Selecting a Suitable Remote Phosphor Configuration for Improving Color Quality of White Led

When compared with two conformal phosphor and in-cup phosphor structures, the remote phosphor structure has higher luminescent performance. However, it is di cult to control the color quality of the remote phosphor structure, so it has become a research target in recent years. So far, there are two remote phosphor structures used to improve color quality including dual-layer phosphor con guration and triple-layer phosphor con guration. This study suggests using those two con gurations to make multi-chip white LEDs (WLEDs) that can achieve adequate values in color rendering index (CRI), color quality scale (CQS), luminous efcacy (LE) and color uniformity. WLEDs with color temperature of 5600 K are applied. Research results show that the triple-layer phosphor con guration is superior in CRI, CQS, LE. Besides, the color deviation decreases signi cantly, meaning that the color homogeneity increases with the triple-layer phosphor con guration. This can be demonstrated by analyzing the scattering characteristics of phosphor classes through Mie theory, thus making the research results more reliable and valuable for producing quality WLEDs.


Introduction
Phosphor-converted white light-emitting diodes (WLEDs) are a potential light source because of their small size, high energy eciency, reasonable cost, and color stability [1][2][3][4]. WLEDs apply the principle of complementary colors: Blue light from a blue chip is linked with yellow light from phosphor [5].
WLEDs have a high probability of being applied in solidstate lighting, but their luminous eciency must be strengthened [6]. Generally, a freely dis- to be controlled easily and reduces much of the cost, it does not produce high-quality WLEDs [7]. Therefore, the conformal coating method can be used as an alternative. This method distributes colors uniformly, resulting in angular homogeneity of correlated color temperature (CCT) [8]. However, the disadvantage of a conformal phosphor structure is the backscattering eect, which reduces luminous eciency. Previous studies have proved the concept of separating the chip and the phosphor layer of remote phosphor structures [9,10]. The enhanced light extraction internal reection structure, which uses a polymer hemispherical shell lens with an interior phosphor coating, is known to increase extraction eciency [11]. Furthermore, an air-gap embedded structure can enhance luminous eciency by reecting downward light [12]. Obviously, in addition to luminous ecacy (LE), other optical characteristics including color rendering index (CRI), color quality scale (CQS) and color uniformity are all so extremely important for WLEDs. Therefore, two improved remote phosphor structures were applied to improve the optical properties of LEDs including dual-layer phosphor conguration and triple-layer phosphor conguration. For duallayer phosphor conguration, the yellow phosphor layer below and above is the phosphor red or green layer. For the triple-layer phosphor conguration, the yellow phosphor layer below and above is the red phosphor layer, the middle is the green phosphor. In addition to the structure of the package, the concentration of phosphor plays an important role in luminous eciency. The re-absorption failure in the phosphor layer is obtained when the phosphor concentration increases. Therefore, device luminous eciency would be lessened, specically at lower CCTs [13]. Therefore, it is essential to enhance the emission of blue and yellow rays and reduce the amount of light lost from backscattering and reection. It is dicult for manufacturers to choose a remote phosphor structure to improve the optical properties of their LED products due to the many proposed methods men-

Simulation
The rst idea of the study is to use the  Figure 1 (d). These remote phosphor layers are 0.08 mm thick. To maintain the average correlation color temperature average correlation temperature (ACCTs), YAG:Ce 3+ concentration changes when changing the phosphor yellow or red phosphor concentration. At each different ACCT for each phosphor structure, YAG:Ce 3+ concentration is also different. This makes the difference in scattering properties in WLEDs, resulting in differences in optical properties. In Figure 2, it is easy to see that the highest yellowemitting YAG:Ce 3+ phosphor concentration is in the Y Based on the results of the previous studies [8][9][10][11][12][13][14], the diameter of phosphor particles is xed at an average of 14.5 µm.
In this study, WLEDs with 9 internal chips are used. The output of each of these blue chips is 1.16W, with a peak wavelength emitted at 453 nm. Figure 1( Figure 1(d).
These remote phosphor layers are 0.08 mm thick.  Figure 1 (d). These remote phosphor layers are 0.08 mm thick. To maintain the average correlation color temperature average correlation temperature (ACCTs), YAG:Ce 3+ concentration changes when changing the phosphor yellow or red phosphor concentration. At each different ACCT for each phosphor structure, YAG:Ce 3+ concentration is also different. This makes the difference in scattering properties in WLEDs, resulting in differences in optical properties. In Figure 2, it is easy to see that the highest yellowemitting YAG:Ce 3+ phosphor concentration is in the Y In Figure 2, it is easy to see that the highest According to the findings and requirements above, it seems that triple-layer phosphor structure is the most favorable structure in controlling optical properties. However, there is another crucial aspect that need to consider before concluding, emission spectra. There are significant differences in emission spectra among the four remote phosphor structures. The Yemission spectrum has the smallest intensity compared to the other three remote phosphor structures. This confirms that the Y structure achieves the smallest luminous flux. In contrast, the YRG structure has the largest spectral intensity in the wavelength range of 380 nm -780 nm. In the range of 400 nm -500 nm, YG structure has a higher spectral intensity than YR structure so YG's luminous flux can be higher than YR. However, YR's emission spectral intensity is higher than YG's in the range of 650 nm -750 nm, which helps YR achieves higher color rendering index than YG. However, to confirm the findings mentioned above, it is necessary to consider the results achieved in section 3. Figure 4 shows the CRI comparison between remote phosphor structures. It is easy to see that the YR structure achieves the highest CRI. The outstanding CRI value in YR structure is benefited by the red light component However, CRI is just one of the color quality indicators. In recent years, CQS has become the research target of many studies. CQS is a combination of 3 elements: CRI, person's preference and color coordinates. With the coverage of these three factors, CQS becomes a big target and "seems" to be the most important indicator to assess color quality. Thus, it can be concluded that the higher the CQS value is, the higher the color quality becomes. In this study, the CQS values of the remote phosphor structures are compared in Figure 5. If the YR reaches the highest CRI, the YRG reaches the highest CQS. This can be explained by the balance of 3 basic colors yellow, red and green. Meanwhile, the CQS is the lowest in the Y structure. In general, the Y structure has a high luminous flux, but it is difficult to control the color quality due to the lack of red and green light components. Despite the disadvantage in color quality, Y structure has advantages In contrast, the YRG structure has the largest spectral intensity in the wavelength range of 380 nm -780 nm. In the range of 400 nm -500 nm, YG structure has a higher spectral intensity than YR structure so YG's luminous ux can be higher than YR. However, YR's emission spectral intensity is higher than YG's in the range of 650 nm -750 nm, which helps YR achieves higher color rendering index than YG. However, to conrm the ndings mentioned above, it is necessary to consider the results achieved in section 3.

3.
Results and discussion  structure and the lowest is in the YRG structure.
Regarding to the remote phosphor structures, the higher the YAG:Ce 3+ concentration, the higher the scattering ability, resulting in reduced luminous flux. On the other hand, the imbalance between the three primary colors that produce white light: yellow, red and green appears when the YAG:Ce 3+ concentration is high, causing a decrease in color quality of WLEDs. Therefore, in order to improve the luminous flux and color quality of WLEDs, the backscattering effect must be reduced and the three basic colors yellow, red and green must be balanced. The color rendering index can be controlled by increasing the red light component. Besides, color homogeneity can be controlled by adding the green light component. According to the findings and requirements above, it seems that triple-layer phosphor structure is the most favorable structure in controlling optical properties. However, there is another crucial aspect that need to consider before concluding, emission spectra. There are significant differences in emission spectra among the four remote phosphor structures. The Yemission spectrum has the smallest intensity compared to the other three remote phosphor structures. This confirms that the Y structure achieves the smallest luminous flux. In contrast, the YRG structure has the largest spectral intensity in the wavelength range of 380 nm -780 nm. In the range of 400 nm -500 nm, YG structure has a higher spectral intensity than YR structure so YG's luminous flux can be higher than YR. However, YR's emission spectral intensity is higher than YG's in the range of 650 nm -750 nm, which helps YR achieves higher color rendering index than YG. However, to confirm the findings mentioned above, it is necessary to consider the results achieved in section 3. Figure 4 shows the CRI comparison between remote phosphor structures. It is easy to see that the YR structure achieves the highest CRI. The outstanding CRI value in YR structure is benefited by the red light component added from the red phosphor layer MgSr 3 Si 2 O 8 :Eu 2+, Mn 2+ . The second position in the CRI value achieved is the YRG structure. Meanwhile, CRI is the lowest in YG structure. These results confirm that YR is the best structure for mass production of WLED that focuses on CRI. However, CRI is just one of the color quality indicators. In recent years, CQS has become the research target of many studies. CQS is a combination of 3 elements: CRI, person's preference and color coordinates. With the coverage of these three factors, CQS becomes a big target and "seems" to be the most important indicator to assess color quality. Thus, it can be concluded that the higher the CQS value is, the higher the color quality becomes. In this study, the CQS values of the remote phosphor structures are compared in Figure 5. If the YR reaches the highest CRI, the YRG reaches the highest CQS. This can be explained by the balance of 3 basic colors yellow, red and green. Meanwhile, the CQS is the lowest in the Y structure. In general, the Y structure has a high luminous flux, but it is difficult to control the color quality due to the lack of red and green light components. Despite the disadvantage in color quality, Y structure has advantages Regarding to the remote phosphor structures, the higher the YAG:Ce 3+ concentration, the higher the scattering ability, resulting in reduced luminous flux. On the other hand, the imbalance between the three primary colors that produce white light: yellow, red and green appears when the YAG:Ce 3+ concentration is high, causing a decrease in color quality of WLEDs. Therefore, in order to improve the luminous flux and color quality of WLEDs, the backscattering effect must be reduced and the three basic colors yellow, red and green must be balanced. The color rendering index can be controlled by increasing the red light component. Besides, color homogeneity can be controlled by adding the green light component. According to the findings and requirements above, it seems that triple-layer phosphor structure is the most favorable structure in controlling optical properties. However, there is another crucial aspect that need to consider before concluding, emission spectra. There are significant differences in emission spectra among the four remote phosphor structures. The Yemission spectrum has the smallest intensity compared to the other three remote phosphor structures. This confirms that the Y structure achieves the smallest luminous flux. In contrast, the YRG structure has the largest spectral intensity in the wavelength range of 380 nm -780 nm. In the range of 400 nm -500 nm, YG structure has a higher spectral intensity than YR structure so YG's luminous flux can be higher than YR. However, YR's emission spectral intensity is higher than YG's in the range of 650 nm -750 nm, which helps YR achieves higher color rendering index than YG. However, to confirm the findings mentioned above, it is necessary to consider the results achieved in section 3. Figure 4 shows the CRI comparison between remote phosphor structures. It is easy to see that the YR structure achieves the highest CRI. The outstanding CRI value in YR structure is benefited by the red light component

III. RESULTS AND DISCUSSION
The second position in the CRI value achieved is the YRG structure. Meanwhile, CRI is the lowest in YG structure. These results confirm that YR is the best structure for mass production of WLED that focuses on CRI. However, CRI is just one of the color quality indicators. In recent years, CQS has become the research target of many studies. CQS is a combination of 3 elements: CRI, person's preference and color coordinates. With the coverage of these three factors, CQS becomes a big target and "seems" to be the most important indicator to assess color quality. Thus, it can be concluded that the higher the CQS value is, the higher the color quality becomes. In this study, the CQS values of the remote phosphor structures are compared in Figure 5. If the YR reaches the highest CRI, the YRG reaches the highest CQS. This can be explained by the balance of 3 basic colors yellow, red and green. Meanwhile, the CQS is the lowest in the Y structure. In general, the Y structure has a high luminous flux, but it is difficult to control the color quality due to the lack of red and green light components. Despite the disadvantage in color quality, Y structure has advantages has advantages in production. The production procedure for Y structure WLED is simpler than the rest, which also reduces the production costs.
Based on the result of Figure 5, it can be conrmed that if the manufacturer's goal is color quality, it is recommended to select YRG structure. However, there is an assumption that the luminous ux will be aected if the color quality is better. The comparison between the emitted luminous ux between the single-layer and duallayer structures will help to demonstrate this issue. This part will show and describe the math- (Title of the paper will be placed here) in production. The production procedure for Y structure WLED is simpler than the rest, which also reduces the production costs.
Based on the result of Figure 5, it can be confirmed that if the manufacturer's goal is color quality, it is recommended to select YRG structure. However, there is an assumption that the luminous flux will be affected if the color quality is better. The comparison between the emitted luminous flux between the single-layer and dual-layer structures will help to demonstrate this issue. This part will show and describe the mathematical model of the transmitted blue light and converted yellow light in the double-layer phosphor structure, from which a notable advancement of LED efficiency can be achieved. The transmitted blue light and converted yellow light for single layer remote phosphor package with the phosphor layer thickness of 2h are expressed as follows: The transmitted blue light and converted yellow light for double layer remote phosphor package with the phosphor layer thickness of h are defined as: Where h is the thickness of each phosphor layer. The subscript "1" and "2" are used to illustrate single layer and double-layer remote phosphor package. β presents the conversion coefficient for blue light converting to yellow light. γ is the reflection coefficient of the yellow light. The intensities of blue light (PB) and yellow light (PY) are the light intensity from blue LED, indicated by PB 0 . α B ; α Y are parameters describing the fractions of the energy loss of blue and yellow lights during their propagation in the phosphor layer respectively.
The lighting efficiency of pc-LEDs with the double-layer phosphor structure enhances considerably compared to a single layer structure: The scattering of phosphor particles was analyzed by using the Mie-theory. In addition, the scattering cross section C sca for spherical particles can be computed by the following expression through applying the Mie theory. The transmitted light power can be calculated by the Lambert-Beer law: In this formula, I 0 is the incident light power, L is the phosphor layer thickness (mm) and µ ext is known to be the extinction coefficient, which can be expressed as: µ ext = N r .C ext , where N r is as the number density distribution of particles (mm -3 ). Cext (mm 2 ) is the extinction crosssection of phosphor particles. Equation 5 demonstrates that using multiple phosphor layers is more beneficial to luminous flux than a single layer. Obviously, this is illustrated in the results of Figure  6, the structure Y reaches the lowest luminous flux out of the four structures. In contrast, the highest luminous flux is achieved in the YRG structure. This eliminates any doubt about YRG lumen output when its color quality is the best. The second place in terms of the usefulness in luminous flux development is the YG structure thanks to the green phosphor YAl 3 B 4 O 12 :Ce 3+ ,Mn 2+ . Green phosphor YAl 3 B 4 O 12 :Ce 3+ ,Mn 2+ helps to increase green light composition and increase spectra intensity in the wavelength range of 500 nm -600 nm. Clearly in this wavelength range, YG's intensity is greater than YR and Y. Due to the smallest YAG:Ce 3+ concentration in the YRG structure that can keep the ACCT, the YRG structure reduces the amount of reflected light after the YAG:Ce 3+ concentration decreases. Blue light rays from LED chips are easily transmitted straight through the YAG:Ce 3+ layer to other layers. In other words, the YRG structure helps blue light energy from the LED chip to convert efficiently. Therefore, the YRG spectral intensity is the highest compared to other remote phosphor structures in the same white light wavelength range. Accordingly, the luminous flux of the YRG structure also reached the highest level.  (Title of the paper will be placed here) in production. The production procedure for Y structure WLED is simpler than the rest, which also reduces the production costs.
Based on the result of Figure 5, it can be confirmed that if the manufacturer's goal is color quality, it is recommended to select YRG structure. However, there is an assumption that the luminous flux will be affected if the color quality is better. The comparison between the emitted luminous flux between the single-layer and dual-layer structures will help to demonstrate this issue. This part will show and describe the mathematical model of the transmitted blue light and converted yellow light in the double-layer phosphor structure, from which a notable advancement of LED efficiency can be achieved. The transmitted blue light and converted yellow light for single layer remote phosphor package with the phosphor layer thickness of 2h are expressed as follows: The transmitted blue light and converted yellow light for double layer remote phosphor package with the phosphor layer thickness of h are defined as: Where h is the thickness of each phosphor layer. The subscript "1" and "2" are used to illustrate single layer and double-layer remote phosphor package. β presents the conversion coefficient for blue light converting to yellow light. γ is the reflection coefficient of the yellow light. The intensities of blue light (PB) and yellow light (PY) are the light intensity from blue LED, indicated by PB 0 . α B ; α Y are parameters describing the fractions of the energy loss of blue and yellow lights during their propagation in the phosphor layer respectively.
The lighting efficiency of pc-LEDs with the double-layer phosphor structure enhances considerably compared to a single layer structure: The scattering of phosphor particles was analyzed by using the Mie-theory. In addition, the scattering cross section C sca for spherical particles can be computed by the I = I 0 exp(-µ ext L) (6) In this formula, I 0 is the incident light power, L is the phosphor layer thickness (mm) and µ ext is known to be the extinction coefficient, which can be expressed as: µ ext = N r .C ext , where N r is as the number density distribution of particles (mm -3 ). Cext (mm 2 ) is the extinction crosssection of phosphor particles. Equation 5 demonstrates that using multiple phosphor layers is more beneficial to luminous flux than a single layer. Obviously, this is illustrated in the results of Figure  6, the structure Y reaches the lowest luminous flux out of the four structures. In contrast, the highest luminous flux is achieved in the YRG structure. This eliminates any doubt about YRG lumen output when its color quality is the best. The second place in terms of the usefulness in luminous flux development is the YG structure thanks to the green phosphor YAl 3 B 4 O 12 :Ce 3+ ,Mn 2+ . Green phosphor YAl 3 B 4 O 12 :Ce 3+ ,Mn 2+ helps to increase green light composition and increase spectra intensity in the wavelength range of 500 nm -600 nm. Clearly in this wavelength range, YG's intensity is greater than YR and Y. Due to the smallest YAG:Ce 3+ concentration in the YRG structure that can keep the ACCT, the YRG structure reduces the amount of reflected light after the YAG:Ce 3+ concentration decreases. Blue light rays from LED chips are easily transmitted straight through the YAG:Ce 3+ layer to other layers. In other words, the YRG structure helps blue light energy from the LED chip to convert efficiently. Therefore, the YRG spectral intensity is the highest compared to other remote phosphor structures in the same white light wavelength range. Accordingly, the luminous flux of the YRG structure also reached the highest level. The lighting eciency of pc-LEDs with the double-layer phosphor structure enhances considerably compared to a single layer structure: conversion coefficient for blue light converting to yellow light. γ is the reflection coefficient of the yellow light. The intensities of blue light (PB) and yellow light (PY) are the light intensity from blue LED, indicated by PB 0 . α B ; α Y are parameters describing the fractions of the energy loss of blue and yellow lights during their propagation in the phosphor layer respectively.
The lighting efficiency of pc-LEDs with the double-layer phosphor structure enhances considerably compared to a single layer structure: The scattering of phosphor particles was analyzed by using the Mie-theory. In addition, the scattering cross section C sca for spherical particles can be computed by the following expression through applying the Mie theory. The transmitted light power can be calculated by the Lambert-Beer law: (Title of the paper will be placed here) in production. The production procedure for Y structure WLED is simpler than the rest, which also reduces the production costs.
Based on the result of Figure 5, it can be confirmed that if the manufacturer's goal is color quality, it is recommended to select YRG structure. However, there is an assumption that the luminous flux will be affected if the color quality is better. The comparison between the emitted luminous flux between the single-layer and dual-layer structures will help to demonstrate this issue. This part will show and describe the mathematical model of the transmitted blue light and converted yellow light in the double-layer phosphor structure, from which a notable advancement of LED efficiency can be achieved. The transmitted blue light and converted yellow light for single layer remote phosphor package with the phosphor layer thickness of 2h are expressed as follows: The transmitted blue light and converted yellow light for double layer remote phosphor package with the phosphor layer thickness of h are defined as: Where h is the thickness of each phosphor layer. The subscript "1" and "2" are used to illustrate single layer and double-layer remote phosphor package. β presents the conversion coefficient for blue light converting to yellow light. γ is the reflection coefficient of the yellow light. The intensities of blue light (PB) and yellow light (PY) are the light intensity from blue LED, indicated by PB 0 . α B ; α Y are parameters describing the fractions of the energy loss of blue and yellow lights during their propagation in the phosphor layer respectively.
The lighting efficiency of pc-LEDs with the double-layer In this formula, I 0 is the incident light power, L is the phosphor layer thickness (mm) and µ ext is known to be the extinction coefficient, which can be expressed as: µ ext = N r .C ext , where N r is as the number density distribution of particles (mm -3 ). Cext (mm 2 ) is the extinction crosssection of phosphor particles. Equation 5 demonstrates that using multiple phosphor layers is more beneficial to luminous flux than a single layer. Obviously, this is illustrated in the results of Figure  6, the structure Y reaches the lowest luminous flux out of the four structures. In contrast, the highest luminous flux is achieved in the YRG structure. This eliminates any doubt about YRG lumen output when its color quality is the best. The second place in terms of the usefulness in luminous flux development is the YG structure thanks to the green phosphor YAl 3 B 4 O 12 :Ce 3+ ,Mn 2+ . Green phosphor YAl 3 B 4 O 12 :Ce 3+ ,Mn 2+ helps to increase green light composition and increase spectra intensity in the wavelength range of 500 nm -600 nm. Clearly in this wavelength range, YG's intensity is greater than YR and Y. Due to the smallest YAG:Ce 3+ concentration in the YRG structure that can keep the ACCT, the YRG structure reduces the amount of reflected light after the YAG:Ce 3+ concentration decreases. Blue light rays from LED chips are easily transmitted straight through the YAG:Ce 3+ layer to other layers. In other words, the YRG structure helps blue light energy from the LED chip to convert efficiently. Therefore, the YRG spectral intensity is the highest compared to other remote phosphor structures in the same white light wavelength range. Accordingly, the luminous flux of the YRG structure also reached the highest level.  Due to the smallest YAG:Ce 3+ concentration in the YRG structure that can keep the ACCT, the YRG structure reduces the amount of reflected light after the YAG:Ce 3+ concentration decreases. Blue light rays from LED chips are easily transmitted straight through the YAG:Ce 3+ layer to other layers. In other words, the YRG structure helps blue light energy from the LED chip to convert efficiently. Therefore, the YRG spectral intensity is the highest compared to other remote phosphor structures in the same white light wavelength range. Accordingly, the luminous flux of the YRG structure also reached the highest level.
Where h is the thickness of each phosphor layer. The subscript "1" and "2" are used to illustrate single layer and double-layer remote phosphor package. β presents the conversion coefficient for blue light converting to yellow light. γ is the reflection coefficient of the yellow light. The intensities of blue light (PB) and yellow light (PY) are the light intensity from blue LED, indicated by PB 0 . α B ; α Y are parameters describing the fractions of the energy loss of blue and yellow lights during their propagation in the phosphor layer respectively.
The lighting efficiency of pc-LEDs with the double-layer phosphor structure enhances considerably compared to a single layer structure: The scattering of phosphor particles was analyzed by using the Mie-theory. In addition, the scattering cross section C sca for spherical particles can be computed by the following expression through applying the Mie theory. The transmitted light power can be calculated by the Lambert-Beer law: helps blue light energy from the LED chip to convert efficiently. Therefore, the YRG spectral intensity is the highest compared to other remote phosphor structures in the same white light wavelength range. Accordingly, the luminous flux of the YRG structure also reached the highest level. Thus, the YRG structure can be selected for the superior optical properties of WLEDs including CQS and LE but cannot be ignored in color homogeneity when it comes to color quality factor. There are many methods to improve color homogeneity including methods of using advanced scattering particles such as SiO 2 , CaCO 3 ,... or using conformal phosphor configuration. Although the color uniformity is improved, luminous flux can be significantly reduced if the two methods above are applied. phosphor is not only to increase the scattering properties but also to add a green or red light Therefore, the YRG structure achieves the best color uniformity with the highest luminous ux.
In contrast, the highest color deviation is expressed in the Y structure.