Sunday, June 29, 2008

CRT (cathod ray tube) Features and Attributes

To evaluate the specifications of CRT monitors, here are a few more things you need to know:
A shadow mask is a thin metal screen filled with very small holes. Three electron beams pass through the holes to focus on a single point on a CRT displays' phosphor surface. The shadow mask helps to control the electron beams so that the beams strike the correct phosphor at just the right intensity to create the desired colors and image on the display. The unwanted beams are blocked or "shadowed."
Monitors based on the Trinitron technology, which was pioneered by Sony, use an aperture-grill instead of a shadow-mask type of tube. The aperture grill consists of tiny vertical wires. Electron beams pass through the aperture grill to illuminate the phosphor on the faceplate. Most aperture-grill monitors have a flat faceplate and tend to represent a less distorted image over the entire surface of the display than the curved faceplate of a shadow-mask CRT. However, aperture-grill displays are normally more expensive.
A less-common type of CRT display, a slot-mask tube uses a combination of the shadow-mask and aperture-grill technologies. Rather than the round perforations found in shadow-mask CRT displays, a slot-mask display uses vertically aligned slots. The design creates more brightness through increased electron transmissions combined with the arrangement of the phosphor dots.
Dot pitch
Dot pitch is an indicator of the sharpness of the displayed image. It is measured in millimeters (mm), and a smaller number means a sharper image. How you measure the dot pitch depends on the technology used:
In a shadow-mask CRT monitor, you measure dot pitch as the diagonal distance between two like-colored phosphors. Some manufacturers may also cite a horizontal dot pitch, which is the distance between two-like colored phosphors horizontally.
The dot pitch of an aperture-grill monitor is measured by the horizontal distance between two like-colored phosphors. It is also sometimes are called stripe pitch.

The smaller and closer the dots are to one another, the more realistic and detailed the picture appears. When the dots are farther apart, they become noticeable and make the image look grainier. Unfortunately, manufacturers are not always upfront about dot pitch measurements, and you cannot necessarily compare shadow-mask and aperture-grill CRT types, due to the difference in horizontal and vertical measurements.
The dot pitch translates directly to the resolution on the screen. If you were to put a ruler up to the glass and measure an inch, you would see a certain number of dots, depending on the dot pitch. Here is a table that shows the number of dots per square centimeter and per square inch in each of these common dot pitches:

Dot Pitch

Approx. number of

Approx. number of

.25 mm



.26 mm



.27 mm



.28 mm



.31 mm



.51 mm



1 mm



Refresh Rate
In monitors based on CRT technology, the refresh rate is the number of times that the image on the display is drawn each second. If your CRT monitor has a refresh rate of 72 Hertz (Hz), then it cycles through all the pixels from top to bottom 72 times a second. Refresh rates are very important because they control flicker, and you want the refresh rate as high as possible. Too few cycles per second and you will notice a flickering, which can lead to headaches and eye strain.

more details here:

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