Liquid Crystal Display

In 1968, the Radio Corporation of America developed a liquid crystal display that was lightweight and consumed little power. The most important aspect of the LCD is the materials which have properties of a conventional liquid and a birefringence of anisotropic crystals [80] that let light pass through a substrate by deflecting it into different angles for a display that is as effective

as external electric fields. A polarizing filter is applied to the LCD. Imagine the light as a string wave with two mutually perpendicular vibrating directions. The two polarizing filters are fences that block the perpendicular direction of the string wave and bypass the parallel. It is the same way that the polarization of the passing light is selected through a polarizing filter. The liquid crystal layer becomes a way of changing the light’s polarization through voltage control, as shown as Fig. 4-7. When the voltage is off, the layer’s polarization changes, lets the light pass through, and the display is brightly lit. When the voltage is on, the liquid crystal molecules rotate and cannot change the light polarization, so the display is dark as the light is blocked. Thus, the combination of liquid crystal molecules changes to show gray levels as the external electric fields change with varying optical transmittance.

Figure 4-7: Different effects on visual fatigue by projectors.

The main reason that a LCD is able to show colored images is that red, green, and blue color filters are placed on a liquid crystal cell. The gray levels are formed by a back light passing through a liquid crystal layer control driver IC and through color filters to form red, green and blue and finally a color image for the eyes. Since liquid crystals do not emit light directly, the LCD is arrayed in front of a light source to produce colored images from different colored filters.

A color filter consists of a glass substrate, black matrixes, a color layer, an over coat and indium tin oxide (ITO), shown as Fig.4-8. The main function of the black matrix (BM) is to avoid a color mix effect between the colors. The

purpose of the overcoat is to form a flat surface and keep the liquid crystal uncontaminated. ITO is used as a transparent conducting oxide with electrical conductivity and optical transparency, which is easily deposited as a thin film.

Figure 4-8: The structure of color filters.

In order to display images with good color quality, the arrangement of color filters is important. The color filters are fabricated by applying organic RGB materials to each pixel. Generally, the pixel arrangement is divided into stripes, mosaics, deltas, squares and so on, shown as Fig. 4-9. The RGB pixel is arranged individually into long stripes with three different neighboring colors, so thick stripes appear if the picture is larger, or if the figure is longitudinal. The mosaic places the RGB sideways to get more natural images than the stripe and a sharper oblique line for larger pictures. The RGB is arranged in a triangular shape in the delta and displays the most natural images, with a resolution 1.5 times the stripe but with the same number of pixels. It is often used in mid and small sized panels. The square feature occurs when a pixel consists of four points, like a square, instead of three, and is applied more to the image field. In summary, LCD TVs use liquid crystals to represent gray levels and use color filters to show colors.

Figure 4-9: The geometrical combinations of color filters.

4.2 3D Display Technologies

3D Technology has been developing for over a century. Sir Charles Wheatstone (1833) drew the first stereo picture using a binocular parallax, and made a mirror stereoscope by mistake. Later on, Sir David Brewster (1818) made a prism stereoscope by using two lenses. Anderton (1891) introduced his findings that polarized light can be used to make 3D projectors. The 3D anaglyph became fashionable for a time in the 1950s, and even today there are still various 3D displays. From stereopictures to 3D, it is undeniable that 3D technology has continued to develop and progress.

As early as ancient Greece, people have noticed that although they possess two eyes, and the images received by the two retinas may not be the same, there are no double images. After several years of rigorous experimentation on animal and human bodies, it is evident that there are specific cells that are stereoscoptic [6, 7]. The human brain has the ability to merge two different images and generate depth perception. Stereoscopic vision is caused by binocular and motion parallaxes. Binocular parallax results from the eyes having different locations and visual angles, so their images are slightly different. Then the brain fuses the two into a stereo image. As the head moves, the vision angle changes causing a motion parallax. Using one of these two parallaxes can create a stereoscopic feeling. However, in order to achieve perfect stereoscopic vision, both of these parallaxes should be embedded, as moving the head affects 3D products. But, because viewers do not usually move a lot, binocular parallax is adopted in the present-day 3D displays.

3D displays can be divided into two types according to their appearances:

stereoscopic and auto-stereoscopic [81], shown as Fig. 4-10. Stereoscopic displays can be classified as active and passive glasses, and other types. Active glasses mean shutter glasses in particular, whose purpose is to divide images into a right and left image, so that both eyes see different images. Adding in other factors such as persistence of vision, the viewed image is stereoscopic. The difference between the active and passive glasses is that their state is not altered, and does not require external power or synchronization with the screen. There is the theory that passive glasses are the same as active glasses. Both make the left eye see the left image, and the right eye, the right. Passive glasses can be divided into anaglyph and polarized glasses. The Red-Green and Red-Blue of the anaglyph are common. In this theory color images are processed for both the left and right eye. For example, the green image stays in the left eye while the red stays in the right; then the brain combines the two images so that they become a stereoscopic picture. As for viewers, the glasses send a slightly

different image to each eye, but the brain fuses the images to make stereoscopic vision. There are two types of polarized glasses: linear and circular. The main principle is that it decomposes, distinguishes and preserves the image by the direction of the light waves. The 3D glasses have polarized glass with different directions for each eye. Thus a person’s right and left eye can receive two images which will be combined into a stereo image. The head mounted display (HMD) is another type that puts two screens onto the glasses, and viewers can see a stereo image when the screens receive two different signals.

Figure 4-10: The sort of 3D display technology.

There are four types of auto-stereoscopic display: holographic, volumetric, multi-Planar and 2D multiplex. The holograph uses three laser light sources: red, blue and green, to generate phase grating through an acoustic optical modulator (AOM). The stereo image is created by amalgamating the laser with the hologram and scanning vertically with a vertical scanning mirror and horizontally with a polygonal mirror [82]. The Volumetric type uses a laser to scan stereo images. It uses a rapidly rotating vertical disk coordinated with a laser projector below the disk. When the laser light projects onto the revolving disk, it can scan every point in the space through a scatter effect to generate stereo images [83]. A Multi-Planar requires two liquid crystal displays. When these show images of the same size, the distance is varied between the viewers and objects so that there are differences in color and brightness. Once the images become superimposed, viewers can see the image in stereo. The 2D multiplex provides viewers with different 2D images angled from their right and left, in one system, but their brains combine these two images to generate stereo images.

The 2D multiplex can be divided into space and time. Space divides the screen

into right and left images and uses the parallax barrier or lenticular lens array to project into the right and left eye in order to create stereoscopic vision. The Parallax barrier [84, 85] a black and limpid longitudinal stripe, splits the light so that only the viewers’ right eye receives the image projected by the LCD, and does the same with the left eye. But when the light goes through the black longitudinal stripe area, the brightness stays there because the light is absorbed.

Therefore, the longitudinal stripes make up of two layers of chromium and aluminum replaces the black longitudinal stripes. When the light projects into the original black stripes, the aluminum layer reflects the light back to the stripes.

The light is then recycled and the brightness improved. The lenticular lens projects left and right images into viewers’ eyes irrespectively, and the brain sees the image stereoscopically. The time multiplex uses a special light splitting design to project continuous images into the viewers’ left and right eyes at different times in order to display the image stereoscopically. When the left and right eye images are quick, the brain does not see the images change. The image angles for each eye are slightly different and so they become stereoscopic. One of the split light methods uses the parallax barrier serving the same purpose as the limpid and black longitudinal stripe, and then they switch places. That is, the longitudinal stripe becomes black and limpid. This exchange does not make one eye see just the same image, so it improves the resolution.

This highly developed 3D technology is used in present day stereoscopic displays, and the shutter and polarized glasses are excellent for these displays.

Therefore, the present study utilizes these two 3D displays, which are introduced as follows.