# Measuring Facial Illuminance with Smartphones and Mobile Devices
## 1 Introduction  
The recent dramatic increase in myopia prevalence, reaching almost 100% in some young adult groups (most notably in East Asia) [1, 2, 3], points to the existence of a causal agent in the modern environment. Notably, in the most recent decades, children have changed their behavior, now spending the majority of their time indoors [4]. Urbanization, modern technology, and modern schooling are all implicated in this behavioural change [5, 6, 7]. Studies have shown that myopia prevalence is lower in children who spend more time outdoors [8] even when both their parents were myopic [9], and that increasing the time spent by children outdoors (by approximately 80 min per day) significantly reduced the incidence and progression of myopia in a population of East Asian school children [10].  
The indoor visual environment differs from the one outdoors in three major ways: less light intensity, different spectral composition of light, and generally nearer viewing distances [10, 11]. Experimental studies of young chicks and monkeys have revealed that low light levels can accelerate eye growth, whereas high light levels can slow eye growth [12, 13, 14]. Furthermore, seasonal variations in eye growth in children in the northern hemisphere provide indirect support for the important role of light level in eye growth regulation [15].  
Studies in human children attempting to uncover a relationship between light exposure and myopia onset and progression are complicated because of the challenging nature of environmental light level monitoring, particularly those features of the luminous environment that affect the retinal image. In many situations, single illuminance measures can only have an approximate relationship with light reaching the retina, because although most of the light entering the eye originates as light reflected by the illuminated environment, the eye's visual field is restricted in its extent. For example, in the case of a child viewing a high luminance mobile device's screen in a dark room, a general room illuminance measure will constitute a poor indicator of central retinal illuminance. Since mobile device use is a risk factor for myopia [16], monitoring of environmental light as a surrogate for measures of retinal illuminance should take into account both diffuse environmental illuminance and any local light sources often occurring in the centre of the visual field.  
Small ambient light sensors which can be mounted to the wrists, clothing, or incorporated into spectacle frames [17; 18; 19; 20; 21; 22; 23] have recently been employed to monitor light levels in children's environments and helped reveal that myopic children, on average, tend to be exposed to less light [8; 24]. Since retinal illuminance depends on the eye pupil illuminance, which depends on the direction of the eye's line of sight, the ability of these light measurements to capture the features of the lighting environment relevant to eye growth is disputable. For example, wrist mounted detectors may be covered by clothing or pointed in a different direction than the eye's line of sight. Moreover, wrist-mounted ambient light sensors integrate light over a wide field of view that may not match the visual field.  
The present report examines the feasibility and accuracy of measuring face illuminance as a surrogate for retinal illuminance using cameras included in mobile devices. If feasibility and accuracy can be demonstrated, the devices which are usually in possession of children at risk of myopic eye growth [25] offer an opportunity to collect face illuminance data from very large populations, with the advantage of not requiring any additional equipment. We propose that face illuminance provides a measure of environmental lighting more closely related to retinal illuminance than alternative wide-field ambient light measures that have been used previously.