**3. Results**

As described in the Introduction, the main purpose of this study was to find the feasibility of optical approaches in identifying the specific types of particulate matters among whole particulate mixtures in the air and their influence on other parameters, filters and liquid additives on their selectivity in terms of light intensity, wavelength and chromaticity value. A spectral sensor and chromameter were tested, respectively, under the same conditions by liquid additives. Chromaticity values in a chromaticity diagram are shown in Figures 3–5. Details of the conditions and results are denoted in Tables 1–5.

**Figure 3.** Chromaticity diagram of household dust, and soil dust. All dots are chromaticity values (Y, x, y) and only x, y coordinates are denoted. Each figure is listed as: (**a**) as prepared (-) and water added (-) household dust; (**b**) as prepared (-) and refractive index liquid (RIL) added () household dust; (**c**) as prepared (-) and water added (-) soil dust; (**d**) as prepared (-) and RIL added () soil dust.

**Figure 4.** Chromaticity diagrams of samples under pink cellophane filter; (**a**) as prepared (-) and water added (-) talc powder; (**b**) as prepared (-) and RIL added () talc powder; (**c**) as prepared (-) and water added (-) gypsum powder; (**d**) as prepared (-) and RIL added () gypsum powder.

**Figure 5.** Chromaticity diagram of pine tree pollen; (**a**) as prepared (-) and water added (-) pine tree pollen; (**b**) as prepared (-) and RIL added () pine tree pollen.


**Table 1.** Chromaticity values of household dust and soil powder samples measured under cellophane filter conditions.





**Table 4.** Peak positions of detected reflected light of samples as a function of wavelength by spectral sensor under experimental conditions.


**Table 5.** Peak intensity ratio of detected reflected light of samples as a function of wavelength by spectral sensor under experimental conditions.


#### *3.1. Color Detection*

Chromaticity diagrams of five samples are shown in Figures 3–5. As denoted in these Figures, all samples were prepared as dried, water added and refractive index liquid added. Water and refractive index liquid were utilized to investigate the additional effect of additives on the reflected light by also predicting changes to the refractive indices and associated colors. A cellophane filter was used to block a specific range of wavelength depending on its color. Details of the wavelength and color of each cellophane filter were drawn in consecutive color bars as shown in Figure 1. The prepared

samples, denoted as circles in the diagram, were located with the coordinates x, y, Y in the diagram using matching color circles according to the color of the cellophane filter.

Here, coordinates; x, y, and Y represent the color space and luminance value, respectively, of the reflected light [15]. Details of the chromaticity values for household dust, soil, talc, gypsum and pine tree pollen are summarized in Tables 1–3, which correspond to the chromaticity diagram in Figures 3–5. Water-added samples and refractive-index-liquid-added samples were depicted as a square and filled triangle, respectively, as in the above manner. Prior to a detailed explanation, the five samples can be simply divided into two groups, a white group and non-white group (grey and yellowish samples) after the bare-eye observation. Talc and gypsum powders belong to the white group and household dust, soil and pine tree pollen are regarded as part of the non-white group.

All chromaticity values in the tables represent the positions in the diagram. Seven colors of cellophane filters and no filter case were denoted. For household dust, the initial coordinates of x and y were recorded, ranging from (0.3163, 0.3177) to (0.3229, 0.3244) for the prepared sample. Most cases were observed in x and y ranges between (0.25, 0.2) and (0.45, 0.45), close to the center white regions regardless of the color of the filters. Indices in the range represent the boundary limit of the x and y coordinates. Even with the additions of water and refractive index liquid, no significant shift in coordinates value were measured. For soil dust, the initial coordinates of x and y were observed to be slightly close to the yellowish region as (0.3157, 0.3213), and coordinates corresponding to the seven filters were found to spread out to locate at each color region, which range from (0.22, 0.19) to (0.55, 0.48). For additive cases, the movements of the coordinates in the vector scale from the initial point were di fferent depending on water and refractive index liquid and an increased shift was found in the case of the refractive index liquid. This might be attributed to the intrinsic yellowish color of soil dust and deep yellowish color of refractive index liquid.

In Table 2, the white color group, talc and gypsum powders are characterized. The intensity of color, Y values for the as prepared and additive cases under the yellow filter and no filter were found to be highest and similar to the previous soil dust case. This is also due to the intrinsic color of the two powders. White can easily reflect the colors of the yellow filter and refractive index liquid. Starting points (black dots) of white color powders were noticed to similarly position at (0.3127, 0.3191) and (0.3157, 0.3213), which are closer to the center white region than in previous cases. Seven color points were also found to locate at their own color region in the range of (0.18, 0.11) to (0.56, 0.54) for talc, and (0.18, 0.10) to (0.57, 0.53) for gypsum. Interesting results were observed for the additive cases. In the case of water, not much movement in the coordinates in the vector was seen in both cases, but a noticeable amount of shift was found for the refractive-index-added cases. As noted in the Figures of the talc and gypsum powders, the coordinates of the refractive-index-added cases ranged from (0.24, 0.19) and (0.46, 0.44) for talc and (0.20, 0.14) and (0.55, 0.48) for gypsum, respectively. The only exception was for the yellow filter in used condition and this was due to the similar color of the filter and refractive index liquid. Pine tree pollen was also characterized and it has a yellowish natural color. As expected, the initial coordinates were found to be around the yellow region in the diagram as (0.3938, 0.3796). Its intrinsic color is strong yellow. Thus, it appears that the influence of color and additives did not have an impact on the modulating of the coordinates in the diagram. The indices varied in a relatively narrow band from (0.24, 0.22) to (0.52, 0.45) for the as prepared, from (0.24, 0.19) to (0.53, 0.45) for water and (0.25, 0.44) for refractive index liquid.

Based on the coordinate value analysis, a noticeable di fference was measured for the white-colored powders such as the talc and gypsum samples. For the white-colored powders, more distinct shifts to each color region were measured than for the yellowish pollen, soil and grey household dust. This may be attributed to the intrinsic color of the powder being close to white, more light reflected to the detector and induced to increased intensity to the spectral sensor. Meanwhile, other non-white, yellowish and grey powders were detected at lower intensity.

In addition, overall chromaticity values for the yellowish powders were observed to shift into the yellow region in the diagram, in an upper left direction from the central white region. Similar experiments were studied by Dang et al. In their report, the chromaticity value of five different colored inorganic pigments of drawing points revealed corresponding measured chromaticity value according to their color [16]. For water- and refractive-index-liquid-added cases, obvious differences in chromaticity values were observed. In Figure 3a,b, household dust revealed to shift more in the yellow and red regions, which correspond to a long wavelength range in the light spectrum. However, soil samples as shown in Figure 3c,d were detected to have more movement in the red and green regions. In the case of the pollen sample, no significant change in chromaticity values were observed under additive conditions. For the talc and gypsum powders, an obvious shift for the talc was observed only for the refractive-index-liquid case and chromaticity values were centered in the white region more than any other samples as shown in Figure 4a,b. This is well described in the previous study and in accordance with results [17]. This means that more white light is reflected to the chromameter detector. For the comparison with talc, the same experiments were executed for the gypsum powder as shown in Figure 4c,d. Under the chromameter measurement, no noticeable difference was observed. This means that similar intrinsic colors and particle shapes can be hardly differentiated under the chromameter study.

#### *3.2. Light Spectrum Detection*

A spectral sensor was used to characterize the light spectrum of samples under experimental conditions. Figure 6a,b shows the spectrum of reflected light for household dust and talc powder as a function of wavelength. The same measurement was performed under different conditions. Details of the measurements are summarized in Tables 4 and 5 according to peak position and peak intensity ratio. As shown in Tables 4 and 5, five samples revealed obvious difference in terms of peak position and peak intensity after the additive treatment and cellophane filter usage.

**Figure 6.** (**a**) Spectrum of reflected light of household dust according to the sample as prepared (black), pink filter(red), water + pink filter (blue), and RIL + pink filter (green); (**b**) Spectrum of reflected light of talc powder according to sample status as: as prepared (black), water added (red), RIL added (blue), pink filter (green), water + pink filter (violet), and RIL + pink filter (brown).

In Table 5, peak intensity at each wavelength was calculated in a ratio. The peak intensity values at low wavelength for samples are regarded to "1" as a base, then, peak intensities at other higher wavelengths were divided by base peak intensity. After pink filter usage, the overall light intensity decreased and was calculated with the same method for three household dust, talc and gypsum samples. Therefore, a higher ratio value means a relatively strong peak and vice versa. Two representative samples, household dust and talc powder, were graphed to scrutinize the light spectrum changes by additives and filter and compared by peak position in wavelength and peak intensity as well. In the case of household dust as shown in Figure 6, two peaks at 420 and 678 nm in wavelength were observed. Under the pink filter, the peak at 678 nm was observed to be removed and a peak shift from 420 to 440 nm was also observed. This is due to the light filtering at a long wavelength range by the pink cellophane. Even when water and refractive index liquid are added, no significant shift was observed. These results correspond well with the measured chromaticity values under additives. Chromaticity values are previously discussed in Figure 3,b; overall values are centered to the white region with relatively more shifts for the blue, red, and yellow regions observed after refractive index liquid was added. Meanwhile, the talc powder revealed slightly di fferent results than that of the household dust. The light spectrum for as prepared talc powder shows three peaks at 420, 677, and 720 nm, respectively. Two peaks at 420 and 677 nm showed a relatively low ratio value of 6.32, but no noticeable changes were observed for both additives, which show ratio values of 4.51 and 6.21 in Table 5. Considering the combined conditions of the pink filter and water addition, a peak shift from 420 to 430 nm and an additional peak was observed at 490 nm. Other peaks at a long wavelength range were filtered the same as previously. For comparison, similar gypsum power was also characterized under pink filter and additives conditions. The peak at a short wavelength region shifted from 420 to 453 nm, but this was a big di fference from the additional peak observed for the talc case at approximately 490 nm, which was not detected. Instead, an additional peak at approximately 820 nm was detected for the water-added gypsum sample. These results appear to be correlated with the absolute amount of shift in the chromaticity values which are larger for gypsum than for the talc powder.
