Abstract:
An interference coating between a display and an intended viewing area that selectively absorbs incident optical energy in at least one first wavelength region and selectively transmits incident optical energy in at least one second wavelength region.

Description:
TECHNICAL FIELD 
     The present invention relates in general to selective absorbing of light and more particularly concerns selective absorbing of light for rear-illuminated display systems, such as used in television or home cinema applications. 
     BACKGROUND 
     For background, reference is made to U.S. Pat. No. 6,847,483. 
     SUMMARY 
     In general, in one aspect, an optical device includes a rear-illuminated display and an interference coating between the display and an intended viewing area that selectively absorbs incident optical energy in at least one first wavelength region and selectively transmits incident optical energy in at least one second wavelength region. 
     Implementations may include one or more of the following features. The first wavelength region may correspond to yellow light or cyan light. The second wavelength region may correspond to green light. The first wavelength region may have an average absorption of greater than 50%. The second wavelength region may have an average absorption less than 30%. The second wavelength region may have an average transmission of greater than 60% and the first wavelength region may optionally have an average transmission less than 40%. The average reflection may be less than 5% in the wavelength region between 540 and 560 nm and less than 10% in the wavelength region between 500 and 600 nm. The interference coating may further include an absorptive layer and may optionally include selective absorption in the absorptive layer. The absorptive layer may include a metal compound. The absorptive layer may include a Titanium nitride, Silicon nitride, or Zirconium nitride. The absorptive layer may include a metal. The absorptive layer may include Titanium, Niobium, Zirconium, Chromium, or Silicon. The rear-illuminated display may include a DMD light modulator, a transmissive-LCD light modulator, or an LCOS light modulator. The rear-illuminated display may include a UHP projection lamp or an LED projection lamp. The interference coating may be deposited on a substrate. 
     In general, in one aspect, an interference coating for use with a display that selectively absorbs incident optical energy in a predetermined number of first wavelength regions and selectively transmits incident optical energy in a predetermined number of second wavelength regions. 
     Implementations may include one or more of the following features. The interference coating comprises an absorptive layer. Selective absorption may occur in the absorptive layer. 
     In general, in one aspect, an optical device includes a rear-illuminated display with red, green, and blue emission regions and an interference coating between the display and an intended viewing area that selectively transmits in the wavelength regions that correspond to the red, green, and blue emission regions. The interference coating selectively absorbs in at least one other wavelength region. 
     Implementations may include one or more of the following features. The other wavelength region may correspond to at least yellow light or cyan light. 
     In general, in one aspect, an optical device includes a rear-illuminated display and an interference coating between the display and an intended viewing area that selectively absorbs incident optical energy in a plurality of absorption regions and selectively transmits incident optical energy in a plurality of transmission regions. The transmission regions are distinct from the absorption regions. 
     Implementations may include one or more of the following features. The absorption regions may correspond to yellow and cyan light. The transmission regions may correspond to blue, green, and red light. 
     In general, in one aspect, a method for increasing contrast ratio includes selectively absorbing ambient light in an interference coating placed between a rear-illuminated display and an intended viewing area, and transmitting light from the display through the interference coating to the intended viewing area. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a schematic drawing of a rear-illuminated display with a selective absorbing coating; 
         FIG. 2  is a table of an interference-coating optical design; 
         FIG. 3  is a graph of an interference-coating transmission; 
         FIG. 4  is a graph of an interference-coating absorption; 
         FIG. 5  is a graph of an interference-coating reflection; and 
         FIG. 6  is a graph of plasma and LCD display spectrums. 
     
    
    
     DETAILED DESCRIPTION 
     The cross-sectional views of the drawing are not drawn to scale. Some actual dimensions are as stated in the specification and figures. 
     Displays can be divided two general categories, front illuminated and rear illuminated, for use in, e.g., home cinema and business applications. Light modulators turn on and turn off pixels of pre-selected color spectral bands of visible light (e.g., red light, blue light, and green light) to form images on surfaces, e.g., display screens. Generally, front illuminated displays, such as front projectors, have a light source on the same side of the screen as the viewer and illuminate a reflective display screen that reflects light into the viewer&#39;s eyes. Rear illuminated displays, such as rear-projection televisions and liquid-crystal flat-panel displays, have a light source on the opposite side of the screen from the viewer and illuminate a transmissive display screen to display an image that is viewed from the side of the screen that is opposite the light source. Rear-illuminated displays include flat-panel liquid crystal displays (LCD), flat-panel plasma displays, organic light emitting diode (OLED) displays, cathode ray tube (CRT) displays, and rear-projection displays that use light modulator technology such as digital micromirror devices (DMDs), transmissive LCD devices, or liquid crystal on silicon (LCOS) devices. 
     The display produces monochromatic or color images by decoding received electronic signals and using the signals to independently vary the brightness of each pixel on the display as a function of time. Pixels may be fixed with specific colors or the colors of pixels may be changed depending on the type of display device. For color displays, the pixel colors are typically blue, green, and red. Blue pixels typically correspond to a wavelength region of approximately 450 to 480 nm. Green pixels typically correspond to a wavelength region of approximately 520 to 560 nm. Red pixels typically correspond to a wavelength region of approximately 600 to 640 nm. Ambient light is generally white and can be broken down into blue, cyan, green, yellow, and red wavelength regions. Cyan typically corresponds to a wavelength region of approximately 480 to 520 nm. Yellow typically corresponds to a wavelength region of approximately 560 to 600 nm. 
     Referring to  FIG. 1 , there is shown a schematic drawing of the components of a selective absorbing coating with a video display. A rear-illuminated video display  100  such as an LCD panel generates display light  102 , which consists of narrow bands of blue, green, and red light. Display light  102  passes through a coating substrate  108 , an interference coating  110 , and is then perceived visually by a viewer  106  located in an intended viewing area  118 . Display assembly  120  includes display  100 , substrate  108 , and interference coating  110 .  FIG. 1  shows coating  110  on the viewer side of substrate  108 . Alternatively, coating  110  may be located on the side of substrate  108  that is opposite the viewer. Coating  110  may optionally be deposited directly on display  100 , in which case there is no substrate  108 . Ambient light  114  from sources such as room lighting and sunlight, represented by light source  112 , consists of blue, cyan, green, yellow, and red light. Ambient light  114  passes through interference coating  110 , and reflects off the display as reflected ambient light  116 . Reflected ambient light  116  passes again through interference coating  110  and is then visually perceived by viewer  106  located in intended viewing area  118 . The blue, green, and red wavelength regions of display light  102  and incident ambient light  114  selectively pass through coating  110 . The cyan and yellow wavelength regions of incident ambient light  114  are selectively absorbed in coating  110  so that the intensity of these wavelength regions is reduced in reflected ambient light  116  compared to the light reflected in the blue, green and red bands. The reduced intensity of reflected ambient light  116  is indicated by a dotted line in  FIG. 1 . 
     The contrast ratio of display assembly  120  in the presence of substantial ambient light is defined as the visible intensity of the display light  102  divided by the visible intensity of the reflected ambient light  116 . The contrast ratio is beneficially increased by the coating  110  because the visible intensity of the reflected ambient light is preferentially reduced by the selective absorption of coating  110 . 
     The desired characteristics for a display assembly with a selective absorbing coating include high transmittance of the selective absorbing coating in the wavelength regions that are used for information display (e.g. blue, green, and red) and high absorbance for the wavelength regions that are present in the ambient light but not used for information display (e.g. cyan and yellow). It is also beneficial for the display assembly to have low reflectance of ambient light so that glare is minimized. 
     In some implementations, interference coating  110  is constructed from a multilayer interference coating which is an optical filter that can transmit, reflect, or absorb light depending on the wavelength. Multilayer interference coatings are constructed by depositing adjacent layers of different materials, wherein each layer of material has a different index-of-refraction (n) than the material of the adjacent layer or layers. Multilayer interference coatings operate by using constructive and destructive interference between incident light and light that reflects at various layer interferences. It is usually advantageous to maximize the difference in n between adjacent layers, so it is typical to use alternating layers of high n materials (such as TiO 2 , Nb 2 O 5 , or Ta 2 O 5 ) and low n materials (such as SiO 2 ) in adjacent layers. However, it is also possible to use materials with medium n (such as Al 2 O 3 ), or to use two materials having a high n (or two materials having a low n) in adjacent layers. Further information on multilayer interference coatings can be found in  Thin Film Optical Filters  by H. A. MacLeod, ISBN 0750306882. 
     An interference coating can be mathematically calculated and designed with commercially available computer software such as TFCalc available from Software Spectra Inc. at Internet address www.tfcalc.com. The coating is designed to achieve the desired transmission, reflection, and absorption targets by selecting the layer thicknesses and materials such that ambient light is selectively absorbed in certain wavelength regions e.g. cyan and yellow while light from the display is selectively transmitted in certain wavelength regions, e.g. red, green, and blue. Reflection is generally undesirable for rear-illuminated displays and is minimized by setting low reflection targets during the design process. 
     Referring to  FIG. 2 , there is shown a table of an optical design for one implementation of interference coating  110 . The optical design includes alternating layers of TiO 2  and SiO 2  in adjacent layers. The thickness of each layer is shown in nanometers. Layer  22  is an optically absorptive layer of Titanium metal. The thickness of each layer of the multilayer interference coating in  FIG. 2  is designed to maximize selective absorption of the cyan and yellow wavelength regions. The absorption takes place in the absorptive material of layer  22 . 
     The absorptive material used in interference coating  110  may be an environmentally durable metal such as Titanium, Niobium, Zirconium, Chromium, or Silicon or it may be an environmentally durable compound such as Titanium nitride, Silicon nitride, or Zirconium nitride. Although some of these materials are conventionally considered to be reflective materials, they may also exhibit high absorption when combined together with other layers in an interference coating. 
     Interference coating  110  may be manufactured by sputtering, physical vapor deposition, or other optical coating processes. The coating process may be a batch process or a roll-coating process. Substrate  108  may be a rigid substrate such as glass or plastic sheet or it may be a flexible material such as plastic film. Coating  110  may also be deposited directly on the front surface of display  100  or within display  100 . The dimensions of coating  110  will usually match the dimensions of the display or be slightly smaller if a bezel covers the edge of the display. The diagonal size of the display may be as small as 2 to 10 cm for portable devices or as large as 2.5 m or larger for wall-mounted video displays. 
     Referring to  FIG. 3 , curve  300  shows a graphical representation of the transmittance of the coating design described in  FIG. 2 . The vertical axis represents the transmittance of the interference coating of  FIG. 2  and the horizontal axis represents the wavelength of the light transmitted through the coating. In this implementation, the blue wavelength region is 450 to 490 nm, the green wavelength region is 540 to 560 nm, and the red wavelength region is 620 to 650 nm. The average of an optical property such as transmittance can be calculated for a wavelength region of the coating. The average within the region is defined by adding the transmittance values within the region and dividing by the number of values. The average transmittance in each of the blue, green, and red wavelength regions is approximately 70% for the implementation of  FIG. 2 . The average transmittance should be higher than approximately 60% to allow visible light to transmit through the coating to the viewer. The transition region occurs between the wavelength that corresponds to 90% of the maximum value of the transmission region and the wavelength that corresponds to 110% of the minimum value of the blocking region adjacent to the transmission region. In general, a small transition region is desirable to help maximize the transmission of the coating design. 
     Referring to  FIG. 4 , curve  400  shows a graphical representation of the absorbance of the coating design described in  FIG. 2 . For an optical coating that has a small amount of scattered light, the absorbance (in percent) is defined as 100 minus transmittance minus reflectance. In this implementation, the cyan wavelength region is 505 to 525 nm and the yellow wavelength region is 575 to 605 nm. The average absorbance in each of the cyan and yellow wavelength regions is approximately 55 to 60%. The average absorbance should be higher than approximately 50% to absorb the cyan and yellow bands of light and thus raise the contrast ratio. The transition region is defined for absorption similar to the definition for transmission. 
     The reflectance of ambient light by the selective absorbing coating should be small to reduce glare and visible reflections of the surrounding environment at the surface of the display assembly. The reflectance of the display light by the selective absorbing coating should also be small to avoid ghosting issues from light that is reflected back into the display and then reflected back out to the viewer. Referring to  FIG. 5 , curve  500  shows a graphical representation of the reflectance of the coating design described in  FIG. 2 . The average reflectance is less than 5% in the green wavelength region between 540 and 560 nm and less than 10% in the wavelength region between 500 and 600 nm, which includes the colors of green, yellow, and orange. The wavelength region between 540 and 560 is at the peak of the optical sensitivity of the human eye, so minimum reflectance is desirable in that wavelength region. The wavelength region between 500 and 600 nm covers most of the wavelength region that the human eye can perceive with high sensitivity so it is also desirable to minimize the reflection in this wavelength region. The wavelength regions outside of 500 to 600 nm do not contribute substantially to visible reflection, so the allowable reflection from the coating can be higher than within the 500 to 600 nm region. 
     The interference coating of  FIG. 2  may be incorporated internally to the display device rather than on the front surface. The details of incorporation will depend on the type of display device. For example, with an LCD flat panel display, the interference coating  110  may be deposited on an interior glass or plastic surface rather than on the front surface. The interference coating  110  may be adhesively bonded such that the coating is immersed in materials with an index-of-refraction of approximately 1.5 such as conventional glass or plastic materials that are usually used for optical purposes. The adhesive may be a pressure-sensitive adhesive, ultraviolet-cure adhesive, or any optically transparent adhesive. The index-of-refraction of the adhesive is generally chosen to match the other optical materials so that there is minimal reflection between the adhesive and the other optical materials. The index-of-refraction of the adhesive may be approximately 1.5. The transmittance of  FIG. 3 , absorbance of  FIG. 4 , and reflectance of  FIG. 5  is calculated for a coating that is immersed in materials and adhesives that have an index-of-refraction of 1.5. 
     Referring to  FIG. 6 , curve  600  shows a graphical representation of a typical plasma-display optical spectrum and curve  602  shows a typical LCD-display optical spectrum. The optical spectrum shows the normalized intensity as a function of wavelength. The coating design described in  FIG. 2  has its transmissive regions matched to the blue, green, and red wavelength bands of the plasma and LCD displays shown in  FIG. 6 . The selective absorbing coating may be used with various rear-illuminated displays such as flat-panel liquid crystal displays (LCD), flat-panel plasma displays, organic light emitting diode (OLED) displays, cathode ray tube (CRT) displays, and rear-projection displays that use light modulator technology such as digital micromirror devices (DMDs), transmissive LCD devices, or liquid crystal on silicon (LCOS) devices. The rear-projection displays may use projection lamps based on various sources such as ultra-high-performance (UHP) lamps, xenon lamps, light emitting diodes (LEDs), or lasers. The color pixels in the displays may be produced by various technologies such as fixed color filters, color wheels, sequentially-timed light sources, or separate red, green, and blue light sources. By varying the layer thicknesses and materials, the transmission regions of the coating may be adjusted to match the emission wavelengths of the display. The absorption regions of the selective absorbing coating may be designed to match the regions between the emission regions of the display. The spectrum of a display device may be different depending on the backlight source. For example, LCD displays may have fluorescent backlights or may have LED backlights. Other display technologies may be used such as surface-conduction electron-emitter displays (SEDs), organic LEDs (OLEDs) or other rear-illuminated displays not specifically mentioned here. 
     Conventional methods of increasing display contrast utilize colored materials such as intrinsically colored glass or plastic. Interference coatings are advantageous because interference coatings can be mathematically and predictably designed to absorb in arbitrary wavelength regions and to have arbitrarily small transition regions in the spectral curves whereas intrinsically colored materials have their wavelength regions and the size of their transition regions limited by the chemistry of the colored materials. 
     In some implementations, a monochromatic display, or black and white display may be used. The selective absorbing filter may than be designed so that it matches the single band of light from the display rather than the multiple bands of light that are used in a color display. 
     It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific apparatus and techniques herein disclosed without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and limited solely by the spirit and scope of the appended claims.