Rear-projection TVs will continue to dominate TV sales above 50 in. on the diagonal, but their market share will dwindle if action is not taken to produce thin RPTVs – a task that is now both technically feasible and cost effective.

by Arthur L. Berman and Matthew S. Brennesholtz

CONSUMERS SHOPPING for large-screen TVs base their product selections on a number of factors. Specifically, three criteria – excluding the aesthetics of the industrial design and the cachet associated with certain brand names – dominate the purchase decision once the desired screen size has been determined: cost, visual performance, and physical form factor. In the market for all screen diagonals greater than about 42 in., these factors hold the key in the competition between flat-panel TVs, such as liquid-crystal-display (LCD) and plasma-display-panel (PDP) TVs, and microdisplay-based rear-projection TVs (RPTVs).

How do RPTVs stack up against the flat-TV technologies [grouped together as flat-panel displays (FPDs) for the purpose of this article] in these three criteria? At the low end of this size range, the prices of the two categories are roughly comparable. As the size increases, the RPTVs become progressively less expensive than the corresponding FPDs. The pricing trend is illustrated in Fig. 1¹where the prices of LCD and PDP TVs are compared to microdisplay-based rear-projection TV (MD-RPTV or MDTV) as a function of screen diagonal.

In large-screen TVs, price is an important advantage for RPTVs. The image quality of all flat-panel and RPTV display products is generally very good and is a roughly neutral factor in the comparison. Consumers clearly prefer TVs with a form factor that includes a thin depth, thin bezel, and minimal chin and forehead. Form factor has been a big advantage for FPDs as compared to RPTVs. In fact, form factor has always played a role in RPTV, and some of the initial success of MD-RPTV over CRT-based RPTV was due to the thinner form factor of even normal-depth MD-RPTV compared to CRT RPTV.

Fig. 1: The street price of TVs as a function of screen size.

Due to the central importance of thinning to the future prospects of RPTVs, this article will review some of the technological and business aspects of the issue. A good starting point for this discussion is to identify just what defines a thin RPTV. Our analysis leads to two related but slightly different types of answers to this question. The first is based on absolute enclosure depth. By utilizing this definition:

• A "normal" RPTV has an enclosure depth greater than 11 in. and is intended to be free-standing.
• A "slim" RPTV has an enclosure depth of 8–11 in. and is intended to fit on a bookshelf.
• A "thin" RPTV can be hung on the wall and has an enclosure depth of less than 8 in.

The second definition is based on the ratio of screen diagonal D to enclosure depth d:

• Normal RPTVs have a D/d ratio between 2 and 4.
• Slim RPTVs have a D/d ratio between 4 and 6.
• Thin RPTVs have a D/d ratio greater than 6.

Based on these definitions, Fig. 2 offers a summary of current and previously available slim and thin RPTV product offerings. Although there are a few slim RPTVs on the market, there are no thin RPTVs currently available. This can be attributed to the cost and difficulty that has existed in the production of thin RPTVs.

This raises several questions: What design features are required to produce thin RPTVs? What does it cost to manufacture thinner RPTVs? Finally, what will be the consumer demand for thin RPTV and will consumers be willing to pay the premium required for a slim or thin MD-RPTV?

Thinning Technologies

During the course of the past 5 years or so, there have been many approaches developed to produce thin RPTVs that have succeeded technically and commercially to varying extents. Moving forward, the most promising technical approach is seen to be one that combines several known elements.

The principle features of the proposed approach include an optical system based on a wide-field-of-view projection lens. The lens can be all refractive, although typically it would have one or two aspheric mirrors to help achieve the very wide field of view at reasonable cost. Other likely features of the proposed thin RPTV are the need to offset the image in the lens and/or to orient the projection axis of the light engine upwards. A very large number of optical configurations have been developed that incorporate these features.

Four designs of progressive complexity are illustrated in Fig. 3. In the Peterson design at the upper left, all the power is in refractive elements and the mirrors are flat. This design requires 18 optical elements in the lens, some of them quite large and deeply curved. This is unlikely to be cost-competitive with designs where much of the power is in one or more curved aspherical mirrors. The second design from Sony is similar to a normal RPTV except the projection lens is made to be very wide angle through the use of a single aspheric mirror. While this is a cost-effective approach, it is probably mostly suited for slim RPTV and not thin. The third design from Sanyo actually involves four curved mirrors plus a flat mirror. These mirrors can completely replace the refractive projection lens. The final configuration, proposed by InFocus, incorporates a special screen. Although likely to be expensive, this type of screen design adds additional thinning capability and is included in several proposed designs.

Less of an option for the thin RPTV designer is the need for a special Fresnel lens, one designed specifically for light with an oblique angle of incidence. Conventional Fresnel lenses typically cannot be used when the angle of incidence becomes much more than 45°.²Fresnel lenses for these large angles of incidences typically use total internal reflection (TIR) at the lens instead of conventional Fresnel facets. In most cases, these Fresnel lenses are then used with conventional RPTV screens.

Required properties of a projection lens for a thin or slim RPTV include:

1. The MTF required for a 1080p system,
2. A low f/# for use with a UHP-type lamp, 
3. Little geometric distortion, 
4. Lateral color less than one pixel,
5. A very wide field angle.

These lenses are likely to be much too expensive for the cost-competitive RPTV market, even when they depend on aspherical mirrors to provide much of the optical power.

Among these required properties, lateral color may need a bit of explanation. Lateral color is the projection lens chromatic aberration where the magnifications of red, green, and blue light are slightly different. When a projection lens has lateral color, each of the three colors can be in sharp focus, but the images near the edge of the screen do not lie directly on top of each other. To a casual inspector who only examines one corner of the screen, lateral color looks like a mis-convergence problem. To avoid this appearance of mis-convergence, ideally it is necessary for lateral color in the projection lens system to be less than 1 pixel at the screen, or less than about 0.05% magnification difference between red, green, and blue for a 1080-line 16:9 image. Realistically, two pixels of lateral color for a 0.1% magnification difference would be unnoticed by all but the most critical viewers.

While it is possible to design and build projection lenses that have all these required properties, the cost is likely to be very high. If the geometric distortion and lateral color requirements are relaxed, but if the MTF requirement is maintained, the lens can be manufactured at a much lower cost. The distortion and lateral color can then be corrected in the video signal.³ Even allowing 1% geometric distortion and 10 pixels of lateral color can dramatically reduce the cost of the projection lens, more than enough to compensate for the extra image-processing required.

In the current generation of rear-projection TVs, the dominant light source by far is the UHP-type mercury short-arc-type lamp. It is likely that the light source in future RPTVs will transition first to light-emitting diodes (LEDs) and eventually to lasers. Lasers will be especially important in slim and thin RPTV because they can be used with much higher f/# projection lenses and other optics, another major cost savings. For example, current RPTV sets typically have projection lenses that are in the range of f/2–f/2.4. Laser sets, on the other hand, can use f/9 or higher projection lenses. LEDs have étendues comparable to or higher than a UHP-type lamp so they do not provide this f/# cost advantage of lasers. Therefore, LEDs are most suitable for normal-depth systems.

The wide-angle projection lenses needed for thin and slim RPTV are somewhat simpler to design and manufacture if the lens needs only a short back focal length.⁴

Microdisplay technologies that require only one fold-mirror or prism between the projection lens and the microdisplay are therefore somewhat easier to use in thin and slim RPTV. Technologies that need only one fold-mirror or prism include single-panel DLP and three-panel LCD systems along with some less-common technologies such as single-panel LCoS. Three-panel LCoS and two- or three-panel DLP systems require two fold-mirrors or prisms in the path between the microdisplay and the projection lens, making the projection lens design more complex.

Cost to Make Thinner RPTVs

In order to estimate the additional manufacturing cost of slim and thin RPTV sets, Insight Media has made an estimate of the Bill of Materials (BOM) cost for various designs. The results of this BOM analysis for normal and slim RPTV sets are shown in Fig. 4. A similar analysis was conducted on thin UHP-based RPTVs as well as normal, slim, and thin laser-based RPTVs. As might be expected, the product of the analysis is a sensitive function of the underlying assumptions. Some of the principal assumptions are as follows:

• All sets are 61 in. on the diagonal with a single-panel 1080p smooth-picture DLP microdisplay.
• All BOM costs are as of the end-of-year for the year listed.
• Sets are manufactured by a high-volume first-tier manufacturer receiving best prices from all suppliers.
• All RPTVs have similar basic features.
• The BOM includes electronic geometry correction for slim and thin sets.
• The laser prices used for the analysis of laser-based RPTV are based on targets originally proposed by Novalux, with price declines forecast by Insight Media.⁵

The assumption that all sets will have only basic features is unrealistic in one sense. Thin, slim, and laser RPTVs are likely to be introduced as premium products with premium features. This assumption was made so that it would be possible to evaluate only the added cost of slim, thin, and lasers, without confusion added by different sets of features.

Due to the added complexity of the long back-focal-length projection lens, micro-display technologies that require two fold-mirrors or prisms between the projection lens and the microdisplays are likely to have a higher-cost projection lens compared to the short back-focal-length lenses used with technologies that require only one fold-mirror or prism. The technologies that require two fold-mirrors or prisms are normally considered premium technologies and already have a cost premium, even for a normal-depth RPTV set. The additional cost premium for a long back-focal-length projection lens for slim and thin sets compared to that for normal-depth sets is not expected to be significant.

Six designs were considered in the full analysis.⁶ The results for two designs are summarized in Fig. 4 in which the standard of reference for price comparisons is the UHP-based normal-depth RPTV for each forecast year. Through the end of the forecast period, the lowest priced set is expected to remain the normal-depth RPTV with UHP-type illumination. While initially there is a relatively large premium for a laser-based RPTV, by the end of the forecast period the premium for a normal-depth laser set is low and there may be no premium at all.

The premiums for slim, thin, and laser systems compared to a normal-depth UHP-based RPTV are all below or only slightly above the 29% premium the consumer historically has been willing to pay for other new and desirable features.⁷ Therefore, it can be expected that the consumer will be willing to pay the extra premium for slim, thin, and laser RPTV. Even with the premium for slim and thin, RPTV sizes larger than 50 in. are expected to continue to have a significant street-price advantage over LCDs and PDPs.

Market Analysis for Thin RPTVs

The forecast shown in Fig. 5 is offered for the penetration of slim and thin RPTVs into the available RPTV market as a function of time.

Further conclusions are as follows:

• RPTVs will likely lose market share below 50 in. There is little that can change this market trend.
• RPTVs will continue to dominate above 50 in., but its market share will dwindle if action is not taken to produce thin RPTVs.
• Above 50 in., RPTVs are substantially less expensive than other FPDs.

Due to its modest cost premium for slim compared to thin – about 20% for a lamp-based set and slightly more for a laser-based set – most of the slim or thin RPTV sets forecast in Fig. 5 are expected to be slim rather than thin. While the premium may be modest, RPTV does appeal after all to consumers who are looking for a bargain compared to high-priced large-screen LCD and PDP sets. Functionally, slim sets fit most places thin sets or FPDs will fit, especially if you consider the footprint of the base needed by an FPD or a thin RPTV to prevent it from tipping over.

In our forecast, slim and thin RPTVs will grow to dominate the unit sales of RPTVs. Flat-panel TVs already out-sell RPTVs in the 40- and 50-in. segments, but not in the sector larger than 60 in. It seems unlikely that slim or thin will increase the market share of RPTVs at smaller screen sizes, but it should allow the category to maintain at least flat unit sales for the next several years. In addition, thin RPTVs may attract and allow customers with a fixed dollar figure available for their HDTV to purchase a larger slim RPTV.

Exaimed from the broadest perspective, flat-panel HDTVs will remain relatively expensive for the next several years, at least compared to the TV prices consumers became accustomed to before the digital transition. Currently, the sales of the new television technologies, including HDTV, digital TV, and large-screen TVs, are disproportionately to the middle- and upper-class customer categories. In the U.S., with current requirements for ATSC tuners and the end of analog broadcasting, the sale of flat-panel HDTV receivers is spreading rapidly into the general public. This, in part, is the drive behind the dramatic price reductions in LCD TV, plasma TV, and microdisplay-based RPTV in the past few years. Slim and thin RPTVs have the potential to appeal to cost-sensitive consumers and be a profitable product category for manufacturers.

A point not to be lost is that one of the most important elements in the potential success of RPTVs is the commitment of the brands that must sell them. If the leaders in this category do not commit the development funds and the marketing dollars to educate dealers, retailers, and end-users about the benefits of slim and thin RPTVs, then the momentum that exists with flat-panel TVs is unlikely to be reversed. Our forecast calls for thin RPTVs to substantially replace regular-depth RPTVs going forward, although total RPTV unit sales will remain essentially flat over the forecast period.

If the market leaders fail to commit to slim and thin RPTV production and marketing, this may provide an opportunity for upstart companies, either existing second-tier brands or new startups, to fill the niche for thin high-image-quality large-screen low-cost displays.

References

¹Source: NPD Techworld: This chart is for the U.S. sales of 720p, WXGA, and 1024i HDTVs in 2006.
²S. Shikama, H. Suzuki, and K. Teramoto, "Optical System of Ultra-Thin Rear Projector Equipped with Refractive-Reflective Projection Optics," SID Symposium Digest Tech Papers 33, 1250–1253 (2004).
³G. Ramachandran and G. Prior, "Cost-Effective Ultra-Thin RPTVs Can Reverse Falling Market Share Relative to FPDs," SID Symposium Digest Tech Papers 38, 113–116 (2007).
⁴The back focal length of a projection lens is the distance between the last element of the lens and the microdisplay. Virtually all microdisplay light engines have either one or two fold-mirrors or prisms in this space.
⁵M. Brennesholtz, C. Chinnock, and A. Cugnini, 2007 Laser Projection Systems Report (in press).
⁶For the complete BOM analysis of all six designs, see the Insight Media report, Approaches and Prospects for Thin Rear Projection TVs. For details on this report, see http://www.insightmedia.info/reports/2007thnrptv.php
⁷According to Ross Young of DisplaySearch at the 2007 SID Business conference. •