Objective Monitoring of Tablet Use-Related Optical Exposure and Its Association With Axial Length in Preschool Children: Cross-Sectional Intelligent Monitoring Study.

Background: In recent years, the global prevalence of myopia among children has continued to rise. The preschool years represent a critical period for visual development, and the widespread adoption of electronic screens among young children has brought increasing attention to pediatric visual health. However, the association between visual environmental exposures related to screen use-such as screen brightness and ambient illuminance-and the risk of myopia in preschool children has not been thoroughly investigated. Objective: This monitoring study aimed to investigate the association between electronic screen brightness, ambient illuminance, and axial length in preschool children to provide evidence-based support for developing screen brightness usage recommendations for this population. Methods: This cross-sectional monitoring study was conducted between March and July 2023 in Shanghai, China, involving 2 representative samples of kindergarten children aged 3 to 6 years. Each participant was provided with a tablet preinstalled with intelligent monitoring software, which continuously and objectively recorded real-time data on screen time and screen brightness over a consecutive 7-day period. In addition, comprehensive data collection encompassed standardized ophthalmic assessments, high-precision ambient illuminance measurements, simulated laboratory lighting evaluations, and parental questionnaires. Associations between ambient illuminance, screen brightness, and axial length were analyzed using multivariable linear regression and restricted cubic spline models. Results: Of the 199 children included in the total sample, 124 (62.3%) were boys, and 75 (37.7%) were girls. After adjustment for demographic characteristics, parental myopia, and screen use behaviors, the median ambient illuminance during tablet use was significantly inversely associated with axial length (β=-0.13, 95% CI -0.22 to -0.04; P=.006). A nonlinear dose-response relationship was identified between median screen brightness and axial length (Pnonlinearity=.004), with axial elongation accelerating beyond approximately 27 cd/m² and peaking around 56 cd/m². Boys (P<.001) and greater height (P=.33) were also significantly associated with longer axial length. Conclusions: Higher ambient illuminance during tablet use is associated with shorter axial length in preschoolers, whereas screen brightness exhibits a nonlinear effect on axial elongation. This study highlights the importance of optimizing both environmental lighting and device settings to protect visual health in young children, providing empirical support for guidelines on safe digital device use and ambient lighting conditions in early childhood.