Backlight for a liquid crystal display

Abstract: A backlight system for a liquid crystal device display is equipped with a dual lighting system for day viewing and compatible with night vision systems. The day lighting system uses conventional fluorescent or incandescent lamps (42) for full color display during daytime use. The night lighting system uses a light source (50) which is either filtered to remove infrared and near infrared wavelengths or which is chosen from a class of sources which does not emit such wavelengths. The night lighting source (50) is situated adjacent the edge of a transparent block (46) which directs light to the liquid crystal display (60). The day lighting source (42) may be situated at the edge of the block (46) or behind it. The day lighting system is disabled while the night system is in use.

Description: The present invention relates to display devices and, more particularly, to an improved backlight system for liquid crystal device ("LCD") displays, designed for use in daytime, and with night vision imaging systems. LCD's have been employed as modulators of light, and, as such, have been extensively employed in various devices as displays for information in various forms and formats. Reflective LCD's rely upon impinging illumination which enters the face of the device, is reflected from a rear surface, and exits the face. Reflective LCD's have the advantages of lower power requirements, and reductions in size and weight compared to transmissive LCD's, which must have illumination sources. Typical uses of reflective LCD's are in wrist watch faces and some lap top computer screens. These require ambient light to provide the contrast between the light transmissive and opaque portions of the display. Reflective LCD's do not work effectively in a dimly lighted environment, and are completely ineffective in darkness. Some watchmakers solve that problem by providing a small light source in the watch to illuminate the face, creating the necessary contrast between light and dark. However, some of the benefit of the reflective device ( i.e., lower power and fewer components) is lost thereby. While suffering in the presence of low ambient illumination, the reflective device is also sensitive to the light transmissive ability of the LCD, since the luminance of the display is dependent upon the square of the transmission of light through the LCD. This is because the ambient light must travel through the LCD to the reflective surface and then back out through the LCD again. Typically, a monochrome twisted nematic LCD, such as are used in watch faces, can have light transmission of up to 35%. This is adequate for reflective mode viewing, where the luminance level of the display will be about 10% of the viewing ambient. In contrast, Full Color, active matrix LCD displays must use artificial illumination devices due to the extremely low rate of light transmission through the display. Creating a color display requires the incorporation of absorptive color dye filter mosaics along with complex, integral addressing structures. The absorptive dyes and addressing structures allow light transmissions in the range of less than 5%, and in the reflective mode would have a luminance of approximately 0.25% of the ambient light level, which is unacceptably low. A typical backlight illumination source is a fluorescent lamp, which has the requisite properties of high efficiency, long life and good colour balance ( i.e. , transmits the full range of frequencies within the visible spectrum in their normal concentrations). Transmissive color LCD displays have found application in color television projection systems and in "flat screen" color displays for computer systems. Yet another application for such devices is in avionics multi-function displays, which are required to present both full-color graphics and multi-gray scale video imagery. Such a display must have high brightness and contrast for good sunlight viewing, as well as a wide dimming range for night time operation. Another requirement for avionic displays is compatibility with night vision goggles, which are used to view exterior scenes under totally dark conditions. Such night vision devices employ high gain electronic image intensifiers that are sensitive to light in the near-infrared and infrared spectral regions. To be compatible with such systems, cockpit displays must provide sufficient brightness for direct viewing with the unaided dark adapted eye, while, at the same time not emit energy in that portion of the spectrum that might saturate the night vision equipment or cause a reduction in image intensifier gain. Further, the display should not otherwise adversely effect the ability of such equipment to provide real-world night vision. The quantitative requirements for compatibility with such devices is given in Military Specification MIL-L-85762A, dated 26 August 1988. While these are seemingly contradictory requirements, they are compatible because the spectral region of maximum night vision equipment sensitivity is not coextensive with the spectral region of maximum human visual sensitivity. Night vision equipment is most sensitive to light in the infrared region (approx. 780 nm) while the human eye is most sensitive to light in the yellow-green region (approx. 550 nm). Accordingly, one could filter the display emissions to minimize the longer wavelength components that would otherwise interfere with the night vision equipment without adversely affecting that portion of the visible spectrum to which the human eye is most sensitive. In dealing with a cockpit display that uses a backlit LCD, whose emission is mainly in the shorter wavelengths (i.e. green-blue), the back light can be easily filtered to remove the infrared and near infrared components without adversely affecting the display luminance. However, there is a penalty associated with this scheme whenever a multicolor display is involved. That is because the sensitivity of the night vision systems extends into the "red" region for some devices (Class B of MIL-L-85762A) and into the "red-orange" region for more sensitive devices (Class A of MIL-L-85762A). Some night vision systems are so sensitive that display emissions within the range of sensitivity must be strongly attenuated, including those in the visible red and red-orange region of the spectrum. Attenuation of the longer wavelength portions of the visible spectrum reduces brightness of the colors "red" and "red-orange", and can distort the display color balance. However, the color balance can be partially restored by adjusting the primary color emission ratios in the backlight lamps. The net result is a reduction in luminous efficiency, a lowering of peak display brightness and high ambient contrast, and a degradation of sunlight readability. Driving the backlight lamps "harder" to achieve higher brightness levels for the non night vision applications will partially ameliorate the problem, but at the cost of higher overall power dissipation, increased weight and shorter lamp life. With monochrome LCD display, prior art approaches have utilized a light source with an infrared attenuating filter to achieve compatibility with night vision equipment. One such system is disclosed in the recent patent to Klein, U.S. Pat. No. 4,934,793. Such a system is subject to all of the shortcomings outlined above, should a full color system be employed. Sanai et al., U.S. Pat No. 5,029,045, Horiuchi, U.S. Pat. No. 4,998,804, and Suzawa, U.S. Pat. No. 4,618,216 teach the full-color illumination of LCD's, while Tanaka et al., U.S. Pat. No. 5,040,098 allows illumination of monochromatic LCD displays whose color attribute is adjusted by means of organic fluorescent dyes. The LCD illumination device of Hamada, U.S. Pat. No. 4,914,553 utilizes a Fresnel reflection surface, and can be used with either built-in or external light sources. Masuzawa el al., U.S. Pat. No. 4,642,736, teaches a light diffuser for normal illumination of various types of machinery as well as LCD devices. None of these references are intended for use with modern night vision goggles. Other prior art devices for illuminating conventional, non-LCD panel instruments to be useful with night vision goggles include Garret, U.S. Pat. No. 4,887,189, and Reiter et al., U.S. Pat. No. 4,581,683. The device taught in Garret uses a "hot", incandescent light bulb that requires filters to remove unwanted wavelengths of light in the red and infrared band, and teaches a method of cooling the bulb to prevent premature burnout. The device in Reiter is specifically designed to emit only green light, and, accordingly, each is even less applicable for use with full color LCD's. The primary object of the present invention is to permit the reading of an LCD display at night with or without the use of night vision goggles, and with no diminution of system efficiency or readability during the course of daylight viewing. Figure 1 illustrates a conventional LCD backlight structure 10 using a set of high efficiency, high brightness, fluorescent lamps 12 mounted in a reflector housing 14 to enhance the overall luminous efficiency. The structure also incorporates a diffuser plate 16, which scatters the light rays from the individual lamps 12, to create a uniform luminance plane behind an LCD display 18. If the conventional LCD backlight 10 of Figure 1, were to be used with night vision systems, an optical filter 20 would be placed in proximity to the diffuser plate 16 to eliminate the unwanted infrared and near infrared wavelengths. Typically, this filter 20 could be an infrared absorptive element constructed of CM-500 glass (made by Hoya, Inc.), or a similar material. Alternatively, a filter which is an infrared reflective element made of a custom designed thin film interference filter could be employed. Either filter combination prevents the emission of that portion of the visible light spectrum that could compromise visual performance of the LCD 18 in the presence of a night vision system. The absorptive elements in such glass filters 20, although designed for high absorption at longer wavelengths in the infrared region of the spectrum and high transmission at shorter wavelengths in the visual region, absorb significant amounts of light within the visual spectrum as well, especially toward the longer wavelengths. Thus, where the filter 20 adequately attenuates the infrared wavelengths, it also absorbs light well into the visual spectrum, lowering display brightness and altering color balance of the LCD 18. The alternative interference filter described above has a sharper transition between high absorption and high transmission, but its ability to filter light effectively is angle-dependent. The use of a reflective infrared interference filter for night vision system compatibility would, therefore, alter the viewing angle characteristics of the LCD display 18, increasing the color shift for off-axis viewing. A prior art example of an edge lighting technique is a wedge-shaped edge-lit panel 26, as described in Hehr, U.S. Pat. 4,277,817, and is shown in Figure 2. The panel 26 is usually made of glass or plastic. Incident light 28 from lamp 30 enters the wedge-shaped block 26 through the edge surface 32 of the block 26. Total internal reflection of the light 28 occurs until the critical angle is exceeded, at which point the light 28 exits the block 26 through the face 34 to illuminate an LCD 36 located above the block emission surface 34. Shaping the edge surface 32 into a spherical, parabolic or other curvature is sometimes used to improve uniformity from the edge lighting source, particularly for large area devices, as is more fully described in Hehr, Suzawa, U.S. Pat. No. 4,618,216, and Hathaway, et al, New Backlighting Technologies for LCD's , 91 SID Digest, pp. 751-54. In addition, bottom surface treatments can be used to shape the emission envelope of the wedge block 26 for higher brightness. See Suzawa. These includes microgrooves, microbeads, or the inclusion of a light scattering plate. Edgelit panels are usually described as having a reflective coating 38 or material applied to all surfaces except the one in which light is intended to exit in order to improve efficiency, as described in Hehr, Suzawa, Hathaway and Shaw, U.S, Pat. No. 4,974,122. In order to achieve a full-color LCD display during daytime operation, which is also compatible with night vision goggles when the device is being used at night, the present invention utilizes separate lamps for day and night operation. For daytime viewability, high efficiency "day" lamps are used that need not comply with night vision system requirements. The display so illuminated is characterized by high luminous efficiency, good color separation, high brightness and high contrast, with no reduction in the quality of the any color bands, including red or red-orange. Compatibility with night vision systems is achieved by using dedicated "night" lamps which emit little or no infrared, or near infrared energy. Alternatively, full spectrum "night" lamps can be used in conjunction with infrared and red or red-orange absorbing filters. Such lamps do not need to illuminate the LCD brightly for night viewing and the emission profile in the visible spectrum can be adjusted so that a substantially full color display is experienced. EP-A-364 346 shows a system for illuminating an LCD screen as set out in the pre-characterising portion of claim 1. Light from both the day and night lamps which are used to illuminate the screen, enter the block of light transmitting material through the under face of the block. According to the present invention there is provided a liquid crystal display lighting system as set out in the characterising portion of claim 1. Preferably, the said first block is substantially in the shape of a wedge or truncated wedge. A second light transmitting block may be provided, said light sources being located adjacent respective blocks, said blocks being arranged so that light from said first and second light sources is able to emit from said display surface of said first block. The first light source may be adjacent to an end surface of the second light transmitting block such that light from said first light source enters said second block through its end surface. In this case the system normally includes reflective means for the planar under surface of the second light transmitting block whereby light from said first light source is reflected towards the under surface of the first block. Normally, means for independently adjusting the luminance said first and second light sources are included. Means for disabling said second light source during day time viewing are also normally included. Said selecting means may include a filter placed between said second light source and said block and/or said second light source may include a lamp selected from a class of lamps which only emit light compatible with night vision systems. Preferably also, means are provided between said block and the liquid crystal display for diffusing liqlj _ t , emitted from said block before reaching the liquid crystal display. The following description of several specific embodiments of the invention is given by way of example only and with reference to the accompanying drawings in which Figure 3 is an exploded side view of an LCD backlight device according to the present invention, which uses a backlight for day viewing and is edgelit for night viewing; Figure 4 is an exploded side view of a dual edgelit backlight for day and night viewing of an LCD; and Figure 5 is an exploded side view of a generalized LCD backlight device according to the present invention, which uses a back light for day viewing and is edgelit for night viewing. Figure 3 is illustrative of a display assembly panel 40 of the present invention, and shows the combination of a high brightness fluorescent backlight 42, coupled with reflective surface 44, for day viewing, and an edge-lit, wedge-shaped block 46 illuminated through the edge surface 48 of the block 46 with light from night lamp 50, which is made compatible with night vision goggles. External drive circuitry permits independent adjustment of the luminance of lamps 42 and lamp 50. The two sets of lamps 42, 50 operate under complementary conditions. For day viewing, or night viewing without night vision goggles, only the high brightness, fluorescent backlighting lamps 42 are illuminated. During use with night vision goggles, only the edge lighting night lamp 50 is illuminated . Obviously, though, the night lamp 50 can be operated in combination with the day lamps 42 without untoward effects during daylight viewing because of its low luminance compared to lamps 42. The lighting system 40 is made compatible with night vision systems either by selecting the night lamp 50 from a class of lamps which do not emit light of the unwanted wavelengths in the infrared or near infrared ranges, or by adding either an absorptive or reflective filter 52 between the night lamp 50 and the edge surface 48 of the wedge shaped block 46. The latter would require a change in the emission characteristics of the night lamp 50 (compared to the day lamps 42) to preserve similar colors for both night and day operation since, as has been mentioned, filtration to remove infrared light also tends to attenuate the longer wavelengths within the visible spectrum. In either case, no filtration of the day fluorescent lamps 42 is required, resulting in higher luminous efficiency and better color separation for day viewing. In the present invention as shown in Figure 3, the edgelit wedge-shaped block 46 also functions as a diffuser for the day lamps 42. Therefore, rather than putting a reflective surface behind the wedge-shaped block as in Figure 2, the reflecting surface 44 is instead placed behind the fluorescent day lamps 42. This permits the transmission of light from the day lamps 42 through the under surface 54 of the wedge-shaped block 46 without being impeded by a reflective coating which might otherwise be placed on the underside 54 of the wedge-shaped block 46 as in Figure 2. A diffuser plate 56 is placed at the front surface 58 of the edgelit panel to enable good uniformity of light transmission from the day lamps 42 and the night lamp 50 through the LCD 60. The night lamp 50 for this application could be incandescent, fluorescent, or of other lamp types, any of which are within the scope of this invention. In a preferred embodiment, a miniature, cold cathode fluorescent lamp is used. The cold cathode structure has lower efficiency than the hot cathode fluorescent lamps typically used for the high brightness day lamps, but such brightness is not important for night viewing. A reflector housing could be used for the night lamp similar to that used for the day lamps, but a better solution is to use an aperture lamp, incorporating an internal reflector and only emitting light in a narrow slit facing the edgelit panel. More significant is the absence of the hot cathode structure, which eliminates a significant source of infrared emission and which simplifies the night vision system light filtering requirements. One advantage of fluorescent lamps for both day and night viewing is the broad color gamut that can be tailored by judicious selection of the phosphor components. In another embodiment shown in Figure 4, a display assembly 62 contains a second edge-lit, wedge-shaped block 64 placed behind the first wedge-shaped block 46'. The second wedge-shaped block 64 has a reflective coating 66 on the undersurface 68 of the second block 64 to increase the uniformity and quality of light illuminating the LCD 60'. In this configuration, both the day and night lamps 42', 50' are arranged in edge-lit fashion, allowing the display assembly 62 of Figure 4 to be thinner than the assembly 40 in Figure 3. The reduction in overall depth of the LCD lighting assembly 62 in Figure 4 is due to the elimination of some of the components that are necessary for the backlighting in Figure 3 (i.e. , the backlight assembly 42). As in the first embodiment, the day and night lamps 42', 50' operate in a complementary manner. Both are aperture fluorescent lamps, though the day lamp 42' has a hot cathode structure for higher efficiency. However, the two blocks 46', 64 are sandwiched together to form , essentially a rectangular prism. Figure 5 is illustrative of a more generalized display assembly block 140 of the present invention, and shows, in the broadest scope, the combination of a high brightness fluorescent backlight 142, coupled with reflective surface 144, for day viewing, and an edge-lit, translucent or transparent block 146 illuminated through the edge surface 148 of the block 146 with light from night lamp 150, which is made compatible with night vision goggles. External drive circuitry permits independent adjustment of the luminance of lamps 142 and lamp 150. The two sets of lamps 142, 150 operate under complementary conditions. For day viewing, or night viewing without night vision goggles, only the high brightness, fluorescent backlighting lamps 142 are illuminated. During use with night vision goggles, only the edge lighting night lamp 150 is illuminated. Obviously, though, the night lamp 150 can be operated in combination with the day lamps 142 without untoward effects during daylight viewing because of its low luminance compared to lamps 142. The lighting system 140 is made compatible with night vision systems either by selecting the night lamp 150 from a class of lamps which do not emit light of the unwanted wavelengths in the infrared or near infrared ranges, or by adding either an absorptive or reflective filter 152 between the night lamp 150 and the edge surface 148 of the block 146. The latter would require a change in the emission characteristics of the night lamp 150 (compared to the day lamps 142) to preserve similar colors for both night and day operation since, as has been mentioned, filtration to remove infrared light also tends to attenuate the longer wavelengths within the visible spectrum. In either case, no filtration of the day fluorescent lamps 142 is required, resulting in higher luminous efficiency and better color separation for day viewing. In generalized embodiment of the invention as shown in Figure 5, a translucent edgelit block 146 can function as a diffuser for the day lamps 142. Therefore, rather than putting a reflective surface behind the block as in Figure 2, a reflecting surface 144 is instead placed behind the fluorescent day lamps 142. This permits the transmission of light from the day lamps 142 through the under surface 154 of the block 146 without being impeded by a reflective coating which might otherwise be placed on the underside 154 of the block 146 as in Figure 2. A diffuser plate 156 is placed at the front surface 158 of the edgelit block to enable good uniformity of light transmission from the day lamps 142 and the night lamp 150 through an LCD 160. The night lamp 150 for this application could be incandescent, fluorescent, or of other lamp types, any of which are within the scope of this invention. As in the preferred embodiment, a miniature, cold cathode fluorescent lamp is used. A reflector housing could be used for the night lamp similar to that used for the day lamps, but, as in Figure 3, an aperture lamp, could be utilized. As can be seen the particular shape of the block 146 can be varied. What is significant is the provision of both the broad day illumination which directly illuminates the LCD for day viewing and the night vision illumination which is provided at the edge of the block. While both sources of illumination could be enabled for day time viewing, only the night lamps would be used in conjunction with night vision equipment. Other variations and modifications will occur to those skilled in the art. Accordingly, the present invention should be limited only by the scope of the claims appended hereto.

Claims: A liquid crystal display lighting system compatible with day and night vision equipment comprising: a first block (46;46';146) of light transmitting material, having a display surface (58;158), an under surface (54) and an end surface (48;148) ; a first light source (42;42';142), capable of emitting full spectrum visible light; a second light source (50;50';150), including selecting means (52;52';152) for limiting the emission of infrared lights; and means for disabling said first light source during night viewing; characterised in that the first light source is located such that light from the first light source enters the light transmitting block (46;46';146) through the under surface (54), the second light source being adjacent to the end surface (48;148) such that light from the second light source enters the block through the end surface. A liquid crystal display lighting system as claimed in claim 1 in which said first block (46;46';146) is substantially in the shape of a wedge or a truncated wedge. A liquid crystal display lighting system as claimed in claim 1 or claim 2 further including reflection means (44) located behind said first light source (42). A liquid crystal display lighting system as claimed in claim 1 or claims 2 in which a second light transmitting block (64) is provided, said light sources (42'50') being located adjacent respective blocks (46',64), said blocks being arranged so that light from said first and second light sources (42',50') is able to emit from said display surface of said first block (46'). A liquid crystal display lighting system as claimed in claim 4, in which the second light transmitting block (64) includes reflective means (66) on a planar undersurface (68) thereof, the reflecting means (66) reflecting the light from the first source toward the undersurface (54) of the first light transmitting block (46'). A liquid crystal display lighting system as claimed in any preceding claim, including means for independently adjusting the luminance said first and second light sources. A liquid crystal display lighting system as claimed in any preceding claim, including means for disabling said second light source during day time viewing. A liquid crystal display lighting system as claimed in any preceding claim wherein said selecting means include a filter (52;152) placed between said second light source and said block. A liquid crystal display lighting system as claimed in any preceding claim where said second light source includes a lamp selected from a class of lamps which only emit light compatible with night vision systems.

Inventors: KALMANASH MICHAEL H, KALMANASH, MICHAEL H.
Applicants: KAISER AEROSPACE & ELECTRONICS, KAISER AEROSPACE & ELECTRONICS CORPORATION
Application year: 1993
Main classification: G02F1
IPC codes: G02F1, G02B6, G09G3, F21V8