Random depolarization film design based on modulation transfer function measurements of displays with different pixel pitches.

Polarized light emitted from most displays causes blackout and color change problems when viewed through polarized sunglasses. These problems are addressed by random depolarization films (RDFs) doped with birefringent particles. These dopant particles, however, scatter light and degrade the sharpness of the displayed images. To maintain the image sharpness, we designed an RDF based on modulation transfer function measurements of displays with different pixel pitches. RDFs doped with larger particles achieved higher modulations; however, particles larger than a specific size caused undesirable sparkles on the displayed image. A display with a larger pixel pitch achieved higher modulations, suggesting that the RDF is particularly suitable for large-screen displays.

Edge-based modulation transfer function measurement method using a variable oversampling ratio: erratum.

This erratum corrects an error in Fig. 4 and its description in my published paper [Opt. Express29, 37628 (2021)10.1364/OE.435981].

Edge-based modulation transfer function measurement method using a variable oversampling ratio.

The ISO 12233 edge-based method approximates the modulation transfer function (MTF) as a function of horizontal or vertical spatial frequency by analyzing a 1D supersampled edge gradient obtained from the captured image of a near-vertical or near-horizontal bi-tonal edge, respectively. The method involves the slanted projection of pixels in a square array into a linear array of subpixel-wide bins. It can be modified to accommodate diagonal MTF measurements; however, using a fixed integer oversampling ratio degrades the accuracy and precision of diagonal MTF estimates due to periodic misalignment between the projection paths and the bin array. In this study, an edge-based method-called OMNI-sine-is proposed to counter the misalignment and improve MTF estimates by using a variable oversampling ratio dependent on the slant angle.

Proper application of chromaticity gamut area metrics for displays.

The areal dimension in chromaticity diagrams is often used to evaluate the range of reproducible colors (or color gamut) of a display; however, its meaning is not fully understood. For consistent and meaningful color gamut comparisons, the metric needs a color space with a common chromatically adapted white point, which is not considered in legacy chromaticity gamut area metrics. This study demonstrates the validity of chromaticity gamut area metrics and clarifies the proper way to use the metrics by comparing the color gamuts of synthetic additive displays with a range of sampled RGB primaries with different white points. The study reports high correlations of 0.98-0.99 between the xy chromaticity gamut area and color gamut volume for additive displays with a common white point at 6500 K and above. However, using the "gamut rings" 2D visualization method, this study also demonstrates that use of different white points cause a failure of the chromaticity gamut area metric.

Accuracy and precision limitations in measuring luminance modulation for pixelated displays.

This study examines the constraints in measuring the contrast modulation or line-pair luminance modulation for pixelated displays. Simulation results show that measurement precision is affected by the offset between the display pixels and the detector array of the light measuring device (LMD). The contrast modulation is underestimated if the spatial imaging performance of the LMD is inadequate. A high pixel ratio, i.e., the number of pixels of the measurement device per display pixel interval, is required to increase the measurement accuracy. However, determining the minimum required pixel ratio is not straightforward, as the accuracy depends on the display device architecture as well.

Practical edge-based modulation transfer function measurement.

The accuracy and precision of the modulation transfer function (MTF) of a sampled imaging system are affected by the shift-variant nature of subpixel binning of the pixel values in edge-based methods. This study demonstrates that a binning phase selected from a small number of binning phases can achieve a practical precision criterion for the MTF measurement. Furthermore, the new method proposed in this paper approximates the non-aliased, fundamental MTF without edge angle estimation and the following subpixel binning. The algorithm simply averages the aliased MTFs calculated from the row-by-row edge gradients in the region of a bitonal edge image and removes an assumed aliasing component. This method is also applicable to an oblique and non-straight edge.

Appropriate indices for color rendition and their recommended values for UHDTV production using white LED lighting.

We selected appropriate indices for color rendition and determined their recommended values for ultra-high-definition television (UHDTV) production using white LED lighting. Since the spectral sensitivities of UHDTV cameras can be designed to approximate the ideal spectral sensitivities of UHDTV colorimetry, they have more accurate color reproduction than HDTV cameras, and thus the color-rendering properties of the lighting are critical. Comparing images taken under white LEDs with conventional color rendering indices (Ra, R9-14) and recently proposed methods for evaluating color rendition of CQS, TM-30, Qa, and SSI, we found the combination of Ra and R9 appropriate. For white LED lighting, Ra ≥ 90 and R9 ≥ 80 are recommended for UHDTV production.

Metric of color-space coverage for wide-gamut displays.

Assessing the coverage of the color space of Recommendation ITU-R BT.2020 (Rec. 2020) has become increasingly important in the design of wide-gamut displays, and an appropriate metric for measuring the display gamut size is urgently needed. Display manufactures calculate the area ratios of their displays' RGB triangles to a standard RGB triangle in the CIE 1931 xy or CIE 1976 u'v' chromaticity diagram to indicate the displays' relative gamut size. However, they typically fail to mention which of the two diagrams the metric is based on. This paper shows that the ratios calculated in the two chromaticity diagrams are highly inconsistent, and that the Rec. 2020 area-coverage ratios for wide-gamut displays in the xy diagram are much more correlated to the Rec. 2020 volume-coverage ratios in some color-appearance spaces than the Rec. 2020 area-coverage ratios in the u'v' diagram. This paper recommends the use of the xy diagram for area-coverage ratio calculations for wide-gamut displays.

Designing display primaries with currently available light sources for UHDTV wide-gamut system colorimetry.

The wide-gamut system colorimetry has been standardized for ultra-high definition television (UHDTV). The chromaticities of the primaries are designed to lie on the spectral locus to cover major standard system colorimetries and real object colors. Although monochromatic light sources are required for a display to perfectly fulfill the system colorimetry, highly saturated emission colors using recent quantum dot technology may effectively achieve the wide gamut. This paper presents simulation results on the chromaticities of highly saturated non-monochromatic light sources and gamut coverage of real object colors to be considered in designing wide-gamut displays with color filters for the UHDTV.

Modified slanted-edge method and multidirectional modulation transfer function estimation.

The slanted-edge method specified in ISO Standard 12233, which measures the modulation transfer function (MTF) by analyzing an image of a slightly slanted knife-edge target, is not robust against noise because it takes the derivative of each data line in the edge-angle estimation. We propose here a modified method that estimates the edge angle by fitting a two-dimensional function to the image data. The method has a higher accuracy, precision, and robustness against noise than the ISO 12233 method and is applicable to any arbitrary pixel array, enabling a multidirectional MTF estimate in a single measurement of a starburst image.

Computation of optimal metamers.

A fast, accurate model for computing Logvinenko's optimal metamers is introduced. The spectral reflectance of an optimal metamer is uniquely determined from a class of metamers with common tristimulus values. A set of optimal metamers is useful for evaluating the color reproduction and gamut of object colors under different illuminants. In conventional methods, optimal metamers are calculated by interpolating the coordinates of precomputed optimal colors. This model optimizes the spectral reflectance parameters efficiently without switching between bandpass (Type I) and bandstop (Type II) optimal metamers, or requiring any stored optimal-color datasets. Some optimal metamer loci computed using MATLAB are presented.

Number of discernible object colors is a conundrum.

Widely varying estimates of the number of discernible object colors have been made by using various methods over the past 100 years. To clarify the source of the discrepancies in the previous, inconsistent estimates, the number of discernible object colors is estimated over a wide range of color temperatures and illuminance levels using several chromatic adaptation models, color spaces, and color difference limens. Efficient and accurate models are used to compute optimal-color solids and count the number of discernible colors. A comprehensive simulation reveals limitations in the ability of current color appearance models to estimate the number of discernible colors even if the color solid is smaller than the optimal-color solid. The estimates depend on the color appearance model, color space, and color difference limen used. The fundamental problem lies in the von Kries-type chromatic adaptation transforms, which have an unknown effect on the ranking of the number of discernible colors at different color temperatures.

Fast and accurate model for optimal color computation.

A fast and accurate model to compute optimal colors under a given illuminant is introduced. The model estimates the reflectance (transmittance) distributions of optimal colors, which is bandpass (Type 1) or bandstop (Type 2), at a given luminance factor with user-specified small tolerances for the bandwidth (e.g., 10(-10) nm). The tristimulus values of the optimal colors are obtained by using trapezoidal integration of the product of color-matching functions and illuminant spectrums sampled with small wavelength steps instead of performing summation of the product values. Selecting the distribution type whether Type 1 or Type 2 is avoided in the algorithm to reduce computing cost. Some optimal color solids computed by a MATLAB program have been demonstrated.