The main finding of this study was that high illuminance rather than relatively high short wavelengths is the main factor for the difference of myopia risk between indoor and outdoor environments. Specifically, regardless of whether the content of short wavelengths was increased or not, high illuminance light can significantly retard the progress of myopia compared with normal illuminance light. Under the condition of high illuminance, the effect of light with increased short wavelengths or with normal short wavelengths on refractive state and ocular components were the same. Further grouping analysis also showed the same result. There was no significant difference in refractive state and axial parameters between these two groups which was divided by the content of short wavelengths; At the same time, the result showed that high illuminance (no matter the content of short wavelengths) can significantly retard the progress of myopia and delay the increasing of vitreous chamber compered to the low illuminance group.
Previous studies have shown that individuals who spend more time outdoors have more hyperopic refractive errors and a lower prevalence of juvenile-onset myopia.[31–34] The protective effect of time outdoors is not associated with sporting activities, nor the time spent in near work. Instead, it is the total amount of time outdoors that appears to be important.[34, 35] Although the mechanisms underlying this protective effect are not well understood, a review conducted by Lingham, et al. reported that bright light and spectral composition were relatively significant among all the evidence listed.
In this study, even if the blue LED lamps were added to increase the content of short wavelengths, the difference in refractive development and ocular components between animals raised in lower proportion of short wavelengths lighting and higher proportion of short wavelength lighting were not significant when the illuminance were the same. On the contrary, the decreasing of hyperopia and the increasing of vitreous chamber of animals reared in high illuminance light were obviously smaller than that of the low illuminance group, no matter the spectral composition. Research in chickens has provided that high lighting levels, either from sunlight or intense laboratory lights, all reduces the degree of axial myopia produced by form deprivation by 65% over a 4-day treatment period. In chicks reared with normal visual development, emmetropization is also slowed by high light levels, leading to more hyperopic refractive errors. Though it is not reasonable to extrapolate the results from chickens to humans, because of the differences in species and eye size. Smith et al. found that absolute light levels can have a significant impact on vision-dependent ocular growth in primates (rhesus monkeys). Hua et al. also found that increasing the light levels in classrooms can reduce the incidence of myopia in children and have a protective effect on myopia. Recently, Lanca et al. reported that even if certain protective measures were taken, the illuminance of outdoor light was significantly higher than that of indoor light, which played a protective role on the progress of myopia. Although there will be some differences between the spectral composition after protection with indoor light, it also means that light intensity plays a majority role in reducing the risk for myopia after increasing time outdoors, and the role of spectral composition is relatively limited.
However, not all studies support the protective effect of intense indoor lights on myopia. Smith, et al. assumed that increasing light intensity did not alter the final amount of myopia change in monkeys wearing monocular − 3.0D lenses. But exposure to sunlight for 3 hours a day significantly reduced the progress of refractive myopia in normal eyes and negative lens-induced eyes of young monkeys. Similar findings were also confirmed in the research conducted by Yang et al. Considering the significant difference in light levels and spectral composition between indoor and outdoors, and the theory that relative myopic defocus over a large portion of the retina have been shown to produce clinically meaningful reductions in myopia progression.[43–45] In their opinion, the protective effect of increasing outdoor time on myopia is also related to spectral components and the mechanisms might be both complex and phase-dependent. Because the differences in the spectral composition of ambient lighting could interfere with the eye’s ability to recognize the sign of optical defocus.
In this study, high-blue at normal illuminance lighting had little effect on refractive development compared with normal room light, the refractive response was similar to that seen with control group (Fig. 2D, p = 0. 381). At the end of intervention, the refraction and ocular components were all similar to that of control group (p༞0.05). In the past, a study performed by Rohrer et al. found that refractive development in chickens was not different from controls in white light for either red or near-ultraviolet light. Similarly, animals exposed to ultraviolet light or white light also have no significant difference in compensation for myopia induced by negative lenses. Recently, results from Liu et al. still conformed that no significant difference in mean refraction was observed between the rhesus monkeys raised in blue light and white light. The results of this study were basically consistent with the research above. But study performed by Foulds et al. assumed that chicks developed hyperopia when reared in light consisting mainly of shorter wavelengths, and a large number of studies confirmed that short wavelength light can protect against myopia progression in different species in recent 10 years.[15, 50–52] In their opinion, tipping the balance towards activation of S-cones by enhancing the contrast of short wavelengths could be protective against myopia for enhanced S-syndrome may be more hyperopic. In addition, short wavelengths can focus in front than long wavelengths after passing through the refractive system of the eye to form myopic defocus, which can inhibit the growth rate of eye length. Unfortunately, such protective effects have not been found in this work, though the blue cones are distributed in rabbit retina. Furthermore, Long et al.and Tori et al.[55, 56] found that even in an environment exceeding species’ spectral sensitivity, the animals also experienced different refractive compensation compared with normal room lighting, and this phenomenon can not be explained by vision defocus or the activation of S-cones. Besides, combined with the refractive development of the eye, almost all vertebrates (including humans) show hyperopia in their refractive state in infancy. Even if the content of short wavelengths in illumination is increased, the focus plane is still behind the retina, which cannot form the so-called "myopic defocus effect". Therefore, the reason for spectral differences which affect the emmetropization process need to be explained by other characteristics of light.
As a kind of electromagnetic wave, the propagation of light is also a kind of energy transmission. Combined with the photon energy of different wavelengths and the application of photo biotherapy in recent years, we speculate that the influence of light on refractive development may also be related to the irradiance of light and the energy conversion of different wavelengths received by retina. As shown in Table 1, high illuminance always means high irradiance; under the same illuminance, more short wavelengths is related to more irradiance. The retina of animals exposed to high illuminance or high short wavelengths lighting receives greater energy than ordinary indoor lighting after light passes through the refractive system of the eye. Experiment conducted by Torii et al. confirmed the inhibitory effect of ultraviolet on myopia progress in chickens. Even if the focal plane was behind the retina due to negative lens induction, the myopia changes of chickens raised in lighting with ultraviolet was significantly lower than that of the animals raised in lighting without ultraviolet. Moreover, when the irradiance is the same, no matter the white lighting with ultraviolet or only monochromatic blue light, the inhibition effects on lens-induced myopia of chickens were almost the same. However, in this study, the output of short wavelengths we controlled in high-blue groups were more suitable for the outdoor lighting in real life, significantly lower than other experimental studies. Therefore, even if the content of short wavelengths was increased, the difference of overall irradiance is still low. Which may be one of the reasons for a lack of statistical difference of refractive development between normal blue groups and high-blue groups.