## 2 Experimental Approaches  
### Experimental Method and Procedure  
A test duct and light-scattering chamber (LSC) were assembled as shown in Figure 1 to analyze the optical properties of smoke particles generated during the burning of various combustibles. Figure 1a shows the test duct, fabricated in such a way that the smoke particles generated in the experiment follow a particular path. It was made of carbon steel, had a cross-sectional area of 0.3 m \(\times\) 0.3 m, and was fastened using rubber packing to prevent smoke leakage. As the specific purpose of the LSC was to measure the distribution of scattered light, burners for smoke generation were installed at the front end, and a Sirocco fan was installed at the rear end. The flow rate was regulated through the fan via an inverter. The P-type fire control panel and test section installed in the test duct were not used in the present study. The test duct was designed such that the smoke generated from the burning combustibles would flow into the LSC by traveling a distance of 2.6 m (calculated as a straight path). Figure 1b is a photograph and a schematic diagram of the LSC with one light-emitting source and 11 angles, from 0\({}^{\circ}\) to 165\({}^{\circ}\), at 15\({}^{\circ}\) intervals. When an angle is selected, it is assumed that the light scattering characteristics are axially symmetric to 0\({}^{\circ}\) (at which the light transmission signal is measured), following measurement results from previous studies [18]. In the LSC, smoke follows the suction to flow through a 50-mm hole in the center of the chamber. When the smoke particles are exposed to light radiated from the light emitter, the internal light scattering distribution is assessed by measuring the scattered light with its complex characteristics. Within the dark enclosure of the LSC, a black acrylic material was used to block incident light from outside, and 11 photocells in the light emitter (a laser module) and the light receiver (a photocell) were assembled to be in close contact. Specifically, photocells capable of measuring light scattering at 15\({}^{\circ}\) intervals were employed to confirm the scatter distribution of optical properties inside the LSC; the light-extinction method [29] was applied using a photocell at 0\({}^{\circ}\). A detailed description of the device and its adaptation was provided by [30]. A wavelength of 650 nm was used, which is the main wavelength used to measure the concentration of smoke particles generated in a fire; this was selected as an initial approach to simplify the interaction between wavelength band size and particle size. The overall design is intended to provide basic information for the determination of light scattering characteristics in this preliminary study, in order to confirm the interaction between the light source and the smoke particles according to various variables. As the fire source for generating white smoke, shredded filter paper, as used in the sensitivity test, was placed irregularly inside the receptacle so as to generate a sufficient amount of smoke, and flaming combustion was suppressed through the application of an upper plate. For the generation of black smoke, kerosene was applied to a lamp wick, and the smoke concentration was controlled by varying the number of wicks, their diameter, and their exposed length. Finally, for the generation of gray-black smoke, a polymer-based polyurethane (PU) pellet was placed together with methanol onto a rectangular burner, the minimum energy to sustain combustion was supplied, and smoke was generated. The methanol flame supplied sufficient heat for the pyrolysis of the pellet, but the amount of soot generated was very small and, therefore, did not substantially affect the PU smoke concentration. This method has previously been used in evaluating photoelectric smoke detectors; the specific technique for creating a source of smoke is provided by [9]. The principle used to measure the optical characteristics of the smoke particles is based on the light scattering properties of the particles.