## 4 Discussion  
To design the optical structure of the V-, Mel-, and C-filters, we adjusted certain structural parameters, in this case the refractive index difference, thickness, number, and stacking order of the high- and low-index film. As expected, the full width at half maximum (FWHM) of the passing region of the optical filters was closely related to the difference between the refractive indices of the stacked film structures [22; 23]. Although there are some small mismatched transmission spectra between the simulated and fabricated filters, it is acceptable to use a fabricated filter separately attached to the photodiodes because the percentage spectral mismatches of the V-, Mel-, and C-filters were 7.27%, 7.67%, and 7.74%, respectively [25]. Consequently, illuminance values can be measured directly from the photocurrent generated by the photodiodes, and the output values of the photodiodes are proportional to the intensity of the incident light. Instead of using complex calculations to determine the non-visual effects values, the circadian light meter was designed to allow direct measurements of non-visual effects values when using appropriate optical filters,  
Figure 5: Illuminances of the natural light and light exposure of participants (N = 11), measured by the circadian light meter. (**a**) All 24 h, (**b**) dawn and morning (00:00–08:00), (**c**) daytime (09:00–18:00), and (**d**) nighttime (19:00–23:00) illuminances expressed at hourly intervals. The natural light illuminances were obtained from the roof of a five-story building. The participants acted according to their usual habits without any behavioral restrictions. The illuminance data of the natural light and light exposure of participants were simultaneously obtained from 20 March to 30 April 2020. All data represent mean \(\pm\) SEM; all legends are identical, as shown in (**a**). just as ordinary light meters measure VIL values. This concept can be extended to measure and respond to melanopic EDI and other \(\alpha\)-opic EDI values of light. The proposed circadian light meter was fabricated by combining three types of filter-attached photodiodes as light sensors, amplifying units, and other electronic components. Each sensor operates as an independent channel, and to generate a photocurrent, each photodiode accepts only the light derived from a specific region on the attached filter. In particular, the log-ratio amplifier was introduced to identify more sensitively the illuminance within the range of interest at the level of indoor lighting and to use it even at outdoor lighting levels of tens of thousands of lx. The relationships between the voltage-dependent illuminance of each light sensor established by the orthogonal distance regression method appears to form a single curve well; however, a slight mismatch between the measured value and the fitted curve can be seen in the 1.8-2.0 V range, which is considered to be affected by the natural light illuminance at sunrise and sunset times. Meanwhile, the relatively high \(f_{1}\)\({}^{\prime}\) and \(f_{2}\) clearly need improvement. According to DIN 5043 standard, the maximum tolerance of the \(f_{1}\)\({}^{\prime}\) for a class C instrument is 9.0% [26]. However, although this condition is not satisfied, many devices for the purpose of circadian rhythm studies have been developed and commercialized. For example, the light meter, developed by the Light Research Center, contains a red-green-blue solid-state photosensor package, and the \(f_{1}\)\({}^{\prime}\) value of the photosensor is 25% [20]. In addition, the Acriwatch 2 (Philips Healthcare, Best, Netherlands), a commercial tool for recording the VIL and activity, has a relatively high \(f_{1}\)\({}^{\prime}\) value of 62% [25]. Although the proposed circadian light meter does not satisfy the class C grade, it is expected to be of sufficient value as a circadian research tool like the other cases mentioned above.