Introduction
People’s lives have been deeply affected by COVID-19.1 2 To prevent the spread of COVID-19, traditional teaching modes have been replaced by online teaching at nearly all education levels, and this situation may continue during the postpandemic era.3
The daily online course time is 1 hour for grades 1 and 2 and 2.5 hours for grades 3 –6.4 Children aged 8–12 years in the USA were found to spend 4–6 hours a day watching or using screens.5 One study revealed that the prevalence of myopia after COVID-19 was approximately three times higher than in other years for children aged 6 years, 2 times higher for children aged 7 years and 1.4 times higher for children aged 8 years.6 Although the increased prevalence of myopia precedes the advent of smart devices, it has been suggested that these devices could exacerbate the myopia epidemic.
Shorter reading distance is widely considered a significant environmental risk factor for myopia incidence and progression.7 8 The Chinese Ministry of Education proposed 33 cm as a ‘watershed’ between fast and slow myopia progression7 9 10 and the appropriate distance from the eye to the viewing panel when children are reading or writing.11 For backlit displays, relatively small screens may necessitate close viewing distance (VD) and small text sizes, increasing the accommodation and vergence demand and thus accelerating myopia progression.12 13 Another display parameter, pixels per degree (PPD), referring to the average number of pixels in every 1° field angle, should also be considered. A lower PPD leads to the screen door effect, which describes the visible gaps between actual pixels. This effect is induced by shorter distances and resolved by increasing resolution. PPD should be at least 60 to avoid the screen door effect.14 15
The VD of most studies was acquired by questionnaires or by using rulers and a continuous shooting system, or the participants’ heads were fixed using a chin-support tripod without regard to how people actually read in daily life. This study used a Clouclip device (JingZhiJing Technology, Hangzhou, China), which is attached to the spectacle arm and can measure both light intensity and distance. The collected data reflect the experience of the eye itself.16
This study aims to explore the optimal font sizes and acceptable viewing time during children’s online learning to guarantee a suitable VD and PPD for backlit devices.