# A Novel Fluorescent-Material-Based Simple Method for Sunscreen Evaluation
## 1 Introduction  
Recently, the harmful effects of UV rays on human skin have attracted increasing attention. To protect the skin from these harmful effects, sunscreens have been developed, and the performance of sunscreens is expressed by two values: SPF and PA. SPF stands for the sun protection factor, and the higher its value, the better the performance in shielding ultraviolet B (UVB) radiation in the wavelength range of 290-320 nm. PA stands for the protection grade of UVA, and this index is used in Japan and Korea. UVA is ultraviolet A, in the wavelength range of 320-400 nm. The UVA protection factor (UVAPF) has been used in Europe since 2013. The higher the level of PA or UVAPF, the better the shielding performance against UVA [1, 2]. For many years, SPF and PA (UVAPF) values have been measured according to regulations set by the Comite de Liaison des Industries de la Parfumerie (COLIPA) in Europe and by the Food and Drug Administration (FDA) in the United States [3, 4, 5]. Additionally, ISO 24444 and ISO 24442 were published in 2010 and 2011 [6, 7], respectively. These standards regulate the evaluation method for sunscreens and unify the index of shielding effectiveness against UVA to UVAPF. Before ISO 24444 and 24442, sunscreens were evaluated not only by visual changes in skin color caused by UV rays irradiated from a predetermined light source, but also by hemispherical transmittance measurement [8]. The existing COLIPA and FDA standards do not specify a method for determining UVAPF values but stipulatethat SPF values are determined by hemispherical transmittance measurement of sunscreens using an integrating sphere. Alternatively, the ISO standard specifies that both SPF and UVAPF values should be determined through visual evaluation. However, the visual observation evaluation method may yield different results depending on the nature of the subject's skin, differences in health status, and the evaluator's skill. Therefore, it is necessary to conduct a quantitative evaluation by measuring the hemispherical transmittance of the sunscreens. When measuring the hemispherical transmittance of sunscreens, the integrating sphere has long been used as a basic tool [9], and research has continued to not only expand the range of types of measurement objects and wavelengths [10; 11; 12; 13; 14; 15; 16] but also to improve the accuracy of measurement [17; 18; 19; 20; 21; 22; 23] with the details of a few studies described as follows. [PERSON] et al. [11] studied a system to simultaneously measure the scattering coefficient, absorption coefficient, and anisotropy factor to determine the optical properties of biological tissue. In this study, a measurement system with a scattering sample placed between two integrating spheres was constructed to measure the scattering parameters experimentally and evaluate them theoretically. [PERSON] et al. [15] proposed a method to measure the fluorescence quantum yield using a commercial spectrophotometer with a 150 mm integrating sphere detector. They obtained the fluorescence quantum yield from a combination of absorbance measurements of the buffer and analyte solution inside and outside the integrating sphere. [PERSON] et al. [19] developed a spectroreflectometer/transmissometer that can determine absolute optical characteristics in the wavelength range of 300-2600 nm by using three rapidly interchangeable 20 cm integrating spheres. They showed that these values can be measured with extremely high accuracy. [PERSON] et al. [23] developed an accurate method to determine the optical properties of scattering media using an integrating sphere-based setup based on theoretical investigations of the light propagation within an integrating sphere. They tested their experimental system or model with Monte Carlo simulations and in practice using an integrating sphere made by a 3D printer. Then, they determined the effective scattering coefficient and the absorption coefficient in the wavelength range of 400-1500 nm.