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Chapter 5:
Generating the Pixel
The pixel (picture element) is the building block from which image quality will be derived. A perfect Gaussian-shaped pixel (or spot) is not a realistic expectation for a CRT display. The screen center is the only location where the electron beam is truly perpendicular to the phosphor screen and theoretically capable of providing a Gaussian distribution. Add in veiling glare, halation, and other inherent distortions that make the pixel larger, and the ideal pixel shape becomes awash in unwanted luminance energy. Controlling the formation of the pixel in both the horizontal and vertical dimensions is therefore key to optimizing contrast modulation. If the electron beam were stationary at the time each pixel was formed, the optics would be the only controlling element of concern. But in a raster-scanned display, the beam is being pulled from the top left corner to the bottom right corner in sequentially scanned lines. The added element of velocity imparts a time domain to controlling the beam current reaching the screen at any given pixel location. As such, the vertical dimension of a pixel is controlled by the optics, but the horizontal width, with the element of time, is controlled by the video amplifier’s ability to respond. A slow video amplifier will spread the current out over the available pixel time, reducing the potential contrast modulation with adjacent pixels. Thus, an appropriately matched video amplifier will yield better contrast modulation because more of the beam current (current density) is concentrated at the center of the pixel. The most inferior display can produce “on” pixels, but may never generate an “off” pixel between two on-pixels at maximum luminance. The display with the superior image quality (i.e., contrast modulation) is the one that can produce the off-pixel in a field of on-pixels by controlling the shape of the individual pixel down to the 5% point of luminance energy.
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