Display Standard - Veritas et Visus

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Veritas et Visus Display Standard February 2009 Variations in the transfer function affect not only the picture’s gray scale and contrast, but also the resulting hue and saturation for all color mixtures, because the individual R,G,B intensities simultaneously dip into different portions of the transfer function. The setting of the black level has an increasingly strong affect on the slope of the transfer function at lower signal intensities. Setting the black level too low reduces the local gamma and increasing it increases the local gamma. Many HDTVs come set from the factory with a high black level. This also eliminates some dark and shadow detail in the picture. For digital signals the standard black level is 16, but many HDTVs come set at levels as high as 40. Figure 3 (on the left): Transfer functions for the Samsung unit with LED local dimming on and off; Figure 4 (on the right): transfer functions for the Samsung unit for gamma parameter settings 70 Source: DisplayMate Technologies and Insight Media Figure 3 compares the transfer functions for the Samsung unit with LED local dimming on and off. The transfer functions are very similar down to about 10% signal. At 5% signal (not shown in this figure) the luminance for the local dimming on is 0.100 cd/m 2 and for local dimming off is 0.195 cd/m 2 . The differences between the two transfer functions are due to differences in signal processing between the two modes. The differences are not due to measurement errors. When local dimming is on, the intensity of the LEDs and the internal video gain, which affects the panel transmission, both change with the signal intensity and APL change in opposite directions with the signal intensity and APL in order to deliver the net transfer function.
Veritas et Visus Display Standard February 2009 The Samsung unit has a user menu option labeled “Gamma” with seven different settings labeled –3 to +3 with a factory default of 0. Figure 4 shows how the transfer function varies with this setting. The Gamma +1 setting was used for all of the other figures and tables in this report as well as in the viewing tests because it is the closest fit to the standard gamma of 2.20. Figure 5: Transfer functions for various LG picture modes; Figure 6: Comparison of the transfer functions on a common percent of peak luminance scale Source: DisplayMate Technologies and Insight Media In Figure 5 the transfer functions for the predefined daylight, normal, and night time picture modes for the LG hospitality unit are the out-of-box unmodified factory settings provided by LG. Given that this model has a very high peak luminance and a very low contrast ratio, these transfer functions are the opposite of what is needed to compensate for the unit’s short-comings and actually further decrease its contrast, picture quality and accuracy. The transfer functions do steepen at low signal intensities due to the very high digital black levels of 24 to 32 chosen by LG instead of the standard digital level of 16. For the user mode we properly set the black level, clipping and white point chromaticity and obtained much better results. The user mode is the mode used for all of the other figures and tables in this report and also for the viewing tests. Note that the user mode luminance has been shifted downward in 9 by 40 percent to prevent crowding. The gammas for the daylight, normal, night time, and user picture modes are respectively: 1.68, 1.65, 1.52, and 2.13. The transfer functions for the four large-screen units are plotted on a common 100% of peak luminance scale in order to compare their relative behavior. A pure power-law gamma with a slope of 2.20 is also plotted in black. 71
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Display Standard - Veritas et Visus

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