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Schematics of a Twisted Nematic (TN) pixel element (cell). The left part of the figure represents the cell's OFF state, where no voltage is applied to the electrodes allowing the light from the light source (e.g. LED backlighting or cold cathode fluorescent backlighting) to arrive to the observer's eye. The right part of the screen represents the ON state, where a voltage is applied to the electrodes resulting in most of the light to be blocked by the two orthogonally oriented polarisers.

Schematics of a Twisted Nematic (TN) pixel element (cell). The left part of the figure represents the cell's OFF state, where no voltage is applied to the electrodes allowing the light from the light source (e.g. LED backlighting or cold cathode fluorescent backlighting) to arrive to the observer's eye. The right part of the screen represents the ON state, where a voltage is applied to the electrodes resulting in most of the light to be blocked by the two orthogonally oriented polarisers.

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An improved method for visual gamma correction is developed for LCD displays to increase the accuracy of digital colour reproduction. Rather than utilising a photometric measurement device, we use observers’ visual luminance judgements for gamma correction. Eight half tone patterns were designed to generate relative luminances from 1/9 to 8/9 for e...

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... in the determination of their OETF are [10] luminance (light leakage) (f) cross-talks between neighbouring pixels (g) dependency of display characteristics with temperature (h) need for measurement instruments to capture the narrow-band fluorescent lights used as LCD light sources (i) complex reflection of ambient light from the display screen. Fig. 1 shows the schematics of a pixel element inside one of the most common LCD types, the backlit Twisted Nematic (TN) LCD [11][12][13]. The light is usually produced by LEDs or fluorescent ele- ments and a mosaic of R, G and B filters is aligned to the substrate glass producing coloured cells that are controlled independently, so that the ...
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... popular LCD technology is termed Vertical Alignment (VA) [14][15][16][17]. The main difference with TN technology is that when no voltage is applied, the liquid crystals do not allow the passage of light through the crossed polarisers (see Fig. 1). Given that their natural state is to block light, VA monitors provide good black depth. The OETF is again dependent on the viewing angle, but there is no reason for its dependency to be the same from that of TN ...
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... . 10 illustrates the results for the three chromatic channels (each line corresponds to a particular monitor, as detailed in the caption). Error bars correspond to the standard deviations of the measurements obtained for nine subjects. Notice that there is no difference among channels regarding the shape of the curves for the same monitors. ...
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... curves in Fig. 11 show a close agreement between the data (in grey-levels) obtained by colorimetric methods and that obtained by psychophysical methods. There is a systematic shift Fig. 14. Correlation between the colorimetric measures and psychophysical data for the ASUS VH222D, the LG IPS231P and the BENQ EW2730V monitors. All values were normalized ...
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... curves in Fig. 11 show a close agreement between the data (in grey-levels) obtained by colorimetric methods and that obtained by psychophysical methods. There is a systematic shift Fig. 14. Correlation between the colorimetric measures and psychophysical data for the ASUS VH222D, the LG IPS231P and the BENQ EW2730V monitors. All values were normalized to its maximum and error bars show the standard deviations for nine human observers. Fig. 15. Sum of the absolute differences between the psychophysical measures and the ...
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... methods and that obtained by psychophysical methods. There is a systematic shift Fig. 14. Correlation between the colorimetric measures and psychophysical data for the ASUS VH222D, the LG IPS231P and the BENQ EW2730V monitors. All values were normalized to its maximum and error bars show the standard deviations for nine human observers. Fig. 15. Sum of the absolute differences between the psychophysical measures and the colorimetric measures for each of the monitors tested. The right plot shows how this changed when the two mid-luminance data points (44% and 55% of MAX luminance) are ...
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... BENQ monitor shows the highest agreement between col- orimetric and psychophysical data (the curves in Fig. 12 are prac- tically superimposed, well within the limits of the error bars). The consistency within subjects is also remarkably high for this particular ...
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... LG monitor (see Fig. 13) is the only one where the colori- metrically-measured ''dip'' is also mirrored by the psychophysical results. The most likely cause of this dip is the rapid inversion of polarity in LCD cells, which is applied to prevent permanent dam- age in liquid crystal materials. 2 This inversion of voltage is applied on alternate video frames ...
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... on alternate video frames and in anti-phase regarding nearby pixels, thus approximately cancelling brightness artefacts (flicker). Voltage inversion can become apparent in some monitor/half-tone pattern configurations. We speculate here that these flickering patterns (vis- ible only in the LG screen) are the best explanation for the dip in Fig. 13. A deeper study of this particular technology may reveal the exact causes of this phenomenon. Fig. 14 shows the correlations between the psychophysically collected results and their corresponding colorimetric measure- ments (for each monitor and half-tone pattern). Error bars corre- spond to standard deviations in the case of the ...
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... brightness artefacts (flicker). Voltage inversion can become apparent in some monitor/half-tone pattern configurations. We speculate here that these flickering patterns (vis- ible only in the LG screen) are the best explanation for the dip in Fig. 13. A deeper study of this particular technology may reveal the exact causes of this phenomenon. Fig. 14 shows the correlations between the psychophysically collected results and their corresponding colorimetric measure- ments (for each monitor and half-tone pattern). Error bars corre- spond to standard deviations in the case of the psychophysical measures. The best fitting lines and corresponding R 2 correlation coefficient are shown in ...
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... data, indeed human observers can be used to determine the gamma transfer function of LCD mon- itors. For the ASUS monitor, the slope of the fit is the most different from one, reflecting the linear shift between both sets of data. The differences between the three (RGB) monitor channels are very small except in the case of the BENQ monitor. Fig. 16. Psychophysical data collected for the ASUS monitor in lab conditions and in an ''office-like'' environment and in parallel and sequential presentations (first and second experiments). Black crosses correspond to standard deviations. Fig. 15 shows the absolute difference (the sum of the differences between the colorimetric and ...
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... between the three (RGB) monitor channels are very small except in the case of the BENQ monitor. Fig. 16. Psychophysical data collected for the ASUS monitor in lab conditions and in an ''office-like'' environment and in parallel and sequential presentations (first and second experiments). Black crosses correspond to standard deviations. Fig. 15 shows the absolute difference (the sum of the differences between the colorimetric and psychophysical measures) in terms of percentage of the maximum value. The data shows that if we use this as a quality measure of the correspondence between what observers do and what a photometer does, the LG monitor does almost twice as bad as the ...
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... of the correspondence between what observers do and what a photometer does, the LG monitor does almost twice as bad as the BENQ and the ASUS. This is probably because of the presence of the two central patterns (44% and 55% of maximum luminance) which seem to be problematic in this par- ticular LCD screen technology and model. The right panel in Fig. 15 shows the same results when the two ''problematic'' points are ...
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... we tested how the psychophysical results differ when measured in office conditions (artificial illumination). Only the ASUS LCD screen was tested under both (lab and office) conditions and its results are shown in Fig. 16. A visual inspection of the figure suggests that all channels behaved in a similar way (see dark bars in all three panels). We expanded the plot of the red channel to show these particular results. For comparison, we also included the results obtained for the Experiment 1 ''parallel'' (light bars) and Experiment 2 ''simultaneous'' ...

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