## 4 Discussion
The contributions of spectral lens transmittance to light-induced melatonin suppression are not well understood due to a lack of established methods for measuring spectral lens transmittance in vivo, although a method based on Purkinje images has been recently developed and validated. In this case, lens transmittance at 480 nm was measured with different results in children (approximately 10 years), young adults (approximately 30 years), middle-aged adults (approximately 40 years) and older adults (approximately 60 years) and was 57.6%, 52.9%, 49.1% and 35.5%, respectively [10; 39]. [PERSON] et al. [10] reported that these findings suggest that lens transmittance and pupil size may contribute to the age-related differences in melatonin suppression, and that information on both parameters may be needed to explain the age-related differences in melatonin suppression between children and adults. In a more reduced range of ages from 25 to 32 years, [PERSON] et al. [40] confirmed that there was no change in the relative relation in the three types of cone and melanopsin stimulus intensity using the model proposed by [PERSON] et al. [3], although they suggested that in experiments in which brightness perception is involved, correction of the SPDs reaching the retina would be needed. We note with our calculations that variations in this age range are slight, but performing lighting projects for areas inhabited by the elderly (hospitals or nursing homes) or even by children (kindergarten or schools) should be properly evaluated and corrected when is needed, as we have proposed, with easy calculations. Another important parameter involved in the amount of light reaching the retina is the aperture of the pupil or the visual field due to eyelids; it is different to what a standard measurement device with a 180\({}^{\circ}\) field of view and cosine correction can measure. Pupil size can change the retinal irradiance over a factor of 1.2 log units from approximately 8 mm at full dark adaptation to 2 mm under bright light conditions [41]. This variation in pupil size has different melatonin-suppressive effects because of the light reaching the photoreceptors; even this parameter is age dependent, in addition to the transmittance of an aged lens. Recently, a more realistic approach of the retinal illuminance level has been proposed to be performed by eye-tracker recoding, allowing us to know the dynamic behavior of the subjects [35; 42]. Relative changes in pupillary area related to age have been described as being responsible for a reduction in retinal illuminance by a factor of approximately two in the eye of a 70-year-old subject with respect to the maximal value of a 15-year-old subject [19]. The total effective relative exposure will depend upon the absolute level of illumination and the duration of exposure, on the assumption that individuals of different ages are exposed under the same conditions, to compare the relative effectiveness of the exposure that they receive. The introduction of an eye-tracker to record realistic setupscould be the best way to accurately compute real values. The overall relative effectiveness will be given by multiplying the spectral radiance of the source, the corneal and lenticular transmitances, the pupillary area, and the action spectrum. The literature reports that the relative effectiveness of daylight and tungsten incandescent light for individuals aged 14, 49 and 92 years with natural crystalline lenses is 1.00, 0.52 and 0.55, and 0.11 and 0.13 for daylight and tungsten, respectively [19]. These values show that despite the different spectral contents of daylight and incandescent lighting, the relative effects of the aging crystalline lens are very similar in all cases.