Patent Publication Number: US-2005134164-A1

Title: Optical coupler for projection display

Description:
FIELD OF THE INVENTION  
      This invention generally relates to cathode ray tube (CRT) projection displays. The invention is particularly applicable to CRT projection displays having low thermal drift.  
     BACKGROUND  
      CRT projection displays are commonly used in consumer applications, such as home entertainment centers, and commercial applications, such as video conferencing, information presentation, and data displays.  
      A CRT projection display typically includes a CRT image forming source and a projection system. The CRT image forming source forms a small image, for example, 12 to 25 cm in diagonal, at the output of the source. The projection system includes one or more, typically at least three, projection lens elements, employed to magnify and project the source image onto a projection screen.  
      More commonly, a CRT projection display includes three CRT image sources, one CRT for each primary color (red, green and blue). Typically, each CRT image source has it own dedicated projection system and projection lens elements. The projection systems magnify, project and superimpose the three source images onto a projection screen resulting in a projected color display.  
      A CRT projection display may be a front or rear projection display. In a front projection display, the image source and viewer are located on opposite sides of the viewing image plane. In contrast, in a rear projection display, the viewer is located on one side of the viewing image plane whereas the projection system and the image created by the source are located on the other side of the image plane. The viewing image is typically displayed on a projection screen which is typically reflective in a front projection display and transmissive in a rear projection display.  
      Optical reflections at the CRT glass plate and the first surface of the projection lens element closest to the source can reduce image brightness, contrast and resolution. To reduce optical reflection, a fluid medium is typically used to fill the space between and optically couple the CRT glass plate and a first lens element. Known fluids include ethylene glycol, mixtures of ethylene glycol and glycerol, mixtures of ethylene glycol and water, alkyl diaryl alkanes, liquids including a siloxane polymer having methyl, phenyl, and hydrophilic side groups, and liquids including mixtures of a siloxane polymer having methyl and phenyl side groups and a siloxane polymer having methyl and hydrophilic side groups.  
     SUMMARY OF THE INVENTION  
      Generally, the present invention relates to CRT projection displays.  
      In one embodiment of the invention, an optical element includes a face plate of a cathode ray tube, a lens element facing the face plate, and an optical coupling material disposed between the lens element and the face plate. The optical coupling material includes particles dispersed in a host material.  
      In another embodiment of the invention, a projection display system includes a CRT image source having a face plate, and a lens element facing the face plate. The projection display system further includes an optical coupling material disposed between the lens element and the face plate. The optical coupling material includes particles dispersed in a host material.  
      In another embodiment of the invention, a cathode ray tube projection system includes a face plate of a cathode ray tube and a lens element. The face plate and the lens element are arranged in spaced-apart opposing positions defining a coupler cavity. The cathode ray tube projection system further includes a coupling fluid dispersed within the coupler cavity. The coupling fluid includes nano-particles. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
      The invention may be more completely understood and appreciated in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
       FIG. 1  illustrates a schematic side view of an optical element in accordance with one embodiment of the invention;  
       FIG. 2  illustrates a schematic side view of a CRT projection display in accordance with another embodiment of the invention; and  
       FIG. 3  illustrates a schematic side view of an optical element in accordance with yet another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
      The present invention generally relates to projection displays. The invention is particularly applicable to CRT projection displays, and even more particularly to CRT projection displays that have an optical coupling material between a CRT image source and a projection lens element.  
      Examples of CRT projection systems can be found in U.S. Pat. Nos. 4,838,665; 5,157,554; 5,381,189; 5,625,496 and 5,440,429; and U.S. Patent Publication Nos. 2002/0196556 and 2003/0071929.  
      Known optical coupling materials include coupling fluids that have a low index of refraction and a high rate of change in refractive index with fluid temperature, dn/dT. Examples of such couplers may be found in U.S. Pat. Nos. 5,117,162; 5,115,163; 4,982,289; 4,904,899; 4,780,640; 4,734,613; 4,725,755; 4,665,336; 4,405,949 and 4,651,217; and U.S. Publication Nos. 2003/0034727 and 2003/0098944. The index of refraction of known optical couplers is generally lower than the optical elements positioned on either side of the optical coupler, and which the optical coupler is designed to optically couple. For example, the index of refraction of known coupling fluids is about 1.43. In contrast, index of refraction of a face plate is about 1.56, and index of refraction of lens elements is approximately in the range from 1.49 to 1.53. An index mismatch between an optical coupling material and the CRT glass plate and/or the first projection lens element, the two media the fluid coupler is designed to optically couple, can result in residual optical reflections leading to reduced brightness and contrast at a viewing image plane.  
      Furthermore, known coupling fluids typically have a rate of change of index of refraction with temperature, dn/dT, that is substantially higher than the dn/dT of other optical elements in the projection system. For example, known coupling fluids typically have a dn/dT of about −30.0×10 −5 /° C. In contrast, dn/dT of plastic lens elements is typically about −10×10 −5 /° C., and dn/dT of a glass CRT face plate is typically +0.4×10 −5 /° C. A large dn/dT can lead to an unacceptable change in the overall focal length and magnification of the projection display. For example, contrast and resolution of a CRT projection display that includes a fluid optical coupler, can change substantially during warm up, where indices of refraction of some optical components including the optical coupler, the lens elements, and the CRT face plate, change as the temperature of the components increases from room temperature to the operating temperature. Furthermore, changes in focal length and magnification can be different for each primary color leading to, for example, an error in registration between the images formed by the three CRT image sources on a viewing screen.  
      Change in index of refraction of a plastic or liquid optical element is typically a result of change in density of the element due to, for example, thermal expansion or contraction of the element. For example, as the temperature of the element increases, the element expands leading to a decrease in index of refraction due to a decrease in density. Similarly, a decrease in temperature can result in the element contracting which can lead to an increase in the refractive index of the element due to an increase in density. Accordingly, dn/dT of an optical element is often negative. Some materials, such as some glasses or semiconductors, have a positive dn/dT due to, for example, a change in polarizability as discussed in, for example, U.S. patent application 2002/0120048.  
      Thermal expansion or contraction of an optical lens in a CRT projection system can change the optical properties of the lens. For example, the focal length and magnification of the lens can change. Furthermore, a change in the temperature of the lens element can lead to a change in the shape of the lens. Such a change can affect both on-axis and off-axis properties of the lens. For example, such a change can introduce or increase undesirable aberrations.  
      An optical coupling material is described for optically coupling a face plate of a CRT image source to a projection lens system. The optical coupling material includes a fluid host material housed in a coupler housing. The optical coupling material further includes nano-particles dispersed in the fluid host material. The average size of the nano-particles is preferably no more than 30 nm. The optical coupling material has an index of refraction n 2 . The CRT image source can form an image at the face plate of the CRT, for example, at the input face of the face plate. The index of refraction of the face plate is n 1 . The projection lens system is employed to magnify and project the source image onto a viewing screen, typically a projection screen. The projection lens system can include one or more projection lens elements. Typically, the projection lens system includes at least three, typically four, lens elements, a first lens element being closest to the CRT face plate. The index of refraction of the first lens element is n 3 . The optical coupling material can include small particles dispersed in a host material.  
      An advantage of the present invention is high optical clarity of the optical coupler. Haze or optical scattering in an optical coupler can reduce display resolution and contrast. The optical coupling material of the present invention can have a high degree of optical transmission including a high specular optical transmission. The optical coupling material of the present invention can have very low optical haze. The particles in the optical coupling material can be small enough to introduce little or no optical scattering. For example, the small particles can be nano-particles, meaning that average particle size is in the nanometer range, for example, no more than 500 nm, such as in the range from 10 to 50 nm. As such, the optical coupling material can have little or no adverse effect on the resolution and contrast of the CRT projection display.  
      Another advantage of the present invention is efficient optical coupling of a CRT face plate to a first lens element. For example, the index of refraction of the optical coupling material can be such to efficiently match the index of refraction of the face plate to that of the first lens element. According to one embodiment of the present invention, the index of refraction of the optical coupling material, n 2 , can be equal to or larger than the lower of n 1  and n 3 . Furthermore, n 2  can be equal to or lower than the higher of n 1  and n 3 . The index matching property of the optical coupling material can reduce optical reflection and, therefore, enhance brightness, resolution and contrast of the image displayed on a viewing screen.  
      According to another embodiment of the invention, n 2  can be the average of n 1  and n 3 . In some applications, n 2  may be the square root of the product of n 1  and n 3 . In some other applications, n 2  may be equal to n 1  or n 3 .  
      Another advantage of the present invention is reduced rate of change of index of refraction with temperature, dn/dT, where T is temperature. Known optical coupling materials are typically organic fluids. As such, dn/dT of known optical couplers can be substantially higher than those of some other optical elements in the projection system, such as the CRT face plate and the lens elements that are made, for example, of glass or plastic. For example, dn/dT of known fluid optical coupling materials can be, at least, twice as high as other optical elements in the projection display, such as the CRT face plate and the lens elements. A high dn/dT can lead to a noticeable change in the overall focal length and magnification as a function of temperature, sometimes referred to as thermal drift. The change can affect resolution and contrast of the viewing image as a function of temperature, both on and off axis. Furthermore, in projection displays employing three CRT image sources, each having a dedicated projection lens system, the thermal drift can be different for each projection system. This can lead to misregistration between two or more projected images, both on and off axis. Addition of small particles to a fluid host material of an optical coupling material can reduce dn/dT of the optical coupling material leading to reduced thermal drift.  
       FIG. 1  illustrates a schematic side-view of an optical element  100  in accordance to one embodiment of the present invention. Optical element  100  includes a CRT  110 , a CRT face plate  120  having an index of refraction n 1 , an optical coupling material  180  having an index of refraction n 2 , and a projection lens system  170 . Projection lens system  170  includes a first lens element  130  having an index of refraction n 3 . Projection lens system  170  can further include additional lens elements such as lens elements  140  and  150 . Face plate  120  has an input face  121  and an output face  122 . Similarly, the first lens element  130  has an input face  131  and an output face  132 . Optical coupling material  180  can include small particles  185  dispersed in a host medium  186 . Optical coupling material  180  has high optical transmission and low haze. Haze is typically a measure of cloudiness. Haze, as used in the specification, is a percentage of light diffusely transmitted compared to total transmitted light. Optical coupling material  180  preferably has a haze no more than 2%, more preferably no more than 1%, and even more preferably no more than 0.5%, and still even more preferably no more than 0.2%. Optical scattering and haze can be affected by the size of particles  185 . In general, as the particle size decreases, the optical scattering and haze decrease. Furthermore, optical scattering and haze can be affected by a mismatch between the index of refraction of particles  185  and index of refraction of host material  186 . In general, as the index mismatch decreases, the optical scattering and haze decrease. It is often easier to control the range of particle size than the index mismatch between the particles and the host material. For example, an upper end of particle size range may be controlled by passing the particles through a screen. In contrast, it may be difficult to control the index mismatch between the particles and the host material over a desired wavelength range, such as the visible range. This may be so because of variation in composition of particles and host material. Furthermore, it is generally difficult to match index dispersion of the particles and the host material as a function of temperature and/or wavelength. As such, particles  186  preferably are sufficiently small to reduce optical scattering and haze to acceptable levels. In particular, small particles  185  are preferably nano-particles, meaning that the average particle size is in the nanometer range. Even more particularly, the average size of particles  185  is preferably no more than 500 nm. The average size of particles  185  is more preferably in the range from 10 to 100 nm. The average size of particles  185  is even more preferably in the range from 10 to 50 nm, and even more preferably in the range from 10 to 30 nm. It will be appreciated that larger particles may be used where, for a given application, the index mismatch between particles  185  and host material  186  is sufficiently small to result in low optical scattering and haze. Particle size can be particle diameter where, for example, particles are spherical or approximately spherical, or where, in general, it is reasonable to assign a diameter to a particle. Particle size may be an average of particle dimension along different directions. For example, where particles are rod-shaped, particle size may be an average of particle dimension along its major and minor axes. In some applications, particle size may be the largest particle dimension.  
      According to one embodiment of the invention, particles  185  are colloidally dispersed in the host medium  186  so that at least a majority of particles  185  remain dispersed in host medium  186  for at least the expected lifetime of optical element  100 . Expected lifetime of optical element can be application dependent. For example, an expected lifetime of a consumer rear projection television can be about 20,000 hours. In such an application, a majority of particles  185  can remain dispersed in host medium  186  for at least 20,000 hours.  
      Furthermore, according to one embodiment of the invention, particles  185  do not aggregate for the expected lifetime of optical element  100 , or any aggregation that may occur during the expected lifetime of optical element  100  does not result in a significant light scattering or haze.  
      According to one embodiment of the invention, particles  185  have a positive dn/dT. An example of such a particle system is magnesium oxide described in U.S. Pat. No. 6,441,077. In some embodiments of the invention, particles  185  have a negative dn/dT.  
      According to one embodiment of the invention, surfaces of particles  185  may be treated to facilitate and maintain dispersion. The surface treatment is preferably compatible with the host medium  186 , meaning that the treated particles  186  remain soluble or dispersable in the host material. An appropriate surface treatment can reduce or eliminate particle precipitation and/or aggregation.  
      According to one embodiment of the invention, n 2  is approximately a linear function of weight or volume fraction of particles  185  in optical coupling material  180 . For example, the relationship between n 2 , the index of refraction of the host material  186 , n b , and the index of refraction of particles  185 , n a , can often be estimated by the relation: 
 
 n   2   =n   b  (1 −VF )+ n   a  ( VF )   (1) 
 
 where VF is a volume fraction of particles  185  in the optical coupling material  180 . Volume fraction VF is generally defined as the ratio of volume of particles  185  to the total volume, where the total volume is the volume of the host material  186  with particles  185  dispersed therein. Volume fraction VF can be determined, for example, by first measuring the volume of fluid host material  186 , V 1 . Next, particles  185  are added to the fluid and the volume of the mixture, V 2 , is measured. Accordingly, the volume of added particles  185  is V 2 -V 1 . Volume fraction of particles  185 , VF, is determined by calculating the ratio (V 2 -V 1 )/V 2 . A similar approach may be used to determine weight fraction of particles  185  by using weights rather than volume. Referring back to relation (1), the rate of change of indices of refraction is often estimated by the relation: 
 
 dn   2   /dT=dn   b   dT (1 −VF )+ dn   a   /dT ( VF )   (2) 
 
 where T is temperature. Accordingly, dn/dT of the optical coupling material can be a linear function of the volume fraction of particles  185  in the host material  186 . According to relation (2), as the volume fraction of particles increases, dn 2 /dT approaches dn a /dT. Particles  185  can have a dn/dT that is in the same range as some of the other optical components in optical element  100 , such as face plate  120  or first lens element  130 . For example, face plate  120  may be made of glass and particles  185  may be inorganic, such as silica. Since glass and silica have comparable values of dn/dT, by increasing the volume fraction of particles  185 , dn/dT of the optical coupling material  180  can approach dn/dT of the face plate. Hence, dn/dT of the optical coupling material can be in the same range as that of the face plate. 
 
      A particular advantage of the present invention is reduced dn 2 /dT. According to one embodiment of the invention, dn 2 /dT is preferably at least 15% less than dn b /dt, more preferably at least 20% less than dn b /dt, even more preferably at least 30% less than dn b /dt, even more preferably at least 40% less than dn b /dt, and still even more preferably at least 50% less than dn b /dt. The reduction in dn/dT is preferably over a temperature range from 20° C. to 60° C., more preferably from 20° C. to 70° C., and even more preferably from 10° C. to 80° C. In general, an upper limit of volume fraction VF may be controlled by the ability of particles  185  to remain dispersed in the host material  186  whereas a lower limit of volume fraction may be established by a desired value for dn 2 /dT. According to one embodiment of the invention, volume fraction VF is preferably in the range from 10% to 80%, more preferably 10% to 60%, and even more preferably 10% to 40%. Although the values given above are in relation to volume fraction of particles  185 , same or similar preferred values may be appropriate for weight fraction of particles  185 .  
      Optical coupling material  180  is preferably fluid where by fluid it is meant any material that can flow including, but not limited to, liquids, gels and sol gels. In some applications, a solid optical coupling material may be used. Solid materials incorporating small particles have been previously described. For example, U.S. Pat. No. 6,586,096 describes a polymethylmethacrylate material with magnesium oxide particles, in the ten nm size, dispersed therein. Other examples may be found in U.S. Pat. Nos. 6,552,111; 6,441,077; 4,710,820 and 6,498,208; and U.S. patent Publication Nos. 2002/0123549 and 2002/0123550. An advantage of a fluid optical coupling material is high heat transfer. Temperature of face plate  120  can reach 70° C. or higher when CRT  110  is activated. It is desirable for optical coupling material to transfer the heat generated in face plate  120  away from the face plate to a surrounding area. For a solid optical coupling material, transfer of heat from face plate  120  to optical coupling material  180  is primarily done via conduction. In contrast, in a fluid type optical coupling material, transfer of heat from face plate  120  to optical coupling material  180  is primarily done via convection. In such a case, the convection can be natural or forced. In a forced convection, fluid flow is, at least in part, induced by an external force, such as a circulating pump. In a natural convection, fluid flow is primarily due to the properties of the fluid itself, where, for example, a cooler fluid that is farther away from the face plate displaces a warmer fluid that is closer to the face plate, or vice versa. Thermal conductivity is a measure of the ability of a material to conduct heat. In general, liquids have a higher thermal conductivity than solid polymers. For example, the thermal conductivity of polystyrene is about 0.12 Watts/mK where K is degrees Kelvin. In contrast, the thermal conductivity of ethylene glycol is about 0.26 Watts/mK, and thermal conductivity of water is about 0.6 Watts/mK. Accordingly, in general, fluids are more efficient in transferring heat generated in face plate  120  to a surrounding area.  
      A fluid optical coupling material  180  may include a fluid host material  186 . Exemplary fluid materials for optical coupling material  180  and host material  186  include ethylene glycol, alkyl diaryl alkanes, mixtures of ethylene glycol and glycerol, mixtures of ethylene glycol and water, liquids including a siloxane polymer having methyl, phenyl, and hydrophilic side groups, and liquids including mixtures of a siloxane polymer having methyl and phenyl side groups and a siloxane polymer having methyl and hydrophilic side groups. In general, the fluid material can be any suitable fluid that can be used in a desired application.  
      Fluid host material  186  preferably has a high boiling point and a low freezing point. The boiling point of host material  186  is preferably no less than 120° C., and more preferably no less than 160° C., and even more preferably no less than 200° C. The freezing point of host material  186  is preferably no greater than −20° C., and more preferably no greater than −40° C., and even more preferably no greater than −60° C.  
      Particles  185  are preferably nano-particles, although in some applications and depending on the difference between n 2  and n b , larger particles, such as micro-particles, may be used. In general, it is difficult to reduce the difference between n 2  and n b  to a sufficiently low and acceptable level for all CRT operating temperatures and wavelengths. Thus, according to one embodiment of the invention, the average size of particles  185  are preferably no more than 500 nm, more preferably no more than 100 nm, and even more preferably no more than 50 nm, and still even more preferably no more than 30 nm. Average size can be the mean or median size, or any other average that may be commonly used to characterize size of particles.  
      Face plate  120  is preferably made of any type of an optically transparent glass. Exemplary glass materials include soda lime glass, borosilicate glass, borate glass, silicate glass, oxide glass and silica glass, or any other glass material that may be suitable for use as a face plate. Face plate  120  in  FIG. 1  is shown to have a rectangular cross-section. In general, face plate  120  can have any shape cross-section. In general, face plate input face  121  and output face  122  need not have the same shape. For example, input face  121  can be curved while output face  122  can be straight or flat. As another example, output face  122  can be curved. Furthermore, face plate  120  can have optical power, thereby acting, in part, as a lens. Optical coupling material  180  can be in contact with face plate  120 . In particular, optical coupling material  180  can be in contact with output face  122  of face plate  120 . In some applications or designs, other components may be disposed between face plate  120  and optical coupling material  180 .  
      Optical element  100  may further include a coupler housing  160  for housing the optical coupling material  180 . Coupler housing  160  may be necessary where the optical coupling material includes a fluid. In such a case, the housing may include one or more sealing mechanisms, not shown in  FIG. 1 , to seal in the optical coupling material. Coupler housing  160  may primarily be designed to house optical coupling material  180 . Optical coupler housing  160  may be further designed to house additional elements, such as first lens element  130  and face plate  120 . In general, optical element  100  may include one or more housings to house the various elements and components. Coupler housing  160  may be made of metal or plastic. Exemplary metal materials include aluminum, copper, magnesium and zinc. In general, coupler housing  160  may be made of any material suitable to house the optical coupling material  180 .  
      Furthermore, optical element  100  may include a mechanism, such as a pump, not shown in  FIG. 1 , to circulate a fluid optical coupling material  180 . A circulating optical coupling material can enhance the transfer of heat from face plate  120 . In addition, optical element  100  may include an optional reservoir  190  for housing excess optical coupling material  191 , for example, to help with fluid circulation or prevent formation of bubbles in the optical path in the event of a fluid leak. Reservoir  190  may be partially full to allow expansion of fluid. An example of such an expansion chamber is described in U.S. Pat. No. 4,740,727.  
      For simplicity and without loss of generality, the first lens element  130  in  FIG. 1  is shown to have a curved cross-section. First lens element  130  may have an optical power. First lens element  130  may have field correcting properties for correcting or improving image quality across a viewing screen both on and off axis.  
      First lens element  130  may be made of plastic or glass. It is generally desirable to make a lens element in a CRT projection system, when possible and appropriate, out of plastic rather than glass. Plastic lenses are more cost effective. Furthermore, plastic lenses may be molded to have, for example, an aspherical shape. The use of aspherical lenses can generally reduce the overall number of lenses required in a projection system to produce an acceptable projected image across a viewing screen. Accordingly, when possible, optical elements, such as lenses, in a CRT projection system are made of plastic. It is also desirable for the optical elements, especially the elements having an optical power, to have a low dn/dT. As such, magnifying lenses in a CRT projection system are typically made of glass which has a low dn/dT approximately in the range from 10 −5  to 5×10 −6 /° C. or less.  
      Typical plastic materials used in first lens element  130  include acrylic. Second lens element  140  and third lens element  150  may be made of glass or plastic. Optical coupling material  180  is preferably in contact with lens element  130 . In particular, optical coupling material  180  is preferably in contact with the input face  131  of first lens element  130 . In some applications and/or designs, other optical components may be disposed between the optical coupling material  180  and first lens element  130 .  
      Optical element  100  may include other components that for simplicity and without loss of generality are not shown in  FIG. 1 . For example, optical element  100  can include one or more layers of phosphor disposed, for example, on input face  121  of face plate  120 . As another example, optical element  100  can include mounts for supporting and keeping various elements in place, and additional lens elements.  
      Indices of refraction described in the invention may be measured at a particular wavelength of interest. For example, indices of refraction may be measured at a Sodium D line (approximately 590 nm), or at a desired laser wavelength, such as 633 nm (HeNe laser). Indices of refraction may be measured at one or more CRT primary emission lines, such as red (e.g., 624 nm), green (e.g., 544 nm), or blue (e.g., 455 nm). Indices of refraction may also be average values, for example, over the visible region. The visible region may include the range from 420 to 650 nm. In such a case, index may first be measured at several wavelengths (for example, CRT primary emission lines). The measured values may then be fit to a formula, for example, a Sellmeier dispersion formula, to generate an index versus wavelength dispersion curve. Next, an average for the index may be determined by, for example, calculating the area under the dispersion curve in a desired wavelength range, such as the visible range, and dividing the calculated area by the wavelength range.  
      Particles  185  may be organic or inorganic or combinations thereof. Particles  185  can include oxides, fluorides, and sulfides. For example, particles  185  may include silica, alumina, titania, ceria, zirconia, yttria and zinc oxide. Particles  185  preferably have low optical absorption in the wavelength region of interest, such as the visible region. Some particle materials such as silica can have little or no optical absorption in the visible range. Some other particles, such as titania, can slightly absorb in the visible. Still some other particles, such as ceria, can have high optical absorbance especially in the blue region of the spectrum. Optical transmittance of particles  185  is preferably greater than 90% in the visible region, more preferably greater than 98%, and even more preferably greater than 99.5%. It will be appreciated that a particle that absorbs light, for example, in the blue, but not in the red, may be used with a red CRT without adversely affecting image properties, such as brightness and contrast.  
      Particles  185  may have any shape. Particles  185  may have a random shape or a regular shape. Particles  185  may be oriented relative to a given direction or have a random orientation. Particles  185  may have a narrow size distribution or a large size distribution. Narrow size distribution generally refers to a particle size distribution that centers around a well-defined prominent peak. In contrast, a large or broad size distribution generally means that particle size distribution does not include a well-defined prominent peak, or that particle size distribution includes multiple peaks. Generally, as average particle size increases a narrower particle size distribution is preferred to reduce haze and optical scattering.  
      Particles  185  may include more than one type particles. For example, particles  185  may include particles of two different types A and B. Particles A may be primarily designed to reduce dn/dT of the optical coupling material  180 . Particles B may be primarily designed to increase the index of refraction of the optical coupling material  180 . In general, particles  185  may include one or more types of particles where different particles types may be primarily designed to meet different requirements.  
       FIG. 2  illustrates a schematic side-view of a projection display system  200  according to one embodiment of the invention. In the specification, a same reference numeral used in multiple figures refers to same or similar elements having same or similar properties and functionalities. Projection display system  200  includes an optical element  100  and a viewing screen  220 . In a front projection display system, a viewer may be located on the same side of screen  220  as the optical element  100 , such as position  240 . In a rear projection display system, a viewer may be located on the opposite side of viewing screen  220 , such as position  230 .  
      Optical element  100  in  FIG. 2  is similar to the optical element  100  described in reference to  FIG. 1 . In particular, optical element  100  includes a CRT image source  110  for forming an image. Optical element  100  further includes a face plate  120  having an index of refraction n 1 , and a lens element  130  facing face plate  120  and having an index of refraction n 3 . Lens element  130  magnifies and/or projects the image formed by the CRT image source  110 . Optical element  100  may include additional lens elements to magnify and/or project an image formed by the CRT image source  110 . Face plate  120  and lens element  130  can be arranged in spaced-apart opposing positions defining a coupler cavity  183 . Optical element  100  further includes an optical coupling material  180 , having an index of refraction n 2 , disposed between face plate  120  and lens element  130 . Optical coupling material  180  includes small particles, for example, nano-particles, dispersed in a host material. The host material is preferably fluid although in some applications, the host material may be solid. A coupling fluid may be dispersed within the coupler cavity  183 . In one embodiment of the invention, n 2  is no lower than the lowest and no higher than the highest of n 1  and n 3 .  
      Viewing screen  220  receives a source image magnified and/or projected by lens element  130  and other lens elements that may be included in the optical element  100 . Projection system  200  may have a single CRT-based optical element  100 . In such a case, optical element  100  may be designed to generate a color image. In some applications, optical element  100  may be designed to generate a monochromatic image that is magnified and projected onto viewing screen  220 . Projection display system  200  may include more than one CRT image source. For example, in addition to the optical element  100 , projection system  200  may include optical elements  201  and  202 , where optical elements  201  and  202  may be similar to optical element  100 . In general, each of optical elements  110 ,  201  and  202  may include different lens and/or other elements including different optical coupling materials. In a three CRT projection display system, each of optical elements  100 ,  201  and  202  may be designed to generate a same image in a different primary color. For example, optical element  202  may generate, magnify and project a red image; optical element  100  may generate, magnify and project a green image; and optical element  201  may generate, magnify and project a blue image. The three projected images may be superimposed on the viewing screen  220  resulting in a color image.  
      In a front projection system  200 , viewing screen  220  can be substantially optically reflective. In a rear projection system  200 , viewing screen  220  can be substantially optically transmissive.  
       FIG. 3  illustrates a schematic side-view of an optical element  300  in accordance with another embodiment of the present invention. For ease of illustration and without loss of generality, some of the elements shown in  FIG. 1  are not reproduced in  FIG. 3 . Optical element  300  includes a CRT  110 , a CRT face plate  120 , and a projection lens system  370 . Projection lens system  370  includes a first lens element  130 , a second lens element  140 , a third lens element  150 , and a fourth lens element  155 . Fourth lens element  155  is typically an output lens, primarily designed to correct image aberrations, and is often referred to as an “A” lens. Second lens element  140  and third lens element  150 , often referred to as “B” lenses, typically have optical power and are primarily designed to magnify an image produced by CRT  110 . For example, third lens element  150  is often referred to as a “B1” lens and second lens element  140  is often referred to as a “B2” lens. First lens element  130  is often referred to as a “C” lens or a “C-shell” lens and is primarily designed to optically couple to CRT  110 . An example of lens elements used in a CRT projection system may be found in U.S. Pat. No. 4,776,681; and U.S. Publication No. 2003/0071929.  
      Projection lens system  370  further includes a first gap region  310  disposed between the first and second lens element, a second gap region  320  disposed between the second and third lens elements, and a third gap region  330  disposed between the third and fourth lens elements. Optical element  300  further includes a mount  340  for housing the lens elements, for example, second, third and fourth lens elements, and for keeping the same in place.  
      Any one or more of the first, second, third and fourth lens elements can be made of glass or plastic, although generally, first lens element  130 , second lens element  140 , and fourth lens element  155  are made of plastic, and third lens element  150  is made of glass.  
      Third lens element  150  is generally designed to magnify an image, formed by CRT  110 , for display onto a viewing screen (not shown in  FIG. 3 ). As such, third lens element  150  is preferably made of a material with a low dn/dT, such as glass. First lens element  130 , second lens element  140 , and fourth lens element  155  are generally designed to improve properties of an image projected onto a viewing careen. Such image properties include contrast, resolution, and brightness. Each of the first, second, and third gap regions is generally an air gap, meaning that the region includes mostly air, although each of the gap regions may include a material other than air, such as a fluid material or a solid material.  
      According to one particular embodiment of the invention, first lens element  130  faces the face plate  120  defining a coupler cavity  380 . Optical element  300  further includes an optical coupling material  180  disposed between face plate  120  and lens element  130  within the coupler cavity  380 . Optical coupling material  180  includes small particles  185 , for example, nano-particles, dispersed in a host material  186 . The host material is preferably fluid although in some applications, the host material may be solid. A coupling host fluid material  186  may be dispersed within the coupler cavity  380 .  
      Advantages and embodiments of the present invention are further illustrated by the following examples. The particular materials, amounts and dimensions recited in these examples, as well as other conditions and details, should not be construed to unduly limit the present invention.  
     EXAMPLE 1  
      Silica particles were dispersed in propylene glycol coupling fluid. Mean particle size was 20 nm. Particles had an index of refraction of 1.46 measured at about 590 nm (a sodium D line). Index of refraction of propylene glycol was 1.432 at 590 nm at 20° C. Particle loading was 60% by weight. dn/dT of silica filled propylene glycol was about ½ of propylene glycol with no particles.  
      The silica filled propylene glycol had a haze of about 1.5%. Propylene glycol without particles had a haze of about 0.2%. Haze was measured using a Colorquest XE spectrophotometer from Hunterlab.  
      Next, thermal drift of a projection system was measured using propylene glycol with and ethylene glycol without particles as an optical coupling material. A CRT image source, similar to the optical element  100  of  FIG. 1 , with primary emission wavelength in the green (550 nm), was used to magnify and project an optical target onto a projection screen. The target included an opaque square bordering a clear square, sometimes referred to as an edge target.  
      First, an image of the target was projected onto the screen using propylene glycol without particles as an optical coupling material. The position of the source was determined for sharpest image at the screen at two different temperatures: 20° C. and 60° C. Thermal drift was defined as the shift in the source position for the two temperatures. Thermal drift was determined for a projected horizontal edge and a projected vertical edge. Furthermore, thermal drift was determined on-axis, at the 50% point (half way point between the image center and the image edge), and the 90% point (90% from the image center, 10% from the image edge). Next, the same experiments were repeated using the propylene glycol solution with particles dispersed therein as an optical coupling material. The resulting values for the projected vertical edge are given in Table 1 below. Similar values for the projected horizontal image are given in Table 2 below:  
                           TABLE 1                                      Thermal drift (mm)               (Vertical Edge)                                 Solution without   Solution with               particles   particles   % Change                                                 On-axis   0.29   0.22   24           50%   0.46   0.42   13           90%   0.85   0.89   −5                      
 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                   
               
               
                   
                 Thermal drift (mm) 
                   
               
               
                   
                 (Horizontal Edge) 
               
            
           
           
               
               
               
               
            
               
                   
                 Solution without 
                 Solution with 
                   
               
               
                   
                 particles 
                 particles 
                 % Change 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 On-axis 
                 0.29 
                 0.22 
                 24 
               
               
                   
                 50% 
                 0.35 
                 0.32 
                 14 
               
               
                   
                 90% 
                 0.53 
                 0.49 
                 8 
               
               
                   
                   
               
            
           
         
       
     
      Measured values listed in Tables 1 and 2 indicate that addition of nano-particles to propylene glycol can substantially reduce thermal drift.  
     EXAMPLE 2  
      Silica particles were dispersed in an ethylene glycol coupling material fluid. Average size of silica particles was 20 nm. The particles had an index of refraction 1.433 at about 590 nm (a sodium D line). Index of ethylene glycol was 1.431 at 590 nm. Particle loading was 30% by weight. The measured value of dn/dT of silica filled ethylene glycol was −2.3×10 −4 . The dn/dT of ethylene glycol without particles was −2.7×10 −4 . The addition of particles reduced dn/dT by about 14.8%.  
      All patents, patent applications, and other publications cited above are incorporated by reference into this document as if reproduced in full. While specific examples of the invention are described in detail above to facilitate explanation of various aspects of the invention, it should be understood that the intention is not to limit the invention to the specifics of the examples. Rather, the intention is to cover all modifications, embodiments, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.