Display Power Consumption: CRTs Versus TFT-LCDs

Display Details And LCD Panel Types

There's no point in stretching out a CRT discussion, since these are largely obsolete. Suffice it to say that cathode ray tube designs use an electron gun that draws images onto a fluorescent screen, line by line. At refresh rate of at least 75 Hz is desirable for a flicker-free image; 85 Hz or more is better. CRTs are housed in glass envelopes, making them physically deep, heavy, fragile, and susceptible to magnetic interference. They're also environmentally unfriendly, owing to various toxic coatings. High-frequency noise, possible implosion (it’s a vacuum tube), and some radiation also don't help curry favor when comparing to LCD technology. CRTs do have some advantages, but they are overshadowed nowadays.

Unlike CRT displays, every LCD has a native resolution at which it should operate for optimal image quality. Setting a native 1920x1080 display to only 1600x900 results in blurry images, as the output resolution has to be converted to the physical resolution. For best quality results, use digital connections to LCDs, such as DVI, HDMI, and DisplayPort. Avoid the old, analog 15-pin D-Sub connections that converted digital output into an analog signal for transmission, then re-digitize it for display on your LCD monitor. Such conversions always result in image quality loss that can be avoided by a digital link.

Most modern LCD monitors are based on active matrix thin-film transistor (TFT) technology. These displays are based on a TFT array substrate containing transistors, capacitors, wiring, and pixel electrodes, which serve to apply voltage between the TFT substrate and the color filter substrate, which contains red, green, and blue sub-pixels. The two glass substrates are kept apart by spacers and cells filled with liquid crystal material. The outer faces of the TFT panel are equipped with polarizing sheets. Finally, data lines attach to LCD driver chips. Each pixel can be addressed separately in this matrix through the bonding pads at the end of each row and column, as if the monitor were lighting up pixels by playing Battleship.

LCD Panel Types

There are significant differences in performance and characteristics between one LCD display and the next. As with most technologies, you can generally assume that newer products are superior (of course, this isn't always the case). Details like response time and input lag (the time required to change a pixel’s color and to have an input signal change the display, respectively), viewing angle, brightness, and contrast improve from one generation to the next.

Twisted nematic (TN) panels are the most widespread TFT type, offering response times of only a few milliseconds (though response time varies between different color transitions). Contrast, viewing angle, and color reproduction remain issues, especially with low-cost TN devices. Color reproduction can be problematic for image processing or other professional applications, given that each color is typically represented by only six bits, resulting in 18 bits as opposed to the 24-bit color necessary for 16.7 million true colors.

In-plane switching (IPS) panels have the liquid crystals in parallel to the panel, not perpendicular to it. Viewing angles are much wider, and light scatters much less within the matrix, which is why color reproduction can be more precise. Initially, this precision came at the expense of response time. Advanced Super-IPS (AS-IPS) provides an improved contrast ratio, and Horizontal-IPS (H-IPS) works on professional LCDs for a more natural white color. Enhanced-IPS (E-IPS) is more advanced still, bringing response time back to only a few milliseconds, but it’s much more expensive than a TN panel.

Multi-domain vertical alignment (MVA) panels are a compromise between TN and IPS. Colors don’t change as much if you move away from a perfect 90° relative to the monitor's screen plane. Color reproduction and response time are good, too. Patterned vertical alignment (PVA) is a similar technology with higher contrast. S-PVA is considered the most advanced technology of the group, utilizing more than eight bits per color, showing the deepest blacks, and offering the quickest response times.

Recently, conventional fluorescent backlights are being replaced by white LEDs. These typically last longer and consume less power, which is the main motivation for our analysis. Do LEDs really make that much of a difference? Let's find out.