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Abstract
We propose two polarization interference filters (PIF1 and PIF2) used in the backlight unit of a liquid crystal display (LCD) to achieve a wide color gamut. Both PIF1 and PIF2 consist of two polarizers and two 720° super twisted nematic liquid crystal polymer (LCP) layers, where two polarizers are crossed in PIF1, and two polarizers are parallel in PIF2. The PIFs can eliminate unwanted cyan and yellow parts in the output spectrum, which can improve the color gamut of LCD. In our calculation, when the PIF1 is employed in the LCD with normal color filter and QD-LED backlight, the color gamut increases from 107.3% NTSC to 124.6% NTSC, which is 13.7% NTSC larger than that of the LCD with high-performance color filter. When the PIF2 is employed in the LCD with normal color filter and QD-LED backlight, the color gamut of LCD with a normal color filter is improved by 6.8% NTSC larger than that of LCD with high-performance color film, and the transfer efficiency is close to that of the LCD with high-performance color film. We define the color gamut enhancement ratio (CGER) to compare the influence of PIFs and the high-performance color filter on the color gamut enhancement performance of LCD. Compared with the high-performance color filter, the two kinds of PIFs have a higher CGER. The PIFs have a great potential for achieving a wide color gamut.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The color gamut reflects the richness of colors in display, and wide color gamut means more colors can be reproduced [1–6]. In recent years, with the demand of consumers for vivid colors, wide color gamut display has become a hot topic in the display field. The color gamut of display mainly depends on the separation of three primary colors’ spectra. Wide color gamut display technology needs to continuously optimize the three primary colors’ spectra. High-performance luminescent materials and high-performance color filters are two traditional methods to achieve wide color gamut display. Quantum dots (QDs) are high-performance luminescent materials with a narrow emission spectrum, which meet the requirement of the backlight in LCD [7–13]. The color filter (CF) is the key material for color separation in LCD [14–16]. The normal color filter has high transmittance because of the wide full width at half maximums (FWHMs) of blue, green and red spectra, so the wide color gamut cannot be obtained. The high-performance color filter can effectively improve the color gamut of LCD, but leads to a significant decrease in transfer efficiency [17]. The trade-off between color gamut and transfer efficiency is important for the display’s application.
Recently, the optical structure with a filtering effect is a method to achieve wide color gamut display [18–24]. The optical filter can directly change the emission spectrum and eliminate the unwanted spectrum to achieve an accurate filtering effect [25–27]. The filtering characteristics of the optical filter are generally determined by the thickness, refractive index and other parameters, so the filtering effect is easy to adjust. Hornburg et al. proposed multi-twist retarders to obtain a wide color gamut [28]. Chen et al. proposed a functional reflective polarizer with a viewing angle (20°) to enlarge the color gamut of LCD, and this functional reflective polarizer is composed of hundreds of nanometers layers [29]. Sun et al. proposed a bandpass filter based on a 5-layer configuration with a viewing angle (30°) to improve the color gamut of LCD [30]. Zhang et al. proposed a narrow-band birefringent filter consisting of two polarizers and five birefringence phase retarders to achieve wide color gamut display [31]. These optical filters largely improve the color gamut of LCD, but the viewing angle is not wide.
In this work, based on our previous research on PIF [32], we further propose two PIFs (PIF1 and PIF2) used in the backlight unit of LCD to achieve wide color gamut. Firstly, we give the structure and experimental fabrication process of PIF1 and PIF2, and obtain the separated spectra for three primary colors. Next, we calculate the color gamut of LCD with CF1, CF2, CF1-PIF1 and CF1-PIF2, respectively. We also give the concept of color gamut enhancement ratio (CGER) to compare the influence of PIFs and high-performance color filter on the color gamut enhancement performance of LCD. Then, we calculate the influence of the Δnd and twist angle of PIF1 and PIF2 on the transmittance, transfer efficiency and color gamut. Finally, we calculate the color gamut at different viewing angles when PIF1 and PIF2 are added into the backlight unit of LCD with CF1. The simulation software is TechWiz LCD (Sanayi Systems, Korea).
2. Structure and experiment
Figure 1 demonstrates the schematic diagrams of PIF1 and PIF2, which are composed of two polarizers and two LCP layers. For the PIF1, the directions of transmission axes of the first polarizer (P1) and second polarizer (P2) are 0° and 90°, respectively. For the PIF2, the directions of transmission axes of the first polarizer (P1) and second polarizer (P2) are 0°. In the PIF1 and PIF2, the first LCP layer is uniformly twisted from 0° to 720° and the second LCP layer is uniformly twisted from 720° to 0°. The polarization interference principle is adopted to describe the optical principle of PIFs. The unpolarized light passes through the first polarizer to become linearly polarized light, then two LCP layers change the polarization state of linearly polarized light, and the second polarizer allows only specific polarized light to pass through to obtain the desired transmittance spectrum.
In the experiment, we used two homogeneous alignment liquid crystal cells with 20 µm cell gap to fabricate the two LCP layers in PIF1 and PIF2. The LCP consists of three monomers (HCM020, HCM021 and HCM009 from Jiangsu Hecheng Display Technology Co., Ltd.), as shown in Fig. 2(a). Firstly, we configured two precursors with 16.58 wt.% HCM020, 33.17 wt.% HCM021, 49.75 wt.% HCM009 and 0.5 wt.% photoinitiator Irgacure 651, and the chiral dopant (1.0 wt.% S811 or 1.0 wt.% R811) was added for obtaining the opposite twisted LCP layers. Next, we heated two precursors and two liquid crystal cells to 90°C, and the two precursors became isotropic liquid. Then, the two isotropic precursors were injected into two liquid crystal cells and then cooled to 53°C for obtaining the two LCP layers in PIF1, and the other two isotropic precursors were injected into two liquid crystal cells and then cooled to 68°C for obtaining the two LCP layers in PIF2. In this process, two precursors transform from isotropic to nematic phase. Under the surface rubbing condition, the liquid crystal molecules at the top and bottom surfaces are arranged in parallel, and the liquid crystal molecules are uniformly twisted 720° (one is left-hand, the other is right-hand). Finally, we cured two precursors by ultraviolet (UV) light for the 60s, and combined two polarizers (G1220DU from Nitto Denko, Japan) and two LCP layers as shown in Fig. 1 to fabricate the PIF1 and PIF2. The Jones matrix method is employed to analyze the optical properties of PIFs [33–35], and the transmittance of the PIF1 is written as [32]
and the transmittance of the PIF2 is written as where $\Gamma = {{2\pi \Delta nd} / \lambda }$ is the phase retardation, which d is the thickness of the LCP layer, $\varDelta n$ is the birefringence of the LCP material, $\lambda $ is the wavelength of input light, $\phi $ is the twist angle of the LCP layer and ${\rm X} = \sqrt {{\phi ^2} + {{({{\Gamma / 2}} )}^2}} $. The dispersion of birefringence of LCP in PIF1 (red line) and PIF2 (blue line) is shown in Fig. 2(b), and the relationship of $\Delta nd$ and wavelength in PIF1 (red line) and PIF2 (blue line) is shown in Fig. 2(c).
Fig. 2. The experimental fabrication of PIF. (a) Structural formulas of three monomers. (b) Birefringence of the liquid crystal polymer. (c) The Δnd of PIF1 and PIF2.
Figure 3(a) and Fig. 3(b) show the experimental and calculated normalized transmittance spectra of the PIF1 and PIF2, respectively. The experimental results are in good agreement with the calculated results.
3. Result
Figure 4 shows the panel configuration with PIF inserted between the LCD module and backlight unit. Here, the light guide plate (LGP) is combined with inverted prism film to form a directional backlight (within 20° viewing angle cone), and a detailed design can be found in [36,37]. The dual brightness enhancement film (DBEF) can convert unpolarized light into linearly polarized light, thus increasing the transmittance of the LCD module to incident light [38,39]. The diffuser film spreads the incident light to achieve wide viewing angle [40,41]. In this panel configuration, the lower polarizer (P1) of PIF is replaced by DBEF and the upper polarizer (P2) of PIF is replaced by the polarizer of the LCD module. When the polarization direction of linearly polarized light converted by DBEF is perpendicular to the transmission axes of the polarizer in the LCD module, the filtering effect of PIF1 is formed. When the polarization direction of linearly polarized light converted by DBEF is parallel to the transmission axes of the polarizer in the LCD module, the filtering effect of PIF2 is formed.
Fig. 4. Schematic diagrams of the LCD with a PIF inserted between LCD module and backlight unit. (CF: color filter, LC: liquid crystal, TFT: thin-film transistor, LCP: liquid crystal polymer, DBEF: dual brightness enhancement film, LGP: light guide plate)
Figure 5(a) shows the emission spectrum of QD-LED (blue solid line), the theoretical transmittance spectrum of PIF1 (red solid line) and the output spectrum of the QD-LED with PIF1 (blue dashed line). In the emission spectrum, the peak wavelengths (PWs) of blue, green and red spectra correspond to 447 nm, 530 nm and 633 nm, and the FWHMs of blue, green and red spectra are 18 nm, 33 nm and 26 nm, respectively. There are four peaks in the transmittance spectrum of PIF1, and the PWs are 413 nm, 453 nm, 520 nm and 645 nm, respectively. When PIF1 is used in the QD-LED backlight unit, the PWs of blue, green and red spectra correspond to 449 nm, 526 nm and 635 nm, and the FWHMs of blue, green and red spectra are 12 nm, 21 nm and 24 nm in the output spectrum. The PIF1 not only changes the shape of the spectrum but also largely eliminates the cyan (475 nm-500 nm) and yellow (550 nm-600 nm) spectra. Figure 5(b) shows the emission spectrum of QD-LED (blue solid line), the theoretical transmittance spectrum of PIF2 (red solid line) and the output spectrum of the QD-LED with PIF2 (blue dashed line). When PIF2 is used in the QD-LED backlight unit, the PWs of blue, green and red spectra correspond to 447 nm, 529 nm and 635 nm, and the FWHMs of blue, green and red spectra are 16 nm, 29 nm and 25 nm in the output spectrum. Compared with PIF1, the spectral filtering effect of PIF2 is slightly weaker, and the transfer efficiency of PIF2 is higher. Figure 5(c) shows the typical transmittance spectra of CF1 (normal color filter) and CF2 (high-performance color filter). Compared with CF1, CF2 has lower transmittance (B2 and G2) and narrower FWHMs (B2, G2 and R2). Figure 5(d) shows the output spectra of the LCD with CF1 and CF2 when QD-LED is used as the backlight. Compared with CF1, CF2 has almost no influence on the red spectrum and mainly optimizes the blue and green spectra. Figure 5(e) shows the output spectra of the LCD with CF1-PIF1 and CF1-PIF2 when QD-LED is used as the backlight. Compared with PIF2, PIF1 has little influence on the red spectrum and greatly reduces the FWHMs of blue and green spectra. The PWs and FWHMs of these enhanced spectra are listed in Table 1 when the LCD contains CF1, CF2, CF1-PIF1 and CF1-PIF2, respectively. Figure 5(f) shows the color gamut (at the normal direction) of the LCD with CF1 (orange solid line, 107.3% NTSC), CF2 (green solid line, 110.9% NTSC), CF1-PIF1 (red solid line, 124.6% NTSC), and CF1-PIF2 (blue solid line, 117.7% NTSC) in CIE 1931 color space. The results of chromaticity coordinates are listed in Table 2. Taking the color gamut of the LCD with CF1 as a reference, the color gamut of LCD with CF2 is improved by 3.6% NTSC, and the color gamut of LCD with CF1-PIF1 and CF1-PIF2 is improved by 17.3% NTSC and 10.4% NTSC, respectively. Compared with the CF2, CF1-PIF1 and CF1-PIF2 improve the color gamut by 13.7% NTSC and 6.8% NTSC, respectively.
Fig. 5. Spectra and color gamut. (a) The influence of PIF1 on the QD-LED spectrum. (b) The influence of PIF2 on the QD-LED spectrum. (c) The transmittance spectra of CF1 and CF2. (d) The output spectra of LCD with CF1 and CF2 respectively. (e) The output spectra of LCD with CF1-PIF1 and CF1-PIF2 respectively. (f) The color gamut of LCD containing CF1, CF2, CF1-PIF1 and CF1-PIF2 in CIE 1931 color space.
Table 1. The PWs and FWHMs of spectra when the LCD contains CF1, CF2, CF1-PIF1 and CF1-PIF2 respectively
Table 2. The color gamut and TE of LCD containing CF1, CF2, CF1-PIF1 and CF1-PIF2, respectively
4. Color gamut enhancement ratio of CF2, PIF1 and PIF2
Here, we calculate the transfer efficiency (TE) of CFs and PIFs [29]
where Pin (λ) represents the spectra power density of the backlight and Pout (λ) represents the output spectra power density. The TE of CF1 and CF2 is 25.7% and 21.6%, respectively. When PIF1 and PIF2 are used in LCD with CF1, the TE of CF1-PIF1 and CF1-PIF2 is 15.8% and 21.0%. The TEs are also shown in Table 2.Improving the color gamut and transfer efficiency simultaneously is a contradictory topic [17,29]. The improvement of color gamut inevitably leads to a decrease of transfer efficiency. The less or same decrease of transfer efficiency for achieving the larger improvement of color gamut is the better choice in different color gamut improving methods. In order to further understand the influence of CF2, PIF1 and PIF2 on the color gamut enhancement performance of LCD. We define the color gamut enhancement ratio (CGER) by
where ΔCG represents the enhancement of color gamut, ΔTE represents the variation of TE. CGER represents the increase of color gamut when the TE decreases by 1%. The color gamut and TE of LCD with CF1 are used as the reference standard. When high-performance color filter (CF2) is used, the color gamut of LCD is improved by 3.6% NTSC and the TE is reduced by 4.1%. When PIF1 and PIF2 are used in LCD with normal color filter (CF1), the color gamut of LCD is improved by 17.3% NTSC and 10.4% NTSC, and the TE is reduced by 9.9% and 4.7%, respectively. Figure 6 shows the CGER of CF2, CF1-PIF1 and CF1-PIF2. Among the three methods, the CGER of CF1-PIF1 and CF1-PIF2 is significantly higher than that of the CF2, especially the CGER of CF1-PIF2 is more than twice that of CF2. Therefore, the PIFs can effectively solve the trade-off question between color gamut and transfer efficiency.5. Influence of Δnd and twist angle in PIF1 and PIF2
According to Eq. (1) and Eq. (2), the optical properties of the PIF1 and PIF2 are determined by the Δnd and twist angle of the LCP layer. Here, based on the results of PIF1 in Fig. 3(a) and PIF2 in Fig. 3(b) as reference standards, the influence of the Δnd and twist angle of PIF1 and PIF2 on the transmittance is calculated. Figure 7(a) and 7(b) show the transmittance spectra of PIF1 and PIF2 when the twist angle is 720° and the Δnd varies from -3% to 3%. With the increase of Δnd, the transmittance spectra shift largely toward the long wavelength, and the shape of the transmittance spectra is almost unchanged. Figure 7(c) and 7(d) show the transmittance spectra of PIF1 and PIF2 when the Δnd is fixed and the twist angle varies from 710° to 730°. With the increase of twist angle, the transmittance spectra shift slightly toward the long wavelength. The influence of twist angle variation on the transmittance spectra is less than that of Δnd variation.
Fig. 7. The influence of PIF. Transmittance spectra of the PIF with different LCP layers Δnd and the fixed twist angle 720° (a) for PIF1 and (b) for PIF2. Transmittance spectra of the PIF with different LCP layers twist angles and the fixed Δnd (c) for PIF1 and (d) for PIF2.
We further calculate the influence of the Δnd and twist angle of PIF1 and PIF2 on the normalized TE and color gamut when the twist angle variation of PIFs is ±10° and the Δnd variation is ±3%. Figure 8(a) and 8(b) show the iso-normalized TE contour of LCD with PIF1 and PIF2. The variation of normalized TE with Δnd is obvious for PIF1, and not obvious for PIF2. Figure 8(c) and 8(d) show the iso-color gamut contour of LCD with PIF1 and PIF2. The color gamut variation (less than 2% NTSC) is not obvious when the twist angle and Δnd vary within a certain range (bold black line). In the industrial process, the precise control of LCP layer thickness and twist angle is less than 0.1 µm and 1° [42]. Moreover, the transmittance spectrum can be monitored and controlled in a reasonable range during the curing production. In short, the normalized TE of LCD is more than 0.98 and the color gamut variation of LCD is less than 2% NTSC can be realized.
Fig. 8. The influence of PIF. Iso-normalized TE contour (a) for PIF1 and (b) for PIF2. Iso-color gamut contour (c) for PIF1 and (d) for PIF2, the twist angle variation is ±10° and the Δnd variation is ±3%.
6. Color gamut of LCD with PIF1 and PIF2 at different viewing angles
Here, we calculate the color gamut of LCD with CF1 at different viewing angles when PIF1 and PIF2 are added into the LCD, respectively. The iso-color gamut contour of the LCD using PIF1 is shown in Fig. 9(a). The color gamut variation of the LCD is less than 1.10% NTSC within 20° polar angle and less than 2.24% NTSC within 30° polar angle. The iso-color gamut contour of the LCD using PIF2 is shown in Fig. 9(b). The color gamut variation of the LCD is less than 1.00% NTSC within 20° polar angle and less than 1.98% NTSC within 30° polar angle. The color gamut variation caused by PIF1 and PIF2 in the range of 30° polar angle is similar to that caused by functional reflective polarizer in the range of 20° polar angle (2.00% NTSC) [29]. In order to further understand the influence of PIF1 and PIF2 on the color gamut of LCD at different viewing angles. We calculate the transmittance spectra of the PIF1 and PIF2 when the polar angle is 30° and the azimuth angle varies from 0° to 180° with 45° intervals. Figure 9(c) shows the transmittance spectra of PIF1 when the polar angle is 30° and the azimuth angle varies from 0° to 180°, where the black line represents the transmittance spectrum of PIF1 at the normal direction (0° polar angle). The transmittance spectrum of PIF1 has a slight peak shift, and the shape of the transmittance spectrum also has a slight change. Figure 9(d) shows the transmittance spectra of PIF2 when the polar angle is 30° and the azimuth angle varies from 0° to 180°, where the black line represents the transmittance spectrum of PIF2 at the normal direction (0° polar angle). The transmittance spectrum variation of PIF2 is similar to that of PIF1.
Fig. 9. The color gamut of LCD and transmittance spectra of PIFs at different viewing angles. Iso-color gamut contour of LCD with CF1 (a) using PIF1 and (b) using PIF2. Transmittance spectra of PIFs when polar angle is 30° and azimuth angle varies from 0° to 180°. (c) PIF1 and (d) PIF2. The black line represents the transmittance spectra at normal direction (0° polar angle).
7. Conclusion
In this paper, we propose two kinds of PIFs (PIF1 and PIF2) used in backlight unit of LCD to achieve wide color gamut. These two kinds of PIFs not only change the shape of the three primary colors’ spectra but also eliminates the cyan and yellow parts in the output spectrum, which greatly improves the color gamut of LCD. Compared with the high-performance color filter (CF2), the color gamut of LCD increases by 13.7% NTSC and 6.8% NTSC when PIF1 and PIF2 are added into the LCD with normal color filter (CF1), respectively. We use the concept of CGER to compare the influence of PIFs and CF2 on the color gamut enhancement performance of LCD. Compared with the CF2, both PIF1 and PIF2 have higher CGER, and the CGER of PIF2 is more than twice that of CF2. We calculate the influence of Δnd and twist angle variation of PIF1 and PIF2 on the transmittance, normalized TE and color gamut of LCD, and conclude that the normalized TE of LCD is more than 0.98 and the color gamut remains large when the Δnd and twist angle of PIF1 and PIF2 vary in the tolerance range. We also calculate the color gamut of LCD with CF1 at different viewing angles when PIF1 and PIF2 are added into the LCD, respectively. The color gamut variation caused by PIF1 and PIF2 in the range of 30° polar angle is acceptable. These two kinds of PIFs will have a broad application prospect in the LCD, and the high-performance filter can be used in other optical devices.
Funding
National Key Research and Development Program of China (2018YFB0703701).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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