Backlighting apparatus

Field of the Invention

This invention relates generally to backlighting. Specifically, this invention concerns the backlighting of liquid crystal displays.

Background Art

Two types of liquid crystal displays (LCD) in common use are positive and negative image displays. The positive image display allows the background to emit or reflect light while the segment area is dark. The segments block the light emitted or reflected only where the segments are darkened, thus forming the characters. The negative image display has a dark background while the open segmented area forms the characters by emitting or reflecting light. The background of a positive image LCD or the segments of a negative image LCD can be colored by using a filter with the incandescent bulbs or by changing the color of the LED lamps.

The LCD acts as a light shutter rather than a light emitting display requiring light behind the LCD to define which segments are turned on. This lighting of an LCD from behind is called backlighting.

Backlighting can be implemented using two techniques. The first technique is to place a reflector behind the LCD. The reflector causes the ambient light entering the top of the LCD to be reflected back through the LCD from behind. This is very effective in high ambient lighting conditions such as sunlight. The brighter the ambient light, the brighter the backlighting and, therefore, the more readable the LCD. In low ambient lighting conditions, such as at night or twilight, a reflector is not effective since there is not enough light entering the top of the LCD for backlighting.

The second backlighting technique uses an artificial light source behind the LCD. This technique is effective in low ambient lighting conditions. This technique does not function in high ambient lighting conditions since the high ambient light overpowers the artificial light. As a result, the LCD segments are either dim or completely unreadable. Artificial light sources may include: incandescent bulbs, light emitting diodes (LED), electrolumuniscent panels, fluorescent tubes or halogen bulbs.

Typically, a negative image display uses artificial backlighting to emit light in low ambient lighting conditions and also a diffuser sheet on top of the light pipe to reflect the ambient light in high ambient lighting conditions. If the backlighting is not bright enough, the display will look poor in average ambient lighting conditions. This is caused by the lack of enough ambient light to utilize the reflective properties of the diffuser and not enough artificial backlighting to overcome the average ambient light.

When the backlighting is combined with the LCD and the drive circuitry it is called a display module. Some display modules are so thin that there is limited space behind the LCD for the backlighting. Some display modules also have length and width restrictions. These small, low profile display modules require that the illumination sources be on the sides of a light pipe to provide the LCD backlighting.

Both negative image and positive image LCDs suffer from similar problems. These problems can include: uneven lighting, dim lighting and low lamp reliability. Uneven lighting is generally characterized by hot spots. The hot spot is a local area where the light is more intense than the other areas of the display. The hot spot is generally confined to the area surrounding the illumination source itself. For low profile displays with the illumination sources at the edge of the light pipe, the LCD usually has hot spots at its edge and is dim in its center.

The dim lighting problem occurs when the entire area of the display is at an unacceptable brightness. Backlighting a negative image display is more difficult than a positive image display. For a negative image display, the viewer only sees the light emitting from the segmented area rather than the entire background area as in a positive image LCD. Therefore, the quantity of the backlighting illumination must be great to produce an aesthetically pleasing display in average ambient lighting conditions. Because of the display module's size restrictions, typically incandescent bulbs or LEDs have been used. Incandescent bulbs would be the preferred choice for negative image, low profile display modules since they put out more light than LEDs. A disadvantage of incandescent bulbs is that they are inherently less reliable than LEDs.

The low reliability of the incandescent bulbs usually deters their use for backlighting the displays. The life expectancy of a lamp is inversely proportional to its efficiency. In other words, increasing an incandescent lamps efficiency decreases its life span. A high rated voltage also reduces the lamp's resistance to mechanical shock and vibration. Typically, therefore, a bright negative image LCD using incandescent backlighting has either a short life span or high power requirements and is susceptible to failure during mechanical shock and vibration.

U.S. Patent No. 4,673,254 discloses a back-reflection type light diffusing apparatus suitable for use in liquid crystal display panels. The apparatus is formed of a light guiding plate having a top, bottom and sides. Opposing sides of the plate each have two notches. A light source is positioned in each notch. The bottom of the plate has a light reflecting surface and a non-light reflecting surface arranged in a pattern so as to radiate light from the bottom surface uniformly over its entire area. A light reflecting plate is mounted on the periphery of the light guiding plate except for the notches.

German Patent DE-A-2 416 621 discloses a transparent light guide including a side having a notch for receiving a light source. The light guide has a plurality of angles between the notch and a corner of the light guide for changing the direction of the light.

There is therefore a need for a backlighting device that has even illumination, aesthetically pleasing brightness under all viewing conditions, and high reliability.

Summary of the Invention

The present invention provides an apparatus for backlighting a liquid crystal display including a substantially transparent means for distributing light having sides, a top, and a bottom, wherein at least one side of the sides has at least one recess, wherein the at least one recess has at least one light source positioned therein for providing light, wherein a diffuse reflecting material is disposed upon the bottom of the means for distributing light, and wherein first reflecting means is disposed upon at least one side of the means for distributing light for changing a path of the light exiting the at least one side of the means for distributing light, the apparatus characterized by: a portion of the at least one side, between the at least one recess and a corner of the transparent means for distributing light, having a plurality of angles for reflecting the light; and second reflecting means , positioned in at least one plane around the at least one light source, for reflecting the light.

Brief Description of the Drawings

FIG. 1 shows a block diagram of the preferred embodiment in accordance with the present invention.

FIG. 2 shows the preferred embodiment of lamp connection in accordance with the present invention.

FIG. 3 shows the illumination pattern of the lamps in accordance with the present invention.

FIG. 4 shows the preferred embodiment of a pattern for painting the bottom of the light pipe in accordance with the present invention.

FIG. 5 shows the configuration of the preferred embodiment of the light pipe in accordance with the present invention.

Best Mode for Carrying Out the Invention

This invention provides an even distribution of bright light across a low profile liquid crystal display. The preferred embodiment of this invention, as illustrated in Fig. 1, is comprised of an acrylic light pipe (101), four incandescent lamps (301 - 304), two diffusing reflectors (103), and two elastomeric connecting strips (104). A bright, evenly lit display is achieved by coupling the light from lamps (301 - 304) into the light pipe (101). Light emitted from the end of the lamp is directed toward the center using internal reflection of the light pipe (101).

The four lamps (301 304) are positioned in pairs on opposing sides of the light pipe (101) and placed on their sides to maintain a narrow side profile for the apparatus. The lamps (301 - 304) are placed as close to the light pipe (101) as possible to insure maximum luminous flux entering the light pipe (101) from the lamps (301 - 304). The lamp (301 - 304) bottoms are positioned close enough together to minimize the dim area between the lamps (301 - 304) at the edge of the light pipe (101).

The lamps can be electrically connected in a diagonal series parallel configuration illustrated in Fig. 2. Either pair of series connected lamps (301 - 302, 303 - 304) alone can adequately illuminate the entire display. The illumination pattern of each pair is illustrated in Fig. 3. A benefit of this configuration is that if one of the lamps (301 - 304) burns out, the other lamp (301 - 304) in series with it also goes out leaving the other two lamps (301 - 304) to adequately evenly light the display. The average brightness will only be half when operating with the two lamps (301 - 304) out, but the illumination will still be an adequately even distribution. Another benefit of this configuration is that each lamp (301 - 304) is operated at a lower voltage. The lower voltage makes the lamps (301 - 304) less susceptible to shock and gives them a longer life expectancy without increasing power consumption.

A white diffuse reflector is placed around the three sides of the lamps which do not face the light pipe. This reflects both the light from the adjacent lamps (301-303, 302-304) into the light pipe and also light which has passed through the light pipe from the other lamps (301-303, 302-304).

The light pipe (101) must be appropriately constructed for proper operation of this invention. All sides, the top and the non-painted area of the back must be smooth, allowing internal reflection of light to occur. The back of the light pipe (101), is selectively painted white. A pattern in the form of a border, half-sun and ovals around each lamp is left unpainted. The preferred embodiment of such a pattern is illustrated in Fig. 4. The smooth unpainted surfaces of the light pipe internally reflect light that has an angle of incidence less than the critical angle, 42° for acrylic. The angle of reflection equals the angle of incidence. The white paint provide diffuse reflection, scattering the light. Some of this light escapes the light pipe and passes through the LCD. The border, ovals and half-sun pattern, therefore, has the effect of dimming the hot spots caused by the proximity of the lamps, and increasing the brightness in the painted area.

The inside radius of the light pipe (101) near each lamp (301 - 304) follows the outside radius of the lamps' (301 - 304) lenses. This allows the light leaving the lamps' (301 - 304) lens to enter the light pipe (101) without refracting. A straight path for the light leaving the lamp (301 - 304) and entering the light pipe (101) improves the evenness of the illumination in the light pipe (101) near each lamp (301 - 304).

The corners of the light pipe (101) near each lamp are angled as shown in Fig. 5. A significant portion of the lamps output exits from the end of the lamp. This light would not be captured, and therefore wasted, if a simple rectangular light pipe were used. The angled surfaces (501, 502) reflect, this light onto surface (503) of the light pipe. The path of the light reflecting off surfaces (501, 502, 503) is easily plotted using a graphical method. Using such a method, the angles have been determined so that this light is directed toward the center area of the light pipe, which would otherwise be dim. In the preferred embodiment, the first surface (501) is angled 18° from the side and the second surface (502) is angled 30° from the side. Any light striking the angled surfaces (501 502) or side (503) that exceeds the critical angle will be refracted outside the light pipe (101).

Light that is refracted outside the light pipe (101) is reflected back into it by the two white elastomeric connecting strips (104) and the two white diffusing reflectors (103). The positioning of these reflectors (103) and connectors (104) is illustrated in Fig. 1. The elastomeric connecting strips (104) also have the purpose of connecting the LCD driving circuitry to the LCD.

The top surface of the backlighting apparatus should not be positioned in direct contact with any other surface. An air gap should be maintained to preserve the internal reflection properties of the light pipe (101). A colored filter between the LCD and the light pipe (101), a typical negative image LCD application, will not degrade the operation as there is only intermittent contact between the light pipe (101) and the filter. Only a few molecules of air above the light pipe (101) are required for proper operation of this invention.

The backlighting apparatus can evenly light both negative and positive image LCDs. The combined improvement in optical efficiency from all aspects of this invention requires less illumination to achieve a given brightness. As incandescent lamp efficiency is inversely related to its operating life, either an increase in lifetime, a decrease in power consumption and heat, an increase in overall brightness or some combination of these advantages is achieved by this invention.

The light pipe, with angled corners to direct light toward the middle of the display, provides even lighting throughout the display with no hot spots. Using the backlighting apparatus, a negative image LCD can attain the appearance of an LED display and be readable under all lighting conditions, but at a lower cost than an LED display.

An alternate embodiment of this invention is to use other light sources such as LEDs. Different numbers of light sources could be used depending on the size of the display and brightness required. The angles and size of surfaces (501, 502) as well as the selective pattern illustrated in Fig. 4 would be optimized for the emission pattern of the light source. The back side of the light pipe could be treated in any manner that produces diffuse reflection. The light pipe could be any material that provides internal reflection principles. The material around the light sources could be any material that provides reflection.