# A Novel Fluorescent-Material-Based Simple Method for Sunscreen Evaluation
## 3 Measurement System  
Figure 4a shows a detailed view of the region, where the incident excitation radiation was converted to fluorescence. The fluorescent layer was prepared by mixing powdered fluorescent material UV-G (TERRENAVI Co., Tokyo, Japan) and transparent UV-curable resin HI-LOCK UV-591 (Toho-Kasei Co., Tokyo, Japan). This fluorescent material emits green fluorescence (517 nm) by absorbing UV rays as the excitation radiation with a recommended wavelength in the range of 200-365 nm (Figure S1). Although UV-G was used as a target to evaluate the sunscreen agents in this study, it is also possible to measure the hemispherical transmittance at other wavelengths by selecting a fluorescent material that matches the measurement target. The viscosity of HI-LOCK UV-591 was reduced by heating it to approximately 70 \({}^{\circ}\)C in a hot water bath so that UV-G could be sufficiently dispersed while stirring, suppressing the inclusion of air bubbles. The mixture of resin and fluorescent material was sandwiched between two glass slides and cured or, in other words, the mixture was hardened by exposure to UV light. The curing process fixes the fluorescent material within the resin. Consequently, the fluorescent material remains uniformly dispersed, ensuring the generation of uniform fluorescence irrespective of the direction of the excitation radiation. Figure 3: Conversion procedure to hemispheric transmittance in the proposed method. Figure 2: Concept of a measurement method for transmitted radiation. Figure 4b shows the overall experimental configuration for measuring the fluorescent radiation. A semiconductor laser emitting UV rays (375 nm, TC20-3720-15, NEOARK Corporation, Tokyo, Japan) was used as the light source. The laser beam was expanded by a beam expander so that the beam had a uniform intensity. An aperture of 10 mm diameter was used to create uniform illumination on the sample surface. Thus, as there is also a zero gap between the sample and the fluorescent material, we can say approximately that the size of the beam incident on the sample surface with a fluorescent layer was 10 mm. This beam, 10 mm in diameter, was incident on the sample surface with a fluorescent layer. The diffuse material as the sample should be applied to a uniform thickness on the surface of the glass slide as shown in Figure 4a. While UV radiation passed through the fluorescent layer, we detected the emitted fluorescent radiation through a sharp cut filter (SCF-50S-42L, Sigmakoki Co., Ltd., Tokyo, Japan) and an optical fiber probe with a receiving lens with a narrow view angle (0.6 \(\times\) 10\({}^{-3}\) sr) (fiber collimator, C-OPCL05G-001/101-L=2, Nippon Sheet Glass Co., Ltd., Tokyo, Japan). The view area on the glass slide surface was approximately 2 mm in diameter. The sharp cut filter was used to remove the excitation radiation transmitted through the fluorescent layer. The light collected by the optical fiber probe was detected by a photomultiplier tube (R374, Hamamatsu Photonics, Shizuoka, Japan) at a specific frequency of a light chopper placed in front of the laser source. A lock-in amplifier (LI5640, NF Corporation, Kanagawa, Japan) was used to selectively measure the signal that varied at the chopper frequency, rejecting the detected signal at other frequencies, and thus removing noise. The output value of the lock-in amplifier yields the hemispherical transmittance obtained using Equation (5). The glass slides used in this study were 1 mm thick soda-time glass slides. Generally, soda-time glass does not have high transmittance for radiation that has wavelengths in the range of 400 nm or less. Figure 5 shows the spectral transmittance of the glass slides used in this study and that of the UV-curable resin. For the UV-curable resin, the transmittance dropped sharply for radiation wavelengths below 400 nm and was approximately 50% at 375 nm. In contrast, the transmittance of the resin was more than 80% at approximately 517 nm, the wavelength of fluorescence, and was sufficiently detectable by our system.