## 1 Introduction  
Before light reaches the retina, it passes through the cornea, lens and ocular media. This prereceptoral filtering alters the spectrum relative to the light arriving at the corneal plane [1]. Figure 1, reproduced from [2], shows how a crystalline lens increases in density as a function of age and specifically attenuates short-wavelength transmitted light [3; 4; 5]. On the other hand, the melanopsin spectra, which absorbs light in the short-wavelength range of the visible spectrum with a maximum at or near 480 nm [6; 7], is modified by the absorption spectra of the anterior media, shifted from 480 nm to 487 nm (for a 20-year-old observer) or 496 nm (for an 80-year-old observer). As a result of the described behavior, retinal illuminance may vary between individuals due to progressive age-dependent changes in the lens, causing a reduction in the amount of short-wavelength light passing through the eye and scattering [3; 4; 5; 8; 9; 10; 11]. Additionally, the number of intrinsically photosensitive retinal ganglion cells (ipRGGs) drops with age, changes their distribution pattern on the retina and changes their morphology [12]. Consequently, retinal photoreceptors receive less light input at older ages, particularly short-wavelength-sensitive photoreceptors (rods, S-cones and ipRGGs). From a technical point of view, the approach to measuring and reporting light was established by the Commission Internationale de l'Eclairage (CIE) [13; 14]. The 10 nm differential between the aforementioned 480 nm peak spectral sensitivity for melanopsin and the 490 nm peak described by the CIE technical note is attributed, as has just been described, to the prereceptoral filtering of the light reaching the retinal plane, and modifications can be found depending on the age (Figure 2). In addition, the parameter of melanopic equivalent daylight illuminance (EDI) was defined by the CIE to compute the melanopic illuminance level instead of the well-known photometric illuminance quantity, since knowing the spectral power distributions (SPDs) of the source is essential to correctly compute both magnitudes, photopic and melanopic illuminances. Figure 1: Spectral transmittance of the crystalline lens depending on age; \(\tau(\lambda,Y)\), being Y = years. This figure shows the transmittance for each decade of life, taking a 32-year-old subject as the standard. It can be observed that the older the individual, the more absorbed the lower wavelengths become by the crystalline lens. Reproduced from [2]. Figure 2: Melanoma spectral relative sensitivity Smel(\(\lambda\)) with a maximum at 490 nm and spectrum modified by the absorption of the anterior media of the eye shifted for aged observers (from 10 to 90 years old). Data show Smel(\(\lambda\)max)*10Y at 487 nm, Smel(\(\lambda\)max)*20Y at 488 nm, Smel(\(\lambda\)max)*30Y at 489 nm, Smel(\(\lambda\)max)*32Y at 490 nm, Smel(\(\lambda\)max)*40Y at 491 nm, Smel(\(\lambda\)max)*50Y at 494 nm, Smel(\(\lambda\)max)*60Y at 496 nm, Smel(\(\lambda\)max)*70Y at 498 nm, Smel(\(\lambda\)max)*80Y at 501 nm, Smel(\(\lambda\)max)*90Y at 504 nm. Abbreviations: Smel(\(\lambda\)), melanopsin spectra modified by ipRGCs’ (intrinsically photosensitive retinal ganglion cells) absorption; Y = years. The specific physiological effect of light of a given melanopic irradiance, due to the wide range of individual differences to the same corneal spectra, is difficult to personalize; it is easier to estimate or predict than to accurately calculate. The melanopsin spectral sensitivity curve standardized in CIE S026/E:2018 comes closest to these types of standard observers. It shows how a spectrum should be weighted to derive an appropriately weighted quantity. In addition to a standard curve, CIE S026/E:2018 also contains information for generating age-adjusted spectral sensitivity curves, accounting for age-dependent changes in lens transmission [13].