Kok S. Chen

17.3: Color Correction in TFTLCD Displays for Compensation of Color Dependency with the Viewing Angle

Most todays twisted nematic TFTLCD displays show a color dependency with the viewing angle. This paper describes a method for compensation of the pixel color variations in these displays relative to a single viewer position. The color correction can be recomputed automatically as the viewer position changes, as long as the position is known. The method determines the color correction required for each pixel of the screen, such that a single viewer for a given position can see the color unaffected by the viewing angle differences to the screen. Color is measured in few points over the screen relative to a viewer position, and then an interpolation procedure is used to compute the color correction for each intermediate pixel, between the measured positions. The measurements for different positions of the viewer are used in a second interpolation procedure for computing color correction for intermediate position of the viewer between the measured positions. The method was experimented for portable and desktop displays. It was found that, for relative small viewing angle variations, with acceptable approximation, the color variation with the viewing angle can be modeled as variations of the transfer function. It was found that the proposed method can also be effectively used to compensate for panel uniformity.

Color characterization issues for TFTLCD displays

This paper describes few issues related to TFTLCD display color rendition and characterization. The paper points out specific aspects of color rendition on TFTLCD devices that differentiate the traditional CRT self luminous devices from the TFTLCD transmissive devices. Two TFTLCD technologies are discussed, TN (Twisted Nematic) and IPS (In Plane Switching) with and without dual domain improvement. The paper discusses specific aspects of color rendering for TN and IPS technologies, the display primaries, color gamut and brightness, the transfer function, the white point, some viewing angle issues and the color model. The paper explains why the TFTLCD displays offer perceptually a larger gamut than the CRT displays, even if chromatically their gamut triangle is smaller in the CIE chromaticity diagram than the one of a typical CRT. The paper also explains the hue shift effect of the primaries with the input voltage and the effect of luminance variation with the viewing angle, both encountered in some TN TFTLCD displays. The paper confirms that despite their advantages due to high brightness, high contrast, high sharpness, virtually no geometric image distortion, in terms of color capability, with few exceptions, TFTLCD devices have not yet surpassed the CRT devices with larger chromatic gamut and no color variation with the viewing angle.

Time varying halftoning

For displays with limited color capability, color halftoning is often used to induce the illusion of more color levels than the device can truly render. In such cases, the pattern created by the halftone technique can be visible due to the limited geometrical resolution of the display device. Unlike printing, the halftone pixels of the display devices are still visible and the patterns created on display devices can be easily detected. When animated images are played and the frame rates are played in a mode that uses a limited number of colors and halftoning techniques, the patterns are very visible because they are perceived as a reference system fix to the playing window, while the image content is moving. In this paper we propose a halftone technique that varies the halftone pattern in time such that the pattern visibility is reduced. This conducts to a higher quality of the displayed image, especially noticeable for dynamic images. The technique enables the use of display devices (or displaying modes) with limited color capability for higher quality images.