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Choi et al.: Colorimetric characterization model for plasma display panel Figure 6. a Spectral power distributions of normalized RGB primaries. b Visible emission of neon gas between 580 and 680 nm. Figure 7. Plot of relative luminance values over time for white at 4, 30, 60, and 100% pattern size. Figure 8. a White color patch at 30% pattern size with a black background. b White color patch at 30% pattern size with a white background. largest gap between its initial state and a steady state. For the 4% and 30% color patches, the luminance decreases significantly at the beginning. Although the time to reach a steady state is similar for all four pattern sizes (around 40 min), the decrease in luminance is dependent on pattern size. A decrease in pattern size leads to an increase the number of RGB sustain pulses accompanied by an increase in temperature of the RGB cells. If a small color patch is displayed on a PDP, the initial temperature is higher than that for a larger sized patch; however this is mitigated by a greater rate of temperature change. As a result, the time to reach a steady state is not dependent on pattern size. Conversely, the decrease in luminance ratio is inversely proportional to pattern size due to the shorter time needed to reach a stable RGB cell temperature. Spatial Independence The two color patches shown in Figures 8(a) and 8(b) have the same center square color and different backgrounds. Spatial independence defines by how much the central white color is affected by changes in background color. 11 The central white colors of Figs. 8(a) and 8(b) have quite different CIE XYZ values (445, 444, 550) and (160, 166, 186), respectively, because the colorimetric characteristics of a PDP are dictated by the number of RGB sustain pulses as determined by the APC rather directly from the RGB input values. For example, in order to produce the same light output for the white in Figs. 8(a) and 8(b), there is a need to have different numbers of sustain pulses. Color Gamut The definition of color gamut is the range of colors that is achievable on a given color reproduction medium under a specified set of viewing conditions. Figure 9 shows the color gamut under dark viewing conditions defined by the primary and secondary colors of a PDP. The ranges of tristimulus values displayed differ according to pattern size (as explained in Pattern size effect section). The color gamuts of four pattern sizes are plotted in a CIELAB a * b * diagram [Fig. 9(a)]. In addition, color gamuts of four pattern sizes at a hue angle of 137° are compared using a CIELAB C * L * diagram [Fig. 9(b)]. The CIE L * a * b * color coordinates were calculated based on the peak white at 4% pattern size representing an L * of 100 848.4 cd/m 2 . Therefore, the C * L * coordinates of the white color patches for 30, 60, and 100% pattern sizes show lower values than those at the 4% pattern size. As pattern size decreases, the range of colors achievable on a PDP becomes larger. Tone Reproduction Curve The tone reproduction curve depicts the relationship between RGB input values and their resultant luminance val- 342 J. Imaging Sci. Technol. 514/Jul.-Aug. 2007
Choi et al.: Colorimetric characterization model for plasma display panel Figure 9. a Color gamuts for the 4, 30, 60, and 100% pattern sizes plotted in an a * b * diagram. b Color gamuts at a hue angle of 137° for the 4, 30, 60, and 100% pattern sizes plotted in a C * L * diagram. ues. The RGB luminances of a CRT are controlled by cathode voltages. For PDPs, on the other hand, the number of RGB sustain pulses controls luminance. Figures 10(a) and 10(b) illustrate the intrinsic properties of the PDP studied. Figures 10(a) and 10(b) contain plots of normalized Y values for the 4% size (white luminance is 1) against the normalized number of sustain pulses and input values, respectively. It can be clearly seen in Fig. 10(a) that an increase in the number of RGB sustain pulses leads to an increase in RGB luminance. Furthermore, a larger patch size has a smaller range of sustain pulse numbers. This is because a larger pattern size is constrained to a lower maximum number of sustain pulses in order avoid exceeding power consumption limitations. The points indicated by the arrows correspond to 95% relative luminance, with respect to the maximum white luminance value at each pattern size. The intrinsic TRC of a PDP [Figs. 10(a) and 10(b)] should be modified so that the slope of the low luminance range is much smaller than that at high luminances. Figure 10(c) shows the result after gamma is modified by adjusting the number of sustain pulses. The shape of the TRC after gamma modification is the same regardless of pattern size. The usable range of the number of sustain pulses, however, depends on pattern size. For example, to produce a white patch on this PDP using an input value of 255, either 466, 766, 1376, or 2594 RGB sustain pulses are assigned, respectively, for the 100%, 60%, 30%, and 4% pattern sizes. The number of sustain pulses available for white at 100% pattern size, 0 to 466, are quantized to 256 levels to make white luminance follow a power function of approximately 2.2 (the gamma value). Additivity The channel additivity was evaluated for white at four pattern sizes. The results are given in Table V in terms of percentage change in tristimulus values and color difference E * ab . The tristimulus values of the RGB patches having the same number of sustain pulses as the peak white patch were measured to evaluate additivity for each pattern size. A substantial difference between the tristimulus values for white and the sum of the red, green and blue channels was found. The latter is larger than the former by about 15% * which corresponds to approximately 6 E ab units. The available power to a cell drops as other cells become active, leading to a reduction in brightness. This means that, for example, in terms of luminance, R+G+Bwhite. To counteract the problem of deviation from additivity, several matrix coefficients—including RGB channel cross terms (RG, RB, GB, and RGB)—were incorporated into the transformation matrices of the two-step model. In addition, a 3D LUT model was implemented in which many measured data points were included so as to compensate for the inherent additivity failure of a PDP. COLORIMETRIC CHARACTERIZATION MODEL FOR A PDP Testing the Models’ Performance at 100% Pattern Size As introduced in the third section, three types of characterization models were developed: 3D-LUT, single-step polynomial, and two-step polynomial. These were tested using the 115-test color set. All of the models developed here are based on a 100% pattern size. The results are summarized in Table * VI in terms of mean and 95th percentile E ab units. It can be seen that the 3D-LUT model using tetrahedral interpolation 12 gave a reasonable prediction to the test data * with a mean and a 95th percentile of 1.3 and 2.5 E ab units. The two-step model using the primary matrix in which their coefficients were based on the measurement data gave quite poor performance with a mean difference of 4.3. Comparing different single-step polynomial models, there is a trend that the higher order polynomial models performed better than the lower ones. However, this is only true for the models developed using more training samples. J. Imaging Sci. Technol. 514/Jul.-Aug. 2007 343
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