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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.
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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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Citations
... To check for display distortions that fall outside acceptable deviation from a standard display after the adjustments, all participants also completed a perceptual brightness matching task [19] in which the brightness of a uniform gray disk was matched to a binary pattern of black and white pixels presented in a surrounding annulus. In separate trials, the proportion of black pixels was set to n/9, where n could take values from 1 to 8. A linear model was used fit to each participant's brightness matches (three repeats per gray-level), and those that fell below an R 2 of 0.8 (N = 19) were excluded from all other analyses in this paper. ...
We present an exploratory study on iridescence that revealed systematic differences in the perceptual clustering of glossy and iridescent samples that was driven by instructions to focus on either the material or the color properties of the samples. Participants’ similarity ratings of pairs of video stimuli, showing the samples from multiple views, were analyzed using multidimensional scaling (MDS), and differences between the MDS solutions for the two tasks were consistent with flexible weighting of information from different views of the samples. These findings point to ecological implications for how viewers perceive and interact with the color-changing properties of iridescent objects.
... The spatial independence and the channel independence were tested for accurate characterization using the JETI spectroradiometer. The GOG (gain-offset-gamma) model was then employed to carry out the display color characterize (Berns, 1996;Day et al., 2004;Xiao et al., 2011). The pure red, green, blue patch and a serious of gray scale patches, seen against a neutral background, were used as the training dataset for building the GOG model. ...
The digital archive of cultural heritage provides new opportunities for the protection of the cultural heritage and the development of online museums. One of the essential requirements for the digitization is to achieve accurate color reproduction. Taking the Imperial Chinese robes in the Qing Dynasty as an example, this study aims to develop a digital achieve system to digitize the robes using a high-end imaging system and accurately reproduce their color properties on a display. Currently, there has been very limited study focused on the color reproduction of silk fabrics or other textile materials. The conventional color management process using a traditional color chart, however, may not be suitable for the reproduction of silk fabrics because they have very high gloss. To address this difficulty, a unique “Qianlong Palette” color chart, consisting of 210 silk fabric samples, has been specifically produced for optimizing the color reproduction of silk fabrics and a color image reproduction system has been developed for the digitization and archiving of the clothing fabric for the royal court. Color characterization models using both the “Qianlong Palette” color chart and the traditional color chart, and different mapping methods, are compared and the model with highest accuracy used in a self-programmed interface for automatically processing textile images in the future. Finally, the digital archive system has been validated using six garments of silk fabric relics. The color differences after the color image reproduction are all less than 3.00ΔE*ab, indicating acceptable color reproduction of the system. The images after color reproduction have also been evaluated subjectively by experts from the museum and the results are considered satisfactory. Our results show that the newly designed “Qianlong Palette” color chart exhibits superior performance over the conventional color chart in effectively predicting the color of the silk fabrics. The self-programmed graphical user interface for image color management can serve as a powerful tool to truly reproduce the color of various silk fabric relics in museums in the future and digitally archive those valuable cultural relics for different uses.
... This visual inspection method can be reiterated, by continuously trying to find the RGB intensity that corresponds to different levels of relative luminance (for example, eight levels ranging from 1/9 to 8/9), and thus providing a more accurate characterization of the display gamma (Xiao, Fu, Karatzas, & Wuerger, 2011). This idea can also be extended to any individual color channel, as demonstrated in Xiao et al. (2011). ...
... This visual inspection method can be reiterated, by continuously trying to find the RGB intensity that corresponds to different levels of relative luminance (for example, eight levels ranging from 1/9 to 8/9), and thus providing a more accurate characterization of the display gamma (Xiao, Fu, Karatzas, & Wuerger, 2011). This idea can also be extended to any individual color channel, as demonstrated in Xiao et al. (2011). ...
Materials in research studies are often presented on digital screens to participants across many subfields of psychology, including clinical, developmental, and social/personality psychology. What is often neglected in the current practice is the reporting of critical visual properties, such as luminance, color, contrast, and gamma, which can dramatically affect the appearance of visual materials. Conventional luminance measurement equipment in vision science is both expensive and onerous to operate for novices. A pressing need—if we are to improve current research practice and education in psychology—is to develop affordable and user-friendly tools to measure and calibrate luminance/color. Here we have developed a software package—PsyCalibrator—that takes advantage of low-cost hardware (SpyderX) and makes luminance and color measurement and calibration accessible and user-friendly. Validation of luminance measurement shows that, in addition to excellent accuracy in linear correction, SpyderX performs at the same level compared with professional, high-cost photometers (MKII and PR-670) in measurement accuracy. SpyderX also has very low measurement variances for practical purposes. A tutorial is provided on how to use PsyCalibrator to measure luminance and color and to calibrate displays. Finally, gamma calibration based on visual methods (without photometers) is discussed, together with its own validation and tutorial.
... This was about 50% of the maximum available brightness ( Figure 3). 15,16 When the brightness is properly adjusted, 36-38 dB should be the approximate upper limit for contrast sensitivity in young observers. 12 Contrast sensitivity data between different perimeters are similar, albeit not exactly the same. ...
Purpose:
To present a home-based visual field examination method using a PC monitor or virtual reality glasses and evaluate the reliability of the method by comparing the results with those of the Humphrey perimeter, in order to assess the possibility of glaucoma screening through the Internet.
Materials and methods:
Software implementing a supra-threshold algorithm for the central 24° (52 points) of visual field at three threshold levels: 1) -4 db, 2) -8 db, and 3) -12 db, from the age-expected sensitivity was used for the purpose of testing. The software uses the web camera as a "virtual photometer" in order to detect room luminosity and allows self-testing using a computer monitor or virtual reality glasses using an Android smartphone with a 6-inch display. The software includes an expert system to analyze the visual field image and validate the reliability of the results. It also allows the physician to combine the results from two or more tests into a single test in order to achieve higher statistical accuracy of the final result. A total of ten patients, 20 eyes tested×52 points per eye=1,040 visual field test points, were compared point to point to those obtained using the Humphrey perimeter for the same patients, as they appeared randomly and consecutively at the glaucoma department within hours.
Results:
Good receiver operating characteristic/area under the curve coefficient was found, ranging from 0.762 to 0.837 (P<0.001). Sensitivity ranged from 0.637 to 0.942, and specificity ranged from 0.735 to 0.497.
Conclusion:
The home-based visual field test exhibits a reasonable receiver operating characteristic curve when compared to the Humphrey perimeter, without the need of specialized equipment. The test may be useful for glaucoma screening.
... Phase linearity correction is an essential step for phase modulation panels. This is similar to the gamma correction for linear amplitude modulation in LC displays [42], but is a more complex process, covering the full 2 π radians phase. The linear phase response is a key criterion for satisfying computer-generated hologram (CGH) patterns calculated on the basis of the iterative Fourier transform algorithm (IFTA) or Fresnel (near-field)/Fraunhofer (far-field) diffraction equations, derived from scalar diffraction theory. ...
Fine pixel size and high-resolution liquid crystal on silicon (LCoS) backplanes have been developed by various companies and research groups since 1973. The development of LCoS is not only beneficial for full high definition displays but also to spatial light modulation. The high-quality and well-calibrated panels can project computer generated hologram (CGH) designs faithfully for phase-only holography, which can be widely utilized in 2D/3D holographic video projectors and components for optical telecommunications. As a result, we start by summarizing the current status of high-resolution panels, followed by addressing issues related to the driving frequency (i.e., liquid crystal response time and hardware interface). LCoS panel qualities were evaluated based on the following four characteristics: phase linearity control, phase precision, phase stability, and phase accuracy.
... The display's gamma/brightness/contrast can be visually calibrated. 5,6 Visual calibration is sufficiently reliable to be used as an alternative to calibration using an expensive photometer. 5 The software uses a gray scale step wedge for display adjustment. ...
Purpose:
To present a visual field examination method using virtual reality glasses and evaluate the reliability of the method by comparing the results with those of the Humphrey perimeter.
Materials and methods:
Virtual reality glasses, a smartphone with a 6 inch display, and software that implements a fast-threshold 3 dB step staircase algorithm for the central 24° of visual field (52 points) were used to test 20 eyes of 10 patients, who were tested in a random and consecutive order as they appeared in our glaucoma department. The results were compared with those obtained from the same patients using the Humphrey perimeter.
Results:
High correlation coefficient (r=0.808, P<0.0001) was found between the virtual reality visual field test and the Humphrey perimeter visual field.
Conclusion:
Visual field examination results using virtual reality glasses have a high correlation with the Humphrey perimeter allowing the method to be suitable for probable clinical use.
... However, the relation between the phase modulation and the gray level must be linear in order to perform an accurate waveform correction. To achieve this, a Gamma correction technique was selected that has been widely used with LCDs to correct the nonlinearity between the brightness and the voltage [15]. When we send the gray map to the LCWFC, it is first converted with a look-up table (LUT) as shown in Fig. 5(b), and then the converted gray map is utilized to drive the LCWFC [16]. ...
... A method of Gamma correction is a mature method for liquid crystal displays [12,13]. In order to satisfy the subjective sense of human eyes to object light, it is necessary to generate a power function relationship between the output light intensity E(x, y) and input signal D in the liquid crystal display, which is called a Gamma curve. ...
The ability of phase modulation enables liquid crystal spatial light modulator (LCSLM) to control wavefront. However, the disadvantage of its inherent nonlinear phase response will decrease the wavefront control accuracy. In this paper, a compensation for the nonlinear phase response is proposed based on Inverse Interpolation method. Characteristic curve of phase retardation versus gray levels for a 256×256 pixels phase-only LCSLM has been measured and calibrated by Inverse Interpolation. A mapping relationship between input gray levels and driving gray levels has been built and recorded by a linear look-up table ANTI2.LUT. The nonlinear error of the phase drops from 15.9% to 2.42% by using ANTI2.LUT. Further more, the mapping curve of ANTI2.LUT is almost consistent with 290.LUT from the manufacturer, which proved the efficiency of the compensation of phase nonlinearity. Finally, the distorted wavefront caused by a liquid crystal flake is corrected using LCSLM based on ANTI2.LUT. Experimental results show that the peak-valley value of the distorted wavefront decreases from 1.56λ to 0.26λ (λ = 0.6328 µm), the root-mean-square value decreases from 0.25λ to 0.02λ and the Strehl ratio of diffractive spots increases from 0.08 to 0.97. So LCSLM can be applied to realize high-precision and high-resolution wavefront correction with linear phase response.
... The nonlinear response functions of the LCD screen and CCD camera will introduce nonlinear error to the captured fringe pattern [9][10][11][12]20 and cause error in the demodulated phase. The nontelecentric imaging of the CCD camera will introduce another kind of nonlinear error (aberration) to the measurement. ...
In phase measuring deflectometry (PMD), the inspection accuracy of the defects and height of the specular surface are related to the level of phase errors. The usage of numeric integration in reconstructing the shape and the defocusing capture of the fringe pattern, which will amplify the phase errors, make error discussion more significant in PMD than other shape measurement techniques. Phase error analysis and reduction in PMD are presented. The random noises, nonlinear response function, the nontelecentric imaging of the charge-coupled device camera, and the nonlinear response function of the liquid crystal display screen are the main phase error sources in PMD. The analytical relation between the random phase error and its influence factors in PMD is deduced. From the relation formulation, the influence factors of random phase error are analyzed, and the results are proven by the simulation and experiment. A possible phase error-reduction method, which integrates several methods for congeneric errors in fringe projection profilometry, is investigated to reduce phase errors in PMD. This composite method is proven to have a good performance by a plane mirror experiment. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)
... A method for estimating the non-linear gamma transfer function of liquid-crystal displays (LCD) without the need of a photometric measurement device was described by Xiao et al [1]. It relies on observer's judgments of visual luminance by presenting eight half-tone patterns with luminances from 1/9 to 8/9 of the maximum value of each colour channel. ...
... In this regard, CRT monitor technology has been extensively studied in the past both in terms of their colour characteristics [4,5] and calibration techniques [6,7], including those that rely on visual comparison instead of a photometer [8]. On the other hand, corresponding LCD colour characteristics and calibration methods have started to be reported much later [1,2,9]. ...
... The precise modelling of the OETF is likely to require a photometer with the corresponding cost and relatively higher degree of user expertise. However, a simpler (and cheaper) "perceptual" alternative has been developed and successfully used in CRT [8,20,21] and LCD [1,22] characterization, and in the case of CRT displays, there are several commercial gamma correction software available, e.g. Adobe Gamma (Adobe San Jose CA,US) and EasyRGB (http://EasyRGB.com). ...
A method for estimating the non-linear gamma transfer function of liquid-crystal displays (LCD) without the need of a photometric measurement device was described by Xiao et al [1]. It relies on observer’s judgments of visual luminance by presenting eight half-tone patterns with luminances from 1/9 to 8/9 of the maximum value of each colour channel. These half-tone patterns were distributed over the screen both over the vertical and horizontal viewing axes. We conducted a series of photometric and psychophysical measurements (consisting in the simultaneous presentation of half-tone patterns in each trial) to evaluate whether the angular dependency of the light generated by three different LCD technologies would bias the results of these gamma transfer function estimations. Our results show that there are significant differences between the gamma transfer functions measured and produced by observers at different viewing angles. We suggest appropriate modifications to the Xiao et al paradigm to counterbalance these artefacts which also have the advantage of shortening the amount of time spent in collecting the psychophysical measurements
