LCD monitors

Liquid Crystal Display (LCD) technology has been around since the advent of the personal computer. However, it is only relatively recently that the cost and quality of LCD monitors surpassed CRT technology.^[1] LCDs use the properties of polarized light and polarizing filters.

Polarized light

Light is an electromagnetic wave. It consists of oscillating electric and magnetic fields perpendicular to each other. A polarizing filter only passes light with an electric field oriented on the plane with the same orientation as the filter; if we have a polarizing filter aligned vertically, only light that is polarized vertically will pass through the filter.

A polarizing filter on top of an LCD screen. The screen only allows light that is polarized on one plane to pass. In the left image, the filter is aligned with the screen polarizer. Only light aligned with the screen polarizer passes through the polarizer. That light also passes through the filter because the filter is aligned with the screen polarizer. As the filter is rotated, less light passes through because light aligned with the screen polarizer doesn't readily pass through the rotated filter. Little light makes it through the filter (right image) when the filter is aligned 90 degrees to the screen polarizer.

LCD Assembly

As shown in the above illustration, light can be blocked or unblocked by rotating one polarizing filter on top of another. Making a display using such technology would be impractical. However, there is a substance that can change the orientation of polarized light in response to electricity. That is liquid crystal, a substance that is crystalline in structure but flows like a liquid. Liquid crystal molecules tend to align with an electric field.

Liquid crystal molecules (purple rods) aligned in the electric field between two electrodes in an LCD assembly.

In an LCD assembly, the liquid crystal is sandwiched between two layers of glass that are etched with fine parallel grooves. The grooves on the two glass panels are aligned perpendicular to each other. The glass is also coated with a pattern of transparent conductive material so an electric field can be applied between the panels. The material is shaped to create the individual cells needed to display information.

In the absence of an electric field, the crystal molecules align with the grooves in each panel. Since the grooves on the opposing panels are aligned perpendicular to each other, the crystal molecules between the panels become arranged with a twist to maintain alignment with both panels.

In the absence of an electric field, the crystal molecules twist between the cross-oriented etched glass panels.

The polarization of any light passing through the liquid crystal between the panels will twist with the molecules. With the electric field applied, the light passes through the molecules such that the polarization is unchanged. However, without the electric field applied, the polarization of the light gets rotated as it passes through the twisted series of crystals.

We can use polarizing filters to pass only that light polarized in a particular orientation. There is a polarizing filter on each side of the display. These filters are aligned perpendicular to each other. With the electric field applied, the first filter polarizes the light, which passes through the liquid crystal unchanged. Since the second filter is aligned perpendicular to the first, the second filter blocks the light. With the electric field removed, the liquid crystal molecules align with a twist between the panels. The first filter polarizes the light, but the polarization is rotated 90 degrees while passing through the twisted liquid crystal. The light now passes through the second filter because the light's polarization has been twisted to match the filter. Therefore, with no electric field, the LCD cell^[2] is bright, and with the electric field, the LCD cell is dark.

Here, the top polarizer has been removed from a calculator's LCD panel and then placed on top of the panel rotated 90 degrees. Note that nothing can be seen on the panel where the polarizer is absent. Furthermore, with the polarizer rotated, the numerals, which should be black on white, are now white on black.

Early LCD Displays

Each cell of an LCD screen consists of the essential elements described above. Early LCD assemblies often had predetermined shapes, such as seven-segment displays that could show individual digits. Other specialized shapes were also used. Such displays are still common with specialized devices. When such a device is turned on, all the display elements are often turned on, showing all the specialized shapes.

When turned on, this blood pressure monitor shows all the specialized shapes on its LCD screen.

Early-style LCD assemblies consist of a panel with a reflective background, as described above. A small incandescent light or LED makes the display visible in the dark.

Color LCD Displays

LCDs had to significantly improve before they were suitable for modern color computer monitors. One of the most significant improvements was Thin Film Transistor (TFT) displays. With passive matrix displays, the electrodes were arranged with columns on one side of the panel and rows on the other. An individual cell was selected by electrifying the appropriate column and row. Each cell could only be activated for a relatively short time because each cell had to be activated in turn, and it took a significant time to scan the whole screen. This caused the display to have low contrast, and moving objects (like a mouse cursor) would tend to disappear and reappear. This was partially solved with dual-scan displays where the top and bottom half of the screen were accessed separately and simultaneously.

TFT Displays

TFT displays have a small transistor in each cell.

A microscopic view of a color TFT display. The associated transistor partially blocks each cell.

These transistors are switched on and off when the cell is scanned and remain on or off until it is scanned again. This improves the contrast, reduces ghosting, and improves the image when viewed from an angle.

A color LCD screen requires three filtered subcells for each cell to create the necessary colors. Brightness is controlled by varying the voltage applied to each cell.

Backlighting

Early LCD assemblies had reflective backplanes and often an incandescent or LED light off to the side for viewing in the dark. A computer monitor requires even backlighting. Early LCD monitors used thin fluorescent tubes along the top, bottom, or one side. Light is passed between plates that evenly reflect the light across the panel to provide even lighting. A diffuser further evens the lighting of the display. Modern LCD panels use LEDs instead of fluorescent tubes. Some high-quality monitors use a matric of white LEDs behind the panel. This matrix of LEDs can be manipulated to improve contrast.

Note that a monitor or television labeled "LED" is backlit by LEDs. The only screens currently using LEDs to create an image are Organic Light Emitting Diode (OLED) displays, which will be labeled as such.^[3]

Pixels and cells

With the CRT, there is no rigid relationship between triad dot pitch and pixel size or placement.

An image on a slot-masked color CRT showing that individual phosphors can be partially lit[4].

The relationship is more rigid with LCD screens.

Any particular subcell on an LCD screen is either on or off, with varying shades of brightness. Nevertheless, You cannot have a subcell that is partially lit.

An image on a color LCD showing that subcells cannot be partially lit.

Aliasing and Antialiasing

Notice the stair-step effect on an LCD screen. This effect is called aliasing. The image is sharp, but the aliasing is bothersome to many people. Computers use antialiasing algorithms that reduce jagged appearance by adjusting the brightness of adjacent cells.

A simulation of antialiasing. Note the gray pixels filling in the jagged edges of the anti-aliased image.[5]

Resolution vs Pixels

Many sources call each LCD cell a pixel. This is only true if the image pixels match the screen cells. The pixel size is technically based on the image resolution, not the screen resolution. An image's pixels may be larger or smaller than the LCD cells (or the triads on a CRT monitor). Therefore, although many sources call an LCD cell a pixel (and even I will below when talking about dead and hot "pixels"), this is technically incorrect. For example, if an image's resolution is half of the screen's resolution, each image pixel will cover four LCD cells.

If the image resolution doesn't match the screen resolution or a multiple of it, pixels will cover partial cells. For example, let's say you have an LCD display with a resolution of 1280 x 1024, but your operating system is set for a display with a resolution of 1024 x 768. Each pixel created by the operating system will cover 1.25 cells horizontally and 1.33 cells vertically. This is not a problem with a CRT because phosphors can be partially lit. With a CRT monitor, setting a lower screen resolution just displays a lower resolution. The triad dot pitch merely sets a practical upper limit to resolution.^[6] However, an LCD subcell cannot be split. The entire face of each subcell must be the same brightness.

If a pixel tries to bisect an LCD cell, the software must decide what color and brightness to make the whole cell. How this is handled depends on the software. It may make the entire cell the color and brightness of the pixel that covers more than 50% of the cell. It may average the colors and coverage of the pixels covering the cell. Antiasing software may use a more complex blending of color and brightness.

This image approximates the appearance of an image when the image resolution is lower than the LCD panel resolution and image pixels bisect LCD cells.

Nevertheless, the quality of the image is compromised if the image pixels are not the same size as the LCD cells; the screen will appear blurry. Modern antialiasing algorithms are very good at reducing this problem, but you should always choose the display resolution in your operating system that matches the LCD panel resolution. Newer LCD monitors communicate with the operating system to match the screen and display resolutions. However, you may need to adjust the display settings with older systems manually. Bad pixels also occur with image sensors in cameras. This causes defects in every image created by the camera.

Dead and hot pixels

Occasionally, cells will stop working correctly. If a cell remains dark when it should be light, it is called a dead pixel. If a cell is bright when it should be dark, it is called a hot pixel. Early LCD manufacturers didn't guarantee a screen free of bad pixels. Warranties usually specified the number of allowed bad pixels before the screen would be eligible for warranty service. Bad pixels are currently rare but have not been eliminated.

LCD Projectors

Image projectors commonly use LCD technology. A bright lamp backlights the LCD panel, and a projection lens focuses the image on a screen.

An Epson 705HD LCD projector

LCD projectors usually have inputs for many types of sources, including composite video, component video, VGA, and HDMI. They also have many features to accommodate the geometry of the venue in which they are used. For example, the image can be flipped vertically so the projector can be ceiling-mounted upside down. The image can also be flipped horizontally to accommodate rear projection. Vertical and horizontal keystone^[7] controls also accommodate projecting from various angles.

An LCD projector cannot make dark black. However, this is usually not an issue because they are typically used in environments where ambient light washes out dark colors. The brightest colors are also not as bright as DLP projectors (covered below). Therefore, although LCD projectors create good images, they lack the contrast of DLP projectors.

The light source is a significant maintenance cost of LCD projectors. They can be expensive and have a limited life.^[8]