Published on March 15, 2007

Things to know when buying an LCD

When I intended to buy an LCD monitor, I firstly wanted to see what I should look for (which are the top producers, which are current standards in place, which are the main issues that could appear for LCD monitors).
During my market research I found some information that formed some guidelines for choosing the monitor. Below is presented some of the information I used while looking for an LCD monitor.

There are different types of panels for LCD monitors in the market. The older monitors are of TN type. The next generation of panels is the PVA and SPVA panels. The next one is the one that uses LED as a backlight instead of fluorescent lamp. The backlight of a monitor is the light that makes the LCD panel to be illuminated.

Most of the monitors in the market have TN panel type, and some have PVA or SPVA type. The difference between a TN and a PVA panel can be noticed looking at the viewing angle. For TN monitors when the observer moves on one side (or above) of the monitor and looks at the display, the colors change dramatically.

An interesting thing about LCD panels is that, according to the international standards the warranty procedure is activated only if the defects number goes over some specified values. That is because if all the LCD panels would have to be perfect, the producers would have to throw away some of the panels, leading so to a very high price of the monitors in the market and few people could buy them.

There are three types of defects about pixels: black (dead pixel), white (blocked pixel), and colored (dead sub-pixels). There are more panel classes: 1,2,3,4. First class is intended only for special applications and means no defect panels (guaranteed by the producer), usually some very expensive monitors. Class 2 is typical. Class 3 is for inexpensive models. Class 4 represents the reject criteria in manufacture. It is possible that the monitors in second class to have some defects: 2 white pixels or 2 black pixels or 5 sub-pixels in a million pixels. Most of the producers respect this standard (ISO 13406).

According to this standard:

Considering these things is very important to ask the vendor about the warranty conditions.
When buying the LCD monitor, some tests could be made to test the monitor for dead pixels and other graphical characteristics. To do these tests, one can use programs that perform these tests. These programs could be found on Internet and some of them are for free.
If somehow the monitor has some defect pixels, there is a chance to make them work. Use the finger and press slowly and round on dead pixels and if there is any luck the pixels will begin to work, because warming the defect pixels could lead to their unblocking.

In case of CRT monitors, the refresh rate is the number of times the display is redrawn. As you increase the refresh rate, more displays are done (there is an alternation of display and black screens). When the frequency of frames gets over a certain frequency per second (around 30Hz), the image would appear constant to the human eye. Anyway only from a certain frequency up, the monitor can be usable; otherwise some headaches could appear because of the flicker effect.

The minimum acceptable refresh rate is around 70-72Hz, and this is the recommended minimum refresh rate at all resolutions for CRT monitors.

LCD refresh rate has a different meaning. In case of LCD, the image is not refreshed (there is no black screen, as in CRT monitors). The image is only checked for update to a new displayed image. The frequency of this check is the refresh rate. For example in a static image there is no refresh at all. Because there is no flicker effect, LCD monitors are better for human eyes.

Because LCD panels are made of physical electrodes they take time to turn off and on at each update of the image. If an electrode takes to long to switch, "ghosting" will occur. This is when the previous image is still on the screen just before the new image is appearing. The lower the response time rating, the faster the pixels can switch colors.

There are some characteristics that define the performances of an LCD monitor. Usually these characteristics do not have their peek values, in the same time, for the same monitor (maybe because the price would be too high or because of technical limitations). Some monitors have some peek characteristics, other have other peek characteristics.

When intending to buy a monitor one must know the main purpose of the monitor. Because depending on the usage of the monitor, there can be chosen the characteristics of the monitor that must be the best. There are some groups of monitors based on their usage: entry level, multimedia, gaming, design, TV monitors, text processing. There could be others also.

The main characteristics that give the quality of the image for an LCD panel are: response time, viewing angle, contrast ratio, color supported, brightness, pixel pitch. Depending on the purpose of the monitor some or other of the characteristics are more important.

For multimedia or design purposes (photos, movies) contrast ratio is an important factor. Most of the monitors have around 700:1-800:1. Monitors that focus on high contrast ratio have above 1500:1 up to 2000:1 and some even more.
For gamers response time is very important. A low response time gives the feeling of ghosting, which leads to headaches and a poor quality of the game. Most of the top level monitors have 5-8 milliseconds for response time. For gamers 2 milliseconds would be a good choice.

A normal viewing angle is around 160 degrees. For latest generation of panels (PVA, LED monitors) the angle goes up to 178 degrees.
The pixel pitch for most of the 19’’ LCD monitors is around: 0.250 – 0.290 mm.

For LCD monitors there is a term: native resolution. This is the resolution where the monitor works with the best characteristics. Related to the resolution there is also a term called ratio (ratio between length and height).

The old ratio for monitors is 4:3. The new 19’’ LCD monitors have another ratio based on native resolution of 1280x1024. It is possible that somebody to decide to keep the monitor at a lower resolution than the native resolution.

The lower resolution must have the same ratio as the native resolution, otherwise a distortion of the image will appear, distortion that can be visible also at the letters of text documents. When lower resolution is used, if it is possible (available in operating system), it should be activated also the ClearType method to smooth edges of screen fonts.

If the LCD is used in some native resolutions that are very high, and there is a discomfort because of the too small icons and fonts, there is the possibility to increase the DPI (dots per inch) to compensate.

Before buying the monitor, a good idea would be to read on Internet some reviews about the desired models, so as to see the impressions of some other peoples that used the products.