Surface relief holography for use in display screens.
R.L Shie, C.W Chau, J.M Lerner
ABSTRACT
Holographic surface relief screens for use in either front or rear projection TV systems offer significant advantages due to light weight, easily sculpted viewing area, high contrast and gain. Produced by the holographic construction of randomized refractive structures, there is no possibility of more effects and due to the micron size features are perfectly adapted for very high resolution. When operating in white light there is no color frigning. Reflective and transmission screens have been produced with application specific viewing angles ranging from “Privacy” screens with 20deg circular to 100deg x 30deg elliptical. Reflection screens have been produced up to 12ft x 9ft by tiling smaller holograms with near invisible seams. Work is underway to produce single component rear projection screens with full “Frensel” correction and contrast control.
1. SCREEN FORMATS
Screens comes in various formats, for example, the screen on a laptop or TV monitor is for “Direct” viewing and utilizes a CRT, LCD or even an LED panel, whereas a “ Big screen” TV or other projection system can be formatted for use with a screen to provide either front or rear viewing. Front surface screens are found in cinemas or conference rooms and receive a projected image. Rear projection transmits the image through a rear surface screen with the viewer on the opposite side.
Front projection screens may be comprised of a convenient wall to sophisticated “High brightness” structures that are designed to limit the returning image to a specified horizontal and vertical direction by use of a glass beads and other materials. A normal, white pull down screen returns light in a random manner and is referred to as a Albertan scatterer with the brightness of the image appearing the same, regardless of the location of the viewer. A white screen is typically used as a reference point so that if a screen is able to return an image in a more spatially selective manner then the screens will appear brighter and is said to have “gain”. A white screen has a gain of 1.
The overall brightness of the image is dependent on the ability of the screen to send what light is available to the viewer and not to the surrounding areas where no viewer will be.
The need for application specific screens is obvious; a screen for use in an ATM machine should preferably distribute light exclusively to the individual viewer and could be referred to as a “Security” screen. The same may apply to a computer screen or cockpit environment. As we all know, however, security screens are rarely deliberately equipped on either ATMs or computer.
If a screen is to view in bright ambient light, the gain of the screen may be a critical component in the competition for enough contrast and definition to clearly see the image. It can be said that a Lambertian scatterer will provide the poorest performance in a high ambient light environment which explains why most slide or transparency presentations are made in a darkened room.
The key to optimum screen presentation is governed by the ability of the screen to send the image exclusively into a vertical and horizontal volume in which a viewer may be located. In case of a direct view screen such as an LCD the ability of a backlight to send light efficiently through the LCD will determine the brightness of the image. In all cases the degree of immunity to the onslaught of high ambient light is critical.
2. SCREEN TYPES
The ubiquitous white screen found in every conference room represents by far the most typical front projection format. Rear projection screens, however, as found on “Big screen” TVs make much effort to send light into a proscribed output pattern. To achieve this, sheets of thermo plastic are embossed on both sides with micro-lens arrays. There are a number of drawbacks to this method in that the arrays are always periodic structures that frequently form more patterns (seen as wavy dark bands) as a result of interference with the spatial frequency of the projection pixels. The size of the micro-lenses can vary up to 500 micrometers (micron) and are functionally macro compared to the pixel size of many LCD projection systems (5 micron and greater). This can be a major resolution issue with an HDTV system. Worst of all, however, the output distribution is rarely optimal in both horizontal and vertical directions.
In order to enhance contrast micro-lens arrays are often “painted” with black stripes to reduce the effect of light passing through the array from the projector or from ambient light in the room. The location of the stripes ensures high transmission efficiency of the image signal, however, they are also clearly visible and to many people, annoying. Many commercial screens that use arrays cannot use tinting of the substrate material due to scattering concerns in the array itself.
3. REAR PROJECTION TV GEOMETRY BASICS
Because a TV system in rear projection mode should ideally have a thin profile rather than the traditional bulky models over the last few years, the issue of the numerical aperture of the projection cone becomes a source of concern.
For example, table 1 below illustrates how the effective of the projection system changes as a function of the focal length and the screen diagonal.
Screen ratio 4:3 Diagonal 32” 42” 52” 60” 18” 35deg x 28deg 43deg x 35deg 49deg x 41deg 53deg x 45deg Focal 24” 28deg x 22deg 35deg x 28deg 41deg x 33deg 45deg x 37deg Length 50” 14deg x 11deg 19deg x 14deg 23deg x 17deg 26deg x 20deg 80” 9deg x 7deg 12deg x 9deg 15deg x 11deg 17deg x 13deg
Table 3: Angles of incidence at the horizontal and vertical edges of the screen.
The big screen TV market place would benefit from a package that has a slim profile and light weight. An 18” focal length would enable a theoretical minimum box thickness of only 9” assuming a 45deg turning mirror. If the screen diagonal were 60” and either 4:3 or 16:9 ratio then the frensel lens would have to bring a horizontal angle of incidence, at the centre of a horizontal peripheral edge of between 53deg and 55deg, to near zero degrees before ritting the screen responsible for redistribution to the viewer.
In any event many existing screen systems end up with two elements, a plate of rigid Plexiglas frensel lens followed by a plate containing the micro-lens arrays. The total weight of this combination can be considerable.
4. HOLOGRAPHIC DIFFUSER SCREENS
Present day requirements go beyond the more traditional needs of projection systems. Use in bright ambient conditions, low cost, and high resolution puts a considerable burden on the traditional approaches described above.
Instead of lambertian scattering screens, POC has evaluated holographic solutions to address the issues described. Historically, holographic diffusers have been produced in a volume format either by sandwiching dichromate gelatin between plates of glass or by using flexible polymer films such as that produced by Polaroid, DuPont and other companies. Using these methods, incoming light can be adequately sculpted into the required horizontal and vertical energy distribution; however, weight, cost and size are major limitations. In addition it is very difficult to produce diffuser screen resistant to high ambient light conditions.
Screen ratio 4:3 Diagonal 32” 42” 52” 60” 18” 0.72 0.55 0.44 0.41 Focal 24” 0.96 0.73 0.59 0.51 Length 50” 2.00 1.52 1.23 1.07 80” 3.20 2.44 1.97 1.71
Screen ratio 16:9 Diagonal 32” 42” 52” 60” 18” 0.76 0.58 0.47 0.41 Focal 24” 1.02 0.77 0.63 0.54 Length 50” 2.12 1.61 1.30 1.13 80” 3.39 2.58 2.09 1.81
Table 1. Relationship between f/#, focal length and diagonal. (The f/# was calculated by equating the area of the screen to a circle, and taking the ratio of the focal length to the effective diameter).
The viewer, at best, is located at the center of the screen; however, however the angle of incidence at the periphery of the screen as seen in tables 2 and 3 can be up to 55deg. In order for the viewer to see a uniform, bright, clean picture it is necessary to bend these edge rays towards the viewer. The screen actually demands that all rays emerge from the screen in precisely the same orientation and energy distribution. Present day solutions address this issue by using a Frensel lens to first capture each ray and then collimate them through the micro-lens screen structure to enable redistribution into the correct viewing angles.
It is safe to say that for a 4:3 and especially for 16:9 ratio screens, with a very short focal length, the task for the Frensel lens is daunting.
Screen ratio 16:9 Diagonal 32” 42” 52” 60” 18” 38deg x 24deg 45deg x 30deg 52deg x 35deg 55deg x 39deg Focal 24” 30deg x 18deg 37deg x 23deg 43deg x 28deg 47deg x 32deg Length 50” 16deg x 9deg 20deg x 12deg 24deg x 14deg 28deg x 16deg 80” 10deg x 6deg 13deg x 7deg 16deg x 9deg 18deg x 10deg
Table 2: Angles of incidence at the horizontal and vertical edges of the screen.
POC has approached the problem from an alternate direction, rejecting volume holographic solutions in favor of surface relief holography. Screens produced in these way posses a strictly random, refractive rather than diffractive structure with a grain size around 5 microns. It is functionally impossible to create a moiré effect either in front or rear projection regardless of the projectors pixel size or frequency. The method used in the recording of hologram ensures that there is no diffraction so that the screen works perfectly in white or monochromatic light.
The effects of high ambient light conditions can be minimized by the correct choice of output characteristics. A sculpted horizontal and vertical dispersion for a specified location of the light source (35mm slide, LCD, DMD projection) makes it very difficult for rays not coming from the projector to find their way to the volume of space where a viewer may be located. In rear projection the holographic diffuser also acts very much like a “moth eye” anti reflection coating substantially reducing Frensel reflection and improving throughput. In fact materials such as acrylic and polycarbonate actually improve in transmission efficiency when a holographic surface relief diffuser surface is present due to greatly reduced Frensel reflection.
POC surface relief screens, referred to as a lights-on reflection screens (LORS) for front projection and light shaper viewing screen (LSVS) for rear projection may be mass produced either by embossing or by thin film “printing”. In both cases very large screen may be produced in a light weight, highly cost competitive manner.
Using this technique both front and rear projection screens have been produced with finely sculpted output characteristics. For example, security screens have been produced with only 20deg circular output, see figure 2 to conference screens with 100deg horizontal and 30deg vertical, and see figure 3. Gains of up to 10 have been achieved with very high contrast and insensitivity to ambient light. To further enhance contrast, successfully utilized neutral density tinting of thermo-plastics while maintaining high transmission efficiency. At the present time holographic masters (up to 15” x 15”) are produced in a photopolymer, and then tiled with “hidden seams” for mass production. So far, two 12ft x 9ft reflection screens have been produced with good uniformity and high gain. It is sure however, that each of these parameters will be further improved in future versions. The seam issue in transmission is more troublesome so efforts are underway to produce very large master holograms up to 80” diagonal and 16:9 ratios.
It has also been demonstrated that both LORS and LSVS can be mass produced using a propitiatory form of thin film “Printing”. This technique literally uses a modified printing press to bring the hologram onto plastic material such as polycarbonate or polyester film (typically less than 0.015” thick). The holographic image is transferred at high speed onto the film and the screen itself ends up on roll the user can cut to size as required.
5. HOLOGRAPHIC SURFACE RELIEF VS MICRO-LENS/FRENSEL SCREENS
The fact that many of the micro- lens arrays are two sided makes it is near possible to combine them with a Frensel lens to form a single screen unit. Surface relief holographic screens, however offer some notable advantages:
v LSVS screens may be incorporated directly onto a Frensel lens producing a one piece solution. v Contrast enhancement may be achieved by tinting and substrate plastic, thereby, eliminating the annoying black stripes found on many micro-lens screens. v LSVS screens may be produced with viewing angles to fit almost any need including wide angle viewing for private viewing. v Micron size surface structures are well suited to HDTV and other high resolutions applications. v Random structures cannot produce moiré effects.
Future LSVS screens may incorporate “Diffusions” into the screen to enable the Frensel lens component to be eliminated. The hologram itself would then take care of both focusing and collimation. Prototype small screens exhibiting this effect have been successfully produced but, because of the size of some of the larger screens, trial implementation is planned for the future.
The net result of these efforts should enable the production of extremely light weight, inexpensive screens with good contrast, and well sculpted output characteristics in a variety of formats ranging from rigid to thin flexible film.
6. HOLOGRAPHICAL OPTICAL ELEMENTS IN PROJECTION SYSTEMS
6.1. Circular to rectangular Surface relief holography not only makes good projection screens but elsewhere in the projector system there are also opportunities for significant energy conservation and maximization. Most light valves devices, LCD or otherwise, are rectangular whereas most beams of light illuminating them are circular. The geometric mismatch may be effectively addressed by the use of a holographic beam shaper that takes a circular beam and transforms it into a rectangular beam. It is now possible to project light that fills the corners of the LCD, 35mm slide or whatever. The transmission efficiency depends on the angles required but typically range from 85% to 95%. 6.2. High heat homogenizers for projection systems The light source in most projection systems may be a filament lamp or arc as well as more exotic light sources. In most cases, lower, there is structure that degrades performance. POC developed a range of light shaping diffusers (LSD) that will efficiently destructure a beam of light regardless of light source. Present manufacturing methods utilize thermo plastics; consequently, it is not possible to put them near a very hot lamp. Initial tests molding the LSD diffusers in sol gel are proving most encouraging. Sol gel is actually a hydrated silicate monomer with syrup like consistency. on pure quartz. If everything works well the LCD is transferred into the how solid glass and may be heated just as any glass may be heated. The resulting parts could be incorporated into a hot light source either for simple homogenization or to convert a circular beams to rectangular. Test pieces up to an inch in diameter, have been demonstrated and work in underway to develop the process for large parts.
6.3 LCD backlights Surface relief diffusers also lend themselves to LCD backlight illumination by producing that will accept light either from producing a high transmission diffuser that will accept light either from edge lit or serpentine fluorescent tubes. Either way light must be captured and projected through LCD. Figure XX compares some methods against an LCD. While perfection is exclusive the LSD offers a good less expensive solution to some existing methods.
7. CONCLUSION Holographic surface relief diffusers offer a powerful tool in the design and implementation of both front and rear projection screens. The viewing angles can be sculpted to any application ranging from privacy screens with only a few degrees of angular divergence to conference screens with wide angle horizontal and narrow angle vertical viewing. Random micron size surface structures cannot produce moiré effects and are perfect for high resolution applications especially when pixel sizes are small. Not only are LCD homogenizers well suited to screen applications but also provide excellent high efficiency diffusion elements in LCD backlights and projection system sources.
8. ACKNOWLEDGEMENTS This work was sponsored in part by NIST grant 70NANB4H1527.
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