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Abstract
The optimization of trichromatic white light emitting diodes (LEDs) spectrum for application scenes related to the age of lighting users is proposed and demonstrated. Based on the spectral transmissivity of human eyes at different ages, the visual and non-visual responses of human eyes to different wavelengths of light, we have built the blue light hazards (BLH) and circadian action factor (CAF) related to the age of the lighting user. The BLH and CAF are used to evaluate the spectral combinations of high color rendering index (CRI) white LEDs obtained from different radiation flux ratios of red, green, and blue monochrome spectrum. The best spectra of white LEDs for lighting users at different ages in work and leisure scenes are achieved due to the optimization criterion of BLH proposed by us. This research provides a solution for intelligent health lighting design applicable to light users of different ages and application scenes.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, LEDs have been widely used in general lighting. Compared with other artificial light sources, LED lamps have the advantages of high luminous efficiency, energy-saving, long-life time, and eco-friendliness [1–3]. However, LED also contains rich blue light components, which may cause circadian rhythm disturbance or eye damage [4–7] if a person is exposed to excessive blue light. The BLH has become a risk that must be avoided for photo biological safety [8,9], but blue light is also an indispensable part of the visual processes including color perception [10–12]. Therefore, in the design of LED light source, optimizing the spectrum and reducing the BLH have received great attentions. Spectral optimization of white LEDs by considering both visual and non-visual aspects, the best peak wavelength combination of RGB-LEDs were proposed with high CRI and better non-visual biological effects [13–15]. Low BLH, high luminous efficacy of radiation (LER, ≥ 297 lm/W) and high CRI (≥90) at correlated color temperatures (CCTs) from 2013K to 7845 K were achieved [16]. Purple, azure, green, and red, four monochromatic LEDs or more complex composition mixed hybrid white light with low BLH (<0.06), high levels of visual performance (CRI > 84.9) [17–18]. These studies have made essential explorations for obtaining high-quality healthy light sources.
However, it must be pointed out that LEDs are often used in different application scene for different users. These application scenes include work scene and leisure scene. In the work scene, such as in an office, the lighting users need to keep attention focused to achieve higher work efficiency, while in the leisure scene, such as in a living room, the lighting users need physical and mental relaxation. Light source can make people adapt to different application scene by regulating the secretion of human melatonin [19]. The function of light source in regulating the secretion of human melatonin depends on the non-visual radiation response of human retinal photosensitive nerve cells to the spectrum, which is characterized by the CAF [20]. Due to different ages, the level of light transmittance of the human eye is also different, and the light source produces different BLH and CAF [21,22]. Therefore, for more refined light source design, it is necessary to consider both light source application scene and lighting user age factors. There is no systematic research in this area at present.
In this paper, we systematically explored the optimization of RGB white LED spectrum related to age and application scene. Based on the spectral transmittance difference of human eyes at different ages, we theoretically defined the calculation of age-related BLH and CAF parameters, and built an age-related white light spectrum optimization model based on RGB-LEDs for different application scene. An optimization criterion that BLH of illumination light source should less than the BLH 6500 K day light to 1-year old children is proposed. As the limiting criterion of BLH, we obtained the high quality lighting spectrum of different application scene with significant age-related.
2. Model building
The light transmittance of the human eyes is determined by the crystalline lens, which becomes yellow gradually and the transmittance of human eyes changes significantly. This physiological characteristic of the visible spectral transmittance can be described by the results shown in Fig. 1 [23]. In this figure, the spectral transmittance of all light in the visible wavelength range decreases with age. We can use $\tau ({\lambda ,A} )$ to indicate the relation between the spectral transmittance and wavelength, age, and A refers to the age of a lighting users.
We consider incorporating the above age-related spectral transmission factors into the evaluation of the non-visual performance of the spectrum. The evaluation involves the planning of the BLH and CAF parameters, which are used to characterize blue light hazard and circadian rhythm respectively. The age-related BLH and CAF are defined as follows,
Fig. 2. Eye spectral sensitivity function for blue light hazard $B(\lambda )$, non-visual effect $C(\lambda )$ and photopia vision $V(\lambda )$, the peak wavelength of which are 437 nm, 464 nm, and 555 nm, respectively.
To explore the optimal spectral of white LEDs, a white light source with RGB-LEDs is considered in our optimization process. The peak wavelength and half peak width (WLP, FWHM) of red, green and blue monochrome LED spectra are (460 nm, 20 nm), (536 nm, 30 nm) and (609 nm, 20 nm) respectively. It could provide a good color rendering performance of the lighting source according to previous study [24]. The normalized spectral shape of each monochromatic LED refers to Gaussian shape as shown in Fig. 3. In the spectral optimization calculation, first, we obtain candidate spectra by the fraction of radiation flux for each monochromatic LED is adjusted between 0% and 100%, with a step of 1%, and the sum of three monochromatic LED radiation flux is set as 1. Second, we calculate the BLH, CCT, CRI, Duv value of each candidate spectrum. Third, we screen the spectra from which BLH, CCT, CRI, Duv parameters meet the optimization conditions. Fourth, from the screened spectra, we select the CAF with the largest for work and the CAF with the smallest for leisure.
Fig. 3. The normalized spectra of red, green, and blue LED of white LEDs are composed. The spectrum is Gaussian shaped. The values in the legend are WLP and FWHM for each monochromatic LED.
3. Optimization criteria and results discussion
The effective BLH and CAF for people aging from 1 to 100 years old at different CCTs are calculated according to Eq. (1), (2). The change curve of BLH and CAF are shown in Fig. 4. It can be seen from Fig. 4, that the BLH and CAF of the white LEDs increase with the increase of CCT. These changes are related to the age of lighting users, the younger the age, the higher the growth rate. From the perspective of age, the BLH and CAF decrease with the increase of the age of the lighting user. The lower blue light spectral transmittance of the ocular media reduces the BLH and CAF for the older lighting users and with the increase of CCT, the growth rate of BLH and CAF decrease rapidly with the increase of lighting user age. When the CCT is lower than 3000 K, the BLH and CAF change little with the lighting user age, and the values of BLH and CAF for people aging from 1-100 years old are relatively small. From the above analysis, we can see that the harm of blue light is more serious for young lighting users when they are exposed to high CCT of white LEDs environment. Therefore, in the design of white LEDs light source, lighting users, especially younger lighting users, should be considered to maintain high working efficiency in the working scene while reducing BLH as much as possible. Then, it is necessary to set a limit value of BLH as the criterion for light source optimization to obtain the appropriate CCT. Low CCT light source shall be used as far as possible during leisure scene to reduce the inhibition of light source on human melatonin secretion, so that people can obtain physiological and psychological relaxation.
We use two typical scenes, office and living room, to consider age-related spectral optimization. In order to seek out the optimal spectral of white LEDs for office and living room scene, the optimization criteria of both visual and non-visual factors is considered simultaneously. In visual aspect, the spectral of white LEDs whose CCT between 2700K-6500 K and CRI greater than 80 is the high-quality lighting source, as specified by the ANSI C78.377-2008. In terms of non-visual factors, we proposed an optimization criterion that the effective BLH of all the chosen spectral of white LEDs are almost equal and lower than that of the 6500 K daylight (D65) for 1-year-old observer, i.e. $BLH({A\textrm{, }P(\lambda )} )\le BLH({1\textrm{, }D65} )$. D65 refers to the artificial light source with a color temperature of 6500 K and the spectrum closest to the daylight. Its blue light hazard is also similar to the daylight, and is used by CIE to represent the healthy light source of daylight. When we design the hybrid white LED light source, we choose the blue light hazard value of D65 as the maximum limit value, then we can obtain high-quality health light source with less blue light hazard. Here, the spectrum of hybrid white LED we designed may be completely different from that of D65, but its blue light hazard is lower than that of D65, which is a safer and healthier light source. On the other hand, the blue light hazard is also different for different age light users. Compared with old people, infants are more sensitive to the problem of blue light hazard. Selecting a light source with lower blue light hazard to 1-year-old infants can ensure that the light source is harmless to all age audiences. In combination with the above two considerations, we selected the BLH (1, D65) as the upper limit, which can ensure the safety of the selected LED light sources in terms of blue light hazard. In addition, the effective CAF was another noteworthy aspect. The CAF should be as large as possible in work scene and as small as possible in leisure scene. This ensures that people who work in the office can be alert and excited to work efficiently, while those who rest in the living room can relax and sleep peacefully. Therefore, we can choose the spectrum with the largest CAF for work and the spectrum with the smallest CAF for leisure from the spectrum with the blue light hazard meeting the conditions.
According to the optimization criteria proposed above, we can find out the optimal combinations of the three monochromatic LEDs. The ratios of the three-channel LEDs, the SPDs, as well as the visual and non-visual parameters of the optimal white light for the lighting users of different ages in office scene or living room scene are figured out in Table 1. Typically, the white light for the lighting users of 1, 10, 20, 30,40,50,60,70,80,90, and 100 years old in work scene or leisure scene are listed. The CRI of the optimal white light is above 80 for all spectra. The best spectra for working in the office are the same for 1, 10, and 20 year olds. The radiant flux fraction of red, green and blue LED in the best spectra corresponding to work in the office for 1, 10, and 20 year olds is 48%, 38%, and 14%, respectively. The CCT of the best spectra corresponding to work in the office for 1, 10, and 20 year olds is 3295 K. For ages 30 to 80, the best spectra for office work varies with age. The CCT of the best spectra corresponding to work in the office for 30 to 80 year olds is range from 3572 K to 5750 K. The best spectra for working in the office are the same for 90 and 100 year olds. The radiant flux fraction of red, green and blue LED in the best spectra corresponding to work in the office for 90 and 100 year olds is 28%, 37%, and 35%, respectively. The CCT of the best spectra corresponding to work in the office for 90 and 100 year olds is 6877 K. The best spectra for relaxing in the living room are the same for people of all ages. The radiant flux fraction of red, green and blue LED in the best spectra corresponding to relax in the living room is 55%, 36%, and 9%, respectively. The CCT of the best spectra corresponding to relax in the living room is 2789 K. All optimized spectra can ensure that people of all ages are not exposed to serious blue light hazards, while their corresponding CAF is maximal for work scene and minimum for leisure scene. People who work in the office can be alert and excited to work efficiently, while those who rest in the living room can relax and sleep peacefully.
Table 1. The parameters of optimal white light for lighting users of different ages in work scene and leisure scene
We plot the optimal spectral for lighting users of different ages in office and leisure scenes based on the data in Table 1, as shown in Fig. 5. Figure 5(a) shows the best spectral of lighting users aged 1, 10 and 20 in the work scene. Figure 5(b)-(g) show the best spectral of lighting users aged 30 to 80 in the work scene. Figure 5(h) shows the best spectral of 90 to 100 year old lighting users in the work scene. Figure 5(i) shows the best spectral of all lighting users in the leisure scene. The fraction of radiant flux of red LED, green LED and blue LED in each spectrum has been marked in the position corresponding to their WLP. All these age-related optimized spectra of different scenes can find the corresponding target light color in the color coordinates shown in Fig. 5(j), which shows that it is feasible to consider both the age of the lighting user and the application scenes in the light source design.
Fig. 5. (a)-(i)The spectra and (j) Color coordinates corresponding to the most suitable lighting source for users of different ages in work scene and leisure scene.(a) in office for 1, 10, and 20 year olds (b)-(g) in office for ages 30 to 80 (h) in office for 90 and 100 year olds (i) in living room for all ages. (j) Color coordinates of all the optimal spectra. The fractions of radiant flux of each monochromatic LED in each spectrum have been marked out in the position corresponding to their WLP.
The innovation of this study is that we have proposed the limiting value of BLH as optimization criteria for the spectra of RGB white LEDs, and obtain the best spectra design based on the age and application scenes of the lighting users. This exploration is valuable for the realization of high quality RGB white LEDs illumination.
4. Conclusions
In this paper, we have built the age-related BLH and CAF of light source, and proposed the optimization criterion that BLH of illumination light source should less than the BLH 6500 K day light to 1-year old children. The best spectra of RGB-LEDs white light source for different age lighting users in leisure and work scenes have been found. The results of this study can be used to implement intelligent and adjustable high-quality light source design for the application scenes of lighting users at different ages.
Funding
National College Students Innovation and Entrepreneurship Training Program (202210403010).
Acknowledgments
This work was supported by the 2022 Innovation and Entrepreneurship Training Program for College Students of China.
Disclosures
The authors declare that they have no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available from the corresponding authors upon reasonable request.
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