are spending many hours watching mobile displays. However, the light emitted by these displays exerts some adverse influences on users’ visual comfort and preference, and this becomes more severe in a dark environment. Consequently, the problem caused by inappropriate luminance and chromaticity of displays has become a critical issue. In this regard, this study investigates the optimal luminance and chromaticity when viewing mobile displays under low illuminance by balancing physiological comfort and psychological satisfaction. The study involves three procedures as follows. Firstly, the optimal display luminance was examined by evaluating physiological and psychological responses to diverse levels of luminance. The experimental results indicated that the optimal luminance for first-time viewing of a display differed from the optimal luminance for continuous viewing of a display, and the transition in display luminance between the two viewing conditions should occur slowly. Based on the results, an adaptive luminance model was established in consideration of the time-dependent adaptation of the human visual system. In the model, the display luminance changed gradually with the passage of time spent watching. It began at a luminance of $10 cd/m^2$, and ten seconds later, the luminance started to increase for 20 seconds until it reached $40 cd/m^2$. Secondly, psychophysical experiments were conducted to examine the optimal display chromaticity by reducing blue light without distorting the perceived quality of displays. The subjects judged perceptibility and acceptability of the displays with different types of content and color composition. As a result, a white tinged with yellow was determined to be the optimal chromaticity, but there was some difference depending on the color composition of the content. A slight chromaticity shift to yellow is recommended if a white covers a majority of the display, whereas a large shift is permitted on a full-color display that has no white point. Lastly, the effect of the optimal display was validated by measuring the subjects’ physiological responses such as melatonin concentration, body temperature, and heart rate before and after using the different displays. The experimental result was that the display with low luminance and tinged with yellow did not affect users’ circadian rhythm thus it supports restful sleep, and this confirmed the superiority of the optimal display compared with the current display for using smartphones under low illuminance. We expect that the findings of this study will contribute to the pleasing use of mobile displays by applying to diverse types of visual display terminals, and the optimal display plays a decisive role in the display industry for increasing the design competitiveness of products.