Before you start
Objectives: Learn what is LCD monitor, how does it work, and what are common features.
Prerequisites: no prerequisites.
Key terms: ratio, display, resolution, aspect, angle, hdtv, pixel, time, response, contrast, color, line, backlight, dead, black, vertical, flat, light, power
LCD Features
The static and dynamic contrast ratio is the difference in brightness (light intensity) between the brightest white and the darkest black pixel. The static contrast ratio indicates the difference that can be displayed at the same time. The dynamic contrast ratio indicates the difference that the monitor is capable of producing. A good display will have a 1000:1 or better contrast ratio. A higher initial number indicates a better quality picture. The response time is the time it takes a pixel to go from black, then to white, and then back to black. It is measured in milliseconds. Fast response time is required if we plan to play games or watch videos on our PC. Response time of 5 milliseconds or better is required to produce video without ghosting or blurring effect. Some manufactures use a grey-to-grey (G2G or GTG) measure of response time, which makes the response time faster than a white-to-black response time. The brightness or luminance is the amount of light the monitor produces (how bright it is). A high luminance is required if our computer will be used in sunlight, and it is great if we plan to watch movies. It is measured in candelas per square meter (cd/m2), with a higher number indicating a brighter screen. The viewing angle is the angle at which the image can still be seen. When we view our LCD monitor from some angle, the image will be dimmer and colors distorted. A viewing angle is defined by horizontal and vertical angle. A larger number indicates a wider viewing angle. The pixel pitch is the distance between individual pixels on the screen. A smaller number indicates sharper image and better possible resolution. Unlike CRT monitors, LCD monitors can use entire screen to display the picture. Screen size is measured diagonally corner to corner. One problem that sometimes occurs with LCD displays are dead pixels. Dead pixel is a pixel that fails to display properly. Having a few dead pixels can be common on many displays. Most manufactures have a minimum number of dead pixels that must exist before they will consider an exchange, so we should review the return policy before purchasing. Sometimes gently pressing on the screen can repair a dead pixel. Also there are programs which can identify and sometimes repair dead pixels.
Image 322.1 – Typicall LCD Monitor
Display Technologies
Flat panel displays can use one of three display technologies. Twisted Nematic (TN) LCD monitors are the most common used in computer monitors, especially in smaller sizes. TN LCD monitors have best response times (2ms to 5ms), but have poor color reproduction (only 6-bits per color can be displayed). They mimic 24-bit color using dithering and other techniques. They also have a low contrast ratio and viewing angle. Vertical Alignment (VA) displays have the best contrast and produce better color and have better viewing angle from the TN displays, but have a slower response time. VA panels suffer from color shift that produces uneven colors across the display with loss of detail in dark scenes. In Plane Switching (IPS) displays have the best color reproduction quality and viewing angle, but also have relatively slow response time. IPS panels are the most expensive type of panel. Interesting thing is that CRT monitors still have the best color reproduction, so they’re still sometimes used by people who deal with graphics.
Backlight
The backlight illuminates the pixels in a flat panel display and makes it visible. The backlight is located along the edges of the panel with a special layer that reflects the light throughout the display. Backlight can be produced by either a Cold Cathode Florescent Lamp (CCFL) or Light Emitting Diodes. CCFL is currently the most common backlight source and it requires an inverter to provide AC power to the backlight. LEDs produce better color, greater contrast, a wider viewing angle and lower power consumption. They are also mercury free. Since LEDs use DC power there’s no need for a power inverter in an LED illuminated flat panel display.
Aspect Ratio
The ratio of the display’s width to height is called the aspect ratio. CRT displays have a 4:3 aspect ratio. Widescreen displays have a 16:10 aspect ratio. HDTV screens have a 16:9 aspect ratio. If we watch HDTV content on a 4:3 or 16:10 monitor, the display will either be slightly stretched or have black bars on the top and the bottom. Black bars are areas without video content.
Resolution
A display’s resolution is the number of pixels that can be shown horizontally (horizontal rows) and the number of pixels that can be shown vertically (vertical columns). It is common for CRT monitors to support a wide number of resolutions, but LCD monitors often have a native (optimum) display resolution. Although we might be able to change the resolution to a different setting, the result may not be satisfactory. The original VGA (Video Graphic Array) resolution is 640 x 480. Super VGA (SVGA) resolution is 800 x 600 lines. XVGA or XGA defines 1024 x 768 lines. XGA+ defines 1152 x 864 lines. SXGA defines 1280 x 1024 lines (5:4 aspect ratio). SXGA+ defines 1400 x 1050 lines. UXGA has 1600 x 1200 lines. In WSXGA (W stands for wide) we have 1680 x 1050 lines. WUXGA resolution is 1920 x 1200 lines. With WSXGA and WUXGA displays we have an aspect ratio of 16:10. Some displays are HDTV compatible. HDTV resolutions are 1280 by 720. This resolution is often called 720p. Other HDTV resolution is 1920 by 1080. This is full HDTV resolution, and it is often called 1080p. With HDTV displays we have an aspect ratio of 16:9. The difference between an LCD monitor and an High Definition TV is disappearing. Now we can buy an HDTV that can also function as a monitor and vice versa. Full HD content is designed for a resolution of 1920 x 1080 using progressive scanning (each line is redrawn in order). Because of that Full HD support is referred to as 1080p. Cheaper TVs and monitors at lower resolutions or which are using interlacing (every other line is drawn with each pass) are not capable of displaying full HD. 720p (1280 x 720 progressive scan) and 1080i (1920 x 1080 with interlacing) identify displays that do not support full HD content. LCD monitors which support HDTV often have an HDMI port that accepts video and audio input through the same HDMI port. They can also have built-in speakers and audio-out to external speakers, an HDTV tuner which enables us to watch TV, and support for HDCP to watch copy-protected content.
Screen Size
Monitor size is usually described using a diagonal measurement. CRT monitors usually list the monitor size along with the viewable image size. Because of how the CRT monitor works, the picture actually can’t be displayed on the whole screen. In contrast to that, LCD monitors can use the entire screen for displaying images, so they use a single value for the screen size.
Remember
LCD features include contrast ratio, response time, brightness or luminance, viewing angle, and pixel pitch. Technologies in which LCD monitors can be built are Twisted Nematic (TN), Vertical Alignment (VA), and . In Plane Switching (IPS). The backlight illuminates the pixels in a flat panel display and makes it visible. The ratio of the display’s width to height is called the aspect ratio. CRT displays have a 4:3 aspect ratio. Widescreen displays have a 16:10 aspect ratio. HDTV screens have a 16:9 aspect ratio. A display’s resolution is the number of pixels that can be shown horizontally (horizontal rows) and the number of pixels that can be shown vertically (vertical columns).