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Portable Projection Screens for Use in Uncontrolled Lighting

Front projection systems deliver large diagonal screen sizes and lowest-cost-per-unit-area images of any display technology. A reasonably high quality �20” diagonal front projection system (Projector and screen) can be purchased for about $2500. For comparison, a �03” diagonal plasma system can cost $65,000. New generations of lightweight, low-power projectors can be used to create display systems that are also the lightest weight-per-unit-area. Lowest cost per diagonal inch and portability make projection systems the most flexible display systems for consumer video display and for business applications such as corporate meeting rooms, commercial applications, and retail signage.

With improved components and reduced costs, more front projectors are being sold and more of these projectors are being used in uncontrolled viewing environments. In order to get the best quality pictures from a projection system they must be used in a reduced illumination environment or in conjunction with a high-gain, high-contrast projection screen.

In controlled illumination settings, such as windowless corporate meeting rooms or dedicated home theater installations, it is hard to improve upon a well placed white vinyl screen in a darkened room. When a room is designed so ambient illumination does not fall on the screen and minimal levels of background lighting keep the viewing regions in a darkened state, innovations in screen development are not required�.

In any other viewing environment innovative high-gain2, high contrast screen designs are required to deliver maximum image luminance to the viewer, and reject the maximum amount of ambient light. Screens that perform both functions – increased image luminance and increased rejection of ambient illumination have not been technologically feasible until now.

In this paper we’ll describe the improvements in projection screen performance based on new large-area microstructure technology.

Introduction

Like every display technology, projected images must first be bright enough to be viewable relative to the ambient surround – whatever the size of the projected image. Then the images have to be viewable from a variety of positions, sitting or standing, on-axis or off. Finally the projected image has to have enough contrast that a viewer can retrieve all the information from the images.

The Problem: Projection Systems and Room Illumination

Donald HirshBright View Technologies, Inc.

Morrisville, NC USA

1 A screen only needs to be a good lambertian reflector to deliver an excellent image to the viewer. A simple matte-white painted wall makes an adequate front projection screen in a darkened room.2 For the purposes of this discussion, gain is defined as the ratio of luminance of a particular screen, measured on-axis, to the luminance of a lambertian white reflector under identical illumination.

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Illumination and Displays in the Home and Office

In most homes, rooms are lit with combinations of general lighting (overhead fixtures, sconces), task lighting (lamps, under-cabinet illumination) and accent lighting. Task and accent lighting dominates over general lighting. Projectors tend to be used in rooms that have more task and accent lighting – living rooms, dens, and family rooms. For the most part paint ceilings are painted shades of white that produces a slight bias to ambient illumination impinging from above. In office buildings and schools general lighting tends to prevail in the form of ceiling-mounted fluorescent units. If one were to take a light meter and walk around a variety of homes and offices one would find signals ranging from 50 lux (a darkish living room corner) to 550 lux – a bright window-lit office and all points in between. Display devices can be used in “typical” indoor settings, in an office with overhead fluorescent illumination with the occasional window and task light, or in a residence with task lights and windows. Below is a list of typical on-axis illumination for several of these devices. All of these devices are useable in, but not optimized for, daylight and overhead/task lit rooms.

By way of contrast, the maximum luminance of a projection screen is dependent on the luminance of the projector, the size of the projected image and the gain of the screen. The table below shows the illumination that will be reflected when various size projection screens are illuminated with the luminous

With dozens of products tailored for increasingly specific niches, it is difficult to generalize about current projectors. But there are a few key points that inform our analysis. Improvements in projector illumination have solved the problem of projecting a sufficiently bright image. For several classes of projector, with the basic luminance issue solved, the projection screen manufacturer can attend to the other two problems – optimizing image contrast while maintaining a wide viewing angle. A few vendors have stepped in with just such products.

Mid-luminance projectors vendors are taking two approaches. They maintain price and increase features (a great recent example: a NEC NP60: 3000 lumens, 3.5 lbs, $1299 MLP) or they maintain features and reduce prices (the increasing number of sub $�000 �200 lumen projectors speak to this trend). These mid-level projectors, when used in mixed ambient lighting will require new, innovative, high-gain high-contrast screens to deliver acceptable performance.

The advent of LED illumination for projectors has allowed for the creation of ultra-portable and even battery powered front projectors. Low-power and low(er) luminance projectors enable a new level of portability to projection systems. But even at �00 lumen projectors can’t project a viewable image on current screens. This class of device requires a high gain, high contrast screen in order to be useful (see table 3).

Table 1: Typical Luminance for Displays with display driven to full white

Impor tant Trends in Projection Systems

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Every display technology makes use of ambient rejecting techniques to improve image contrast and particularly image contrast around uncontrolled illumination. Many display technologies employ anti-reflective and/or anti-glare films to cut down on ambient reflections. RPTV screens utilize black mask area to obtain the necessary contrast. For example Bright View’s GIGASCREEN system leaves 85% of the surface area of the screen covered with a black matrix so that a viewer perceives the widest possible contrast. The remaining 15% of the screen consist of microlens apertures that manage the luminance of the projected image. Although most existing Plasma displays employ no targeted bright room contrast enhancement solutions they use a variety of films that reduce transmission and as a result improve contrast by default. Our eyes require a black appearance in display surface for us to perceive black and therefore contrast. We can’t create black from illumination; it can only be created by the absence of projected images and lowest possible reflection of ambient light. There are analogous requirements for front projection screens, which must be balanced against the very nature of the projection screen – it has to reflect the light from the projector back to the viewers.

In order to study and quantify the effects of ambient limiting structures and materials in front projection screens we developed a laboratory procedure to measure the ability of a screen to reject ambient illumination. We describe it in detail in the sidebar “The Methodology of the Ambient Rejection Ratio.” The result is a useful, objective figure of merit that can be used to characterize how well a screen performs the function of ambient minimization.

Table 2: Projection Screen Illuminance (Gain 1)3

output of projectors of varying brightness. We highlight the signals (in nits) that are between 200 and below �000 NITs, since these are signals that we can see in a lit indoor room. This table illustrates what we already know – low luminance projectors shouldn’t be used in brightly lit rooms. We will have more to say about screen illuminance and gain later in this paper.

3 The luminous output of the projector is measured in lumens (power); the lumens when delivered to a particular area are measured in lux (lumens/ m2); the reflected luminous surface of a projected image is measured in nits (lux per steradian or power/area/solid angle). Lux and nits are not interchangeable, but are related. Øv → Εv → Lv ; Lumens → Lux → Nits; lm → lx (lm/m2) → nits (lm/m2/sr), or power → power/area → power/area/solid-angle.

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Solutions Gain Increasing Technology

To improve the brightness of a projected image, projection screens can use gain to increase on-axis luminance in the same way LCD display makers us Brightness Enhancement Films to increase the luminance of their backlights�. Brightness Enhancement Films are used to compensate for a less powerful backlight, collapsing the vertical viewing angles and making a bright, but narrower viewing “cone.” Though implementations differ, the desired effect is the same – to increase on-axis gain at the expense of wide off-axis viewing angles.

For projection screens, gain becomes critically important in uncontrolled viewing environments because it can allow virtually any projector to achieve on-axis luminance that exceeds the on-axis output of most other display systems.

The Problem II: The State of Projection Screen Technology

Most projection screens are made from vinyl coated fabrics. Innovation in this class of screen material has largely been a function of advances in dyes and pigments: white, gray, with multiple layers of opaque and/or reflective coatings, including suspensions of larger particles with optical properties such as glass spheres. Other production techniques include acoustic transparency - adding perforation or woven substrates to their repertoire of fabrics and finishes. Vinyl fabric-based production techniques are used throughout the industry from high end screenmakers like Stewart Filmscreen, to nameless manufacturers producing private-labeled projection screens. These classes of screens have gains ranging from 0.7 to just north of 2 and for the most part have wide and symmetric viewing angles. Combined with sufficiently high-brightness projectors, these screens can provide adequate contrast even in an uncontrolled viewing environment. For low and mid-luminance projectors however, these screens are never optimal. The reason is because none of these screens discriminate between ambient and projected illumination. Any and every illumination striking the surface of the screen is reflected back into the room in proportion to the strength of the signal impacting the screen, and at least some portion of this light will reach viewers eyes.

4 This is especially important in portable computers where power dissipation is at a premium and backlights are one of the most significant power sinks in the system.

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Table 3: Gain-Adjusted Luminance of Projection Screens

In the same way that a laptop computer screen has a narrower vertical region to accommodate a dimmer, lower-power backlight, high-gain projection screens can harvest light from the edges of the viewing field and concentrate it in the center of the viewing field. The trade off is the image becomes dimmer as you move off axis. A sufficiently bright image is a necessary first condition. High gain screens allow us to create larger area projected images with acceptable luminance using lower luminance projectors. Using a high gain screen introduces tradeoffs in viewing angle. The general principle is to optimize and centralize image luminance that is wasted and put it where it can do some good.

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Screen vendors are experimenting with matte aluminum reflectors in conjunction with diffusive and absorptive coatings. If a screen has a “silver” rather than a grey look, chances are good it is a matte or brushed aluminum base material. Aluminum-based reflectors allow for increased gain screens (with gains as high as 4.5 in our measurements). When the aluminum is brushed it can exhibit a directional scattering effect that produces an elliptical light distribution (i.e. the vertical half-angle of the screen becomes smaller than the horizontal half-angle). However, without the introduction of structures and materials to reduce ambient artifacts, the additional gain these screens offer is of no benefit. In a room with ambient illumination, the ambient illumination that impinges the screen is reflected with increased gain too, so there is a brighter image, but no net increase in contrast from the screen.

Some vendors add glass microspheres to the mix. These microspheres act as retroreflectors, enhancing reflectivity back toward the source. With sufficient density, the microspheres offer increased gain and decreased half-angle. Since they are spherical they offer no preferential treatment of projected illumination. Thus, they too increase the gain of every signal that impinges the screen surface.

Bright View’s technology platform allows for the construction of projection screens based on precisely defined and placed astigmatic microlenses which allow for a bias in the gain of the projected image, instead of the ambient illumination.

Figure 1: One Dimensional Distribution of Luminous Flux at Various Gains

Ambient Rejecting Technology

Conventional screen makers have created contrast enhancing screens by using gray reflectors instead of white trading overall reflective efficiency for lower black level. The technique is useful in low ambient illumination settings, but creates screens with a native gain of less than �, which means the projector illumination must be that much brighter to achieve high peak luminance. Within the past two years manufacturers have introduced screens with ambient rejecting technology, presumably through the use of absorptive materials embedded in microstructures designed for preferential absorption of ambient illumination. In conjunction with our microlenses, Bright View is deploying ambient rejecting materials and structures as well. The table below is a representative list of screen materials we have evaluated as part of our ongoing research. The samples are sorted by the ARR – the higher the number, the more ambient rejecting “behavior” is exhibited by the screen.

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Table 4: Representative Screen Performance Metrics, Sorted by ARR

Based on results from a Bright View Technologies focus group study, an ARR less than 1.25 does not discriminate between ambient and projected illumination to be a useful screen for use in mixed ambient lighting.

Advances in Microstructure Technology & Manufacturability

For low-to-mid-luminance projection systems, a successful screen must supply an optimal mix of ambient rejecting absorbers, and gain enhancing materials and structures. Until quite recently the materials to solve this problem included the techniques and materials noted above: plastic and vinyl films, rigid, metallic reflectors, reflective and absorptive coatings and a few initial attempts at microlens creation through non-precision material deposition. Within the past two years, a few companies have begun to experiment with microstructure-based products. Bright View, in particular, brings a new and fundamental advantage to the design of projection screen technology: the ability to design and reproduce very precise microlens structures over very large areas. Bright View’s first technological breakthrough, the GIGASCREEN System is named for the number of microlenses in a typical 50” diagonal rear projection screen. To the material palette of the optical film design we add the proprietary ability to design and mass produce large area arrays of precision lenses (aspheric, astigmatic, cylindrical, et cetera) which allows Bright View’s film to create light distribution in ways that were previously unachievable.

The other important benefit from Bright View’s technology platform is the inherent manufacturability of the film products. All of our technology lends itself to implementation in rolled goods, continuous and semi-continuous processing, which means we are developing products that can scale to market demand.

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Bright View’s Related Work

In Bright View’s development, the company has invested enormous energy developing a generalized technology platform that can form and manufacture arbitrary microstructures. We employ this capability to develop optical films for all major display modalities, often leveraging advances in one display area to improve another area. For example, we have taken what we have learned in designing 2 dimensional optical distribution fields for rear projection television films and used it in the creation of our front projection screen. Moiré minimization and display MTF analysis provide solutions that are directly applicable to multiple film areas. We have taken what we have learned about contrast enhancement in plasma and films and similarly applied it to our work in front projection screens. Our work in LCD brightness films has benefitted from studies in projection screens.

Table 5: Bright View Product Family

Figure 2: Examples of BVT Microstructures

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Uncontrolled ambient illumination calls for new screen technology, a niche that is being filled by manufacturers with expertise in microstructure design and replication. The requirements to deliver product into this space include both high gain and high contrast screens. The requirements for anytime anyplace projection includes portability and simplicity in setting up and stowing of both screen and projector. Bright View is demonstrating preliminary products in spring, 2007 and intends to enter the market in 2008.

Solutions for Anytime Anyplace Projection

Bright View is developing a family of projection screens targeting the emerging markets for anytime anyplace projection. In addition to the performance attributes we have discussed in this paper, another important attribute is portability. This means all of our screens must be packaged for easy set up and take down. We are developing two screen types, differing largely in gain: one, the Super High Gain Screen, has a gain of about 7 and an ARR greater than 1.4. The High Gain screen has a gain of 4.5. We package these screens into 30”, 50” and 70” collapsible enclosures.

Low-Luminance Projectors

For sub-�00 lumen projectors, a useful projection screen should have a gain of greater than 6 and an ARR greater than 1.4. An image size of 30” - 50” diagonal in 4:3 or 16:9 aspect is about is about as large as these projectors are capable of supporting. In the emerging category of pico projectors, delivering on the order of 25 lumens, a 30” image is about as large as can be accommodated with high gain technology.

Business Applications & Home Theater Applications

For mid-market projectors delivering �000-3000 lumens on screen sizes from 50 – �00” can be supported in fairly bright room lighting using either a High Gain Screen or a Super High Gain Screen.

Conclusions

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Sidebar: The Methodology of the Ambient Rejection Ratio

Gain and viewing angle (or half-angle) are the two most commonly cited measures of projection screen performance. In developing our ambient rejecting technology we need an objective measure of a screens ability to give preferential treatment to on-axis illumination. There is recent precedent in contrast measurement for flat panel devices 5,6,7 and we have extended related techniques to measure devices that are only reflective rather than emissive. We have developed two ratios and a figure of merit. The first, called “diffuse reflectance source” (DS), shows how efficiently the screen distributes the projected image back into ambient surround. This is the dimensionless ratio of a luminance signal of the projection screen (numerator) divided by a luminance signal of a white standard (denominator). The second, the “diffuse reflectance ambient” (DA) is an indicator of how efficiently the screen distributes the ambient illumination back to the projection screen viewer. DA is the dimensionless ratio of illuminance signal of the projection screen (numerator) divided by the illuminance signal of a white standard (denominator). Finally, the figure of Merit, the Ambient Rejection Ratio (ARR) is defined by DS/DA. A white lambertian reflectance standard has an ARR of 1. A black body, theoretically, has an ARR of ∞. An projection screen with an ARR between 1.3 and 1.6 generally shows a reasonable tradeoff between an ability to partially reject ambient illumination without compromising the on axis illumination of the projected image.

Figure 3: Measurement Procedures for DS, DA