Monitors: Things You Need To Know

Dot Pitch: The dot pitch is the measurement between adjacent triads or RGB
phosphor dot groups. Smaller the dot pitch, more the number of dots per inch and higher
the maximum potential resolution of the monitor. In the case of an aperture grille
monitor, it is the distance between two adjacent sets of stripes of phosphor.

Bandwidth: The bandwidth represents the ability of the video circuitry in the
monitor to turn a single pixel on or off. The bandwidth needed for a display is
proportional to the number of pixels per second, so higher the resolution or refresh rate
that you are running, higher your bandwidth requirement.

Dynamic Focus: The optimum focal length for an electronic beam varies as the
beam scans from top to bottom and from side to side on the face of the CRT leading to a
difference in focus between the center and edges of the screen. In some monitors, extra
elements in the CRT gun continuously adjust the focal length of the beam and the
convergence points of the three beams to ensure consistent performance. This technique is
called dynamic focus.

Dynamic Beam Forming: Due to the difference between the radii of focus and the
CRT face, at the center of the screen, the circular beam lands on the shadow mask at right
angles, so that the spot formed is circular. At the corner of the screen, the beam lands
at an angle so that a more elliptical shape is formed. This increases the spot size and
reduces the focus performance. To correct this, CRT manufacturers have developed special
focussing lenses that change the shape of the beam as it scans into the corners.

Convergence: The convergence of color beams affects image sharpness and the
convergence of the monitor. The full range of colors including white is only available
when all three beams converge at the same point on the shadow mask. In practice there is
never perfect convergence because of the difficulties in deflecting the three beams
through very marginally different angles using the same deflection coils..

Reflections: The surface treatment needed to reduce reflections and glare from
the screen can affect the sharpness of the image. There are five current CRT surface

  • Polish: A polished screen looks sharp but would not
    be allowed under any ergonomic standards due to reflections.
  • Etch: With an etched screen, the surface of the CRT
    is treated with an acid to make it rough. This diffuses the light that hits the screen and
    also diffuses the light passing through the glass, reducing the sharpness. Quality screens
    should not be etched.
  • Silica Coatings: A further step is to coat the
    screen with a very fine layer of silica which improves the anti-reflective performance
    compared to etching and reduces the image diffusion. The low cost of this process has led
    to wide adoption as a replacement for etching.
  • Anti-Reflective Coatings: A useful process
    where two very fine layers of silica with different refractive indices are coated onto the
    surface of the tube. With this process a substantial part of the benefits of the AR panel
    could be gained at a relatively low cost.
  • AR Panels: The best way to stop reflections is to
    use three layer multicoated filters on a special glass panel bonded onto the face of the
    CRT. This eliminates almost all the visible reflected light, with no degradation of the
    image. Unfortunately it is expensive and so is only used on the highest quality monitors.
  • Glare: Glare refers to reflections from the surface
    of the phosphor on the inside of the glass faceplate. It reduces the contrast of the
    screen image and legibility. Glare is reduced by using a tinted glass where the reflected
    light has to pass through the tinted glass twice, while the screen image only passes
    through it once.

Geometry & Distortions: The CRT is good at supporting multiple image formats
on the same monitor, but this means that it can be very difficult to ensure accurate and
consistent screen geometry. Good geometry and linearity are very important in applications
like CAD, DTP and graphics.

Moire: Moire patterns are interference patterns caused by the interaction
between the pattern of dots from the beam and those on the shadow mask.
They are most often most visible when a screen background is set to a pattern of dots, for
instance a gray screen background consisting of alternate black and white dots. Earlier
the only way to eliminate moire effects was to defocus the beam, but now a number of
monitor manufacturers have developed techniques to increase the beam size, without
degrading the focus.

Color Performance: In order to get the best colors on the screen, it is
important to start with pure red, green and blue. CRT manufacturers use different types
and formulations of phosphor which can give monitors made with those CRTs a distinctive
color performance.

Color Purity: In order to get consistent color performance across the whole
screen, each phosphor color must be pure. Irregularities and ‘patches’ of the
primary colors of red, green and blue on screens may be a problem for graphics
Tilt and Swivel: Most monitors are supplied with tilt and swivel stands. These allow the
monitor to be adjusted so that the work posture of the user is as comfortable and relaxed
as possible.

Flicker: Flickering is particularly common where a window operating system is
being used with black characters on a white background. The peripheral vision is also more
sensitive to flicker than the center of the eye.

Controls: The inclusion of microprocessors into monitors has led to a dramatic
increase in the number of controls that are available to the user. Monitor makers have
developed on-screen displays (OSD), with menus showing all the options available and even
with help screens on some models.

Excerpted from ‘Monitors Matter’
The Computing Suppliers Federation

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