Focus
Controlling the electron beam(s) in such a manner that it converges on a single, designated point on the inside face of the CRT.
◎ 何谓聚焦不良?(762字)小未 (47822)于2001/06/05(13:42:08)..
What is loose or poor focus?
A Loose focus, or poor focus performance of the display, is highly dependent on the quality of both the deflection yoke and the electron beam gun.
Implementation of a high quality deflection yoke ensures the electron beam will strike the proper position on the CRT surface. Implementation of a high quality electron beam gun and focus lens are vital to ensuring that the shape of the beam when it strikes the screen is as close as possible to perfectly round, not oval or elliptically distorted. Loose focus can be caused by the physical properties of the CRT display. As was described earlier, the electron beams traveling to the corners of the display are longer than those traveling to the center. Therefore, the display circuitry must dynamically compensate for shorter and longer focal lengths of the electron beams as they scan the screen. Also, the beam shape can distort as it passes through the magnetic field generated by the deflection yoke. If this distortion is not compensated for by a dynamic focus lens, the focus quality, especially at the edges and corners of the display, will greatly deteriorate. Sharp focus also depends on the amount of current needed to write the image to the screen. When more current is needed to write the image to the screen, the electron beam becomes somewhat thicker. This causes the beam to illuminate areas around the spot for which it is intended. This illumination of stray phosphor dots causes the edges of the intended image to blur. This is an advantage of tension mask CRT displays over shadow mask type displays: more light can pass through the aperture grill of the tension mask CRT than can through a conventional shadow mask; therefore, bright images can be displayed with less current, allowing for better focus levels.
◎ 会聚(Convergence) and 会聚不良(Misconvergence)(200字)小未 (47820)于2001/06/05(13:39:34)..
Convergence:
The coming together and joining of lines of red, green and blue to from a single white line. Horizontal and vertical convergence are controlled by the accuracy of the electron beam gun. Misconvergence:
Misconvergence is characterized by the appearance of individual red, green and blue lines (horizontal or vertical) where there should be single, one-colored lines. The influence of surrounding magnetic fields can cause misconvergence.
更详细的解释:(554字)小未 (47823)于2001/06/05(13:43:13)..
What are convergence and misconvergence?
Convergence is the ability of the three electron beams (R, G & B) to come together at a single spot on the surface of the CRT. Precise convergence is necessary as CRT displays work on the principal of additive coloration: when Red light, Green light and Blue light are added in equal amounts they will form a white spot or line. The absence of all color makes a black spot or line. Variation of the relative strengths of the R, G and B electron beams creates color variation.
When the individual electron beams (R, G & B) do not converge perfectly mis-convergence error becomes readily apparent. Misconvergence shows up as shadows which appear around text and graphic images. If a white line is drawn on screen, for example, blue and red shadows may appear either above and below the line or on both sides. Convergence depends highly on the capability of the deflection yoke to guide each electron beam to the proper location. If a poor quality deflection yoke is employed, or if there is outside magnetic influence affecting the performance of the deflection yoke, misconvergence becomes apparent. As convergence highly depends on the electron beam striking the proper place on the CRT surface, misalignment of the shadow mask could result in misconvergence.