## 3 Test Methodology
### Visible Light Transmittance (VLT) Test  
Generally, an OF consists of two layers of materials, namely the cladding and the core. Figure 11 shows an axial cross-section of a step-index fiber and a light ray that is coupled into the fiber left cross-section [38]. For the light to be coupled into the guided mode in the fiber, total internal reflection has to occur inside the core, requiring \(\theta_{i}>\theta_{c}\), as shown in Figure 11, where \(\theta_{c}=\arcsin(n_{2}/n_{1})\) is the critical angle of the core-cladding interface. With this requirement on \(\theta_{i}\), there is a corresponding  
Figure 8: Light source for solar simulator.  
Figure 10: Test overview (note: indoor and outdoor denote inside and outside the CHB, respectively): (**a**) view from the indoor side; (**b**) view from the indoor side; (**c**) illumination of the TCP.  
Figure 9: Solar simulator radiation spectrum and reference AM1.5 spectra.  
requirement on the incidence angle \(\theta_{a}\) at the fiber end surface making use of [PERSON]'s law and elementary trigonometry; i.e.,  
\[n_{0}sin\theta_{a}=n_{1}sin\theta_{1}=n_{1}\sqrt{\left(1-sin^{2}\theta_{i}\right)} \tag{6}\]  
It should be noted that the fiber core will propagate the incident light rays only when it is incident at an angle greater than the critical angle \(\theta_{c}\). If the total internal reflection occurs at the core-cladding interface, we have \(\theta_{i}>\theta_{c}\), i.e., \(sin\theta_{i}>sin\theta_{c}=n_{2}/n_{1}\). This requires the incidence angle \(\theta_{a}\) to satisfy the following condition:  
\[n_{0}sin\theta_{a}<n_{1}\sqrt{\left(1-sin^{2}\theta_{c}\right)}=n_{1}\sqrt{1- \left(n_{2}/n_{1}\right)^{2}}=\sqrt{n_{1}^{2}-n_{2}^{2}} \tag{7}\]  
The light entering an OF within a cone defined by the half-acceptance angle, \(\theta_{a}\), is converted into guided modes and can propagate along the fiber. The imaginary light cone with twice the acceptance angle as the vertex angle is known as the acceptance cone. Outside this cone, the light coupled into a fiber will radiate into the cladding. Similarly, the light exiting a fiber has a divergence angle defined by the numerical aperture. This property is critical for the fiber to capture daylight. For the OFs in the TCPs, using Equations (6) and (7), the calculated half-acceptance angle is 33.0\({}^{\circ}\).  
The objective of this test was to measure the VLT of six TCPs at a normal incident angle and two TCPs at five oblique incident angles (20\({}^{\circ}\), 30\({}^{\circ}\), 40\({}^{\circ}\), 50\({}^{\circ}\), and 60\({}^{\circ}\)) by moving the light source held by the arm fixed to its bottom (Figure 12). An integrating sphere of 1.25 m in size, designed and constructed according to CIE 38 (Commission Internationale de l'Eclairage, 1990) and CIE 130 (1998) [39; 40], was used to measure the directional-to-hemispherical total VLT. The light source spectrum was close to the CIE standard illuminant D65. The illuminance level in the integrating sphere was measured by a photo-detector. Its sensitivity was close to the CIE photonic luminosity function (CIE 1990). The measurement was carried out with the sample port open and subsequently was covered by the test specimen. The ratio of the two photo-detector readings was the specimen's VLT (NFRC, 2014) [41]. The same procedures were repeated five times for each TCP at five different randomly selected positions on the surface.  
Figure 11: Light transmission in the step index fiber [8].  
Figure 12: Layout of the test.