## 2 Performance Metrics
### Color Gamut and Color Shift  
Color gamut and color shift are two important metrics to evaluate the color performances of a display panel. A good display should exhibit a wide color gamut and a weak color shift. The color gamut of a display is better represented by a 3D volume instead of a chromaticity gamut area. The gamut envelope is calculated in the human perceptual color space CIE 1976 \(L^{*}\)_a*b*_ due to its good perceptual homogeneity. Our measurement procedure follows the reference method that the ICDM recommends. In this method, the full 602-color reference sets should be used if the behavior of the display panel is not well known. The measurement procedure and setting are explicitly illustrated in Figure 4a. The first step was to generate 602 color images in the Rec.2020 color space. These 602 unique colors were evenly distributed on the linear RGB (red, green, blue) signal space. Then, we measured the tristimulus values (\(X\), \(Y\), \(Z\)) of each color using the LMK with full glass filters for color matching functions. Next, a chromatic-adaptation transform was built to adapt the measured values from the display white point to the standard illuminant D50 using the modified Bradford adaptation transform method. The last step was to calculate the CIE 1976 D50 \(L^{*}\)_a*b*_ coordinates from the converted tristimulus values as follows:  
\[L^{*}=116\times f\bigg{(}\frac{Y}{Y_{n}}\bigg{)}-16, \tag{1}\]  
\[a^{*}=500\times\bigg{[}f\bigg{(}\frac{X}{X_{n}}\bigg{)}-f\bigg{(}\frac{Y}{Y_{n} }\bigg{)}\bigg{]}, \tag{2}\]  
\[b^{*}=200\times\bigg{[}f\bigg{(}\frac{Y}{Y_{n}}\bigg{)}-f\bigg{(}\frac{Z}{Z_{n }}\bigg{)}\bigg{]}, \tag{3}\]  
Figure 3: Measured 2D luminance distributions at the second gray level of the (**a**) mLCD and (**b**) OLED panels. Figure 2: (**a**) Gamma curves for the mLCD and the OLED display and (**b**) gamma curves at low gray levels. \[f(t)=\left\{ \begin{array}{cc}t^{1/3}&t>(6/29)^{3}\\ \frac{1}{3}(\frac{29}{6})^{2}t+\frac{16}{116}&\text{otherwise}\end{array} \right. \tag{4}\]  
where (\(Xn\), \(Yn\), \(Zn\)) is the D50 with a certain white luminance. To compare the color gamut volumes of the mLCD and the OLED display, their different brightnesses should be considered as well. For this reason, the white luminance was set to the white brightness of the mLCD to reflect that the brightness can greatly impact the color gamut volume. The color gamut volumes of the mLCD and the OLED display are shown in Figure 4b. According to the gamut envelope, the color space of the mLCD and the OLED display had a CIELAB gamut volume of 1191300 \(\Delta E^{3}\) and 650680 \(\Delta E^{3}\), respectively. In terms of the reference Rec.2020 color space, the percentages of coverage for the mLCD and the OLED display were 64.3% and 35.0%, respectively. The mLCD exhibited a larger color gamut volume than the OLED display due to its higher peak brightness. Color shift is another important parameter to evaluate the color uniformity of a display panel. The angular color shift can be impacted by both the variations of primary RGB colors and unmatched angular distributions of primary colors according to the color mixing principles. For an mLCD, its angular color shift arises from the angle-dependent accumulated phase retardation in the birefringent LC layer. When the incident light transverses the LC layer at different angles, the accumulated phase retardation varies, giving rise to the angle-dependent transmittance and color shift. For this reason, the color coordinates of primary colors vary with viewing angles. For an OLED display, the variations of primary colors are attributed to the cavity effect. As the viewing angle increases, the emission spectrum shifts toward a shorter wavelength because of the reduced effective cavity length. On the other hand, if the angular distributions of primary colors are not matched well, the mixed colors will vary with the viewing angle due to angle-dependent RGB ratios [24; 25].