Digital cinema projection: choosing the right technology - Christie

Digital cinema projection: choosing the right technology - Christie

White PaperDigital cinema projection:choosing the right technologyBy: Alen Koebel

Digital cinema projectionIt’s always good to have choices.Today, when deciding on which digital cinema projection system will best serve the needs of yourbusiness and the expectations of your customers, you have more choices than ever. First, youhave a choice between projection technologies; two today and potentially more in the future. Youcan choose between two approved pixel formats (resolutions). You have a choice of technologiesfor 3D, a high value add-on for digital cinema projection. And lastly, you have a choice of vendors.Not all of these choices are independent. And it’s not always apparent which of them is right for agiven theater. To clear up the confusion, this whitepaper will discuss the relevant technologies,their fundamental differences and show how they impact the factors that are most important fortheatrical exhibition: image quality, functionality, reliability and cost of ownership.What makes a good image?What makes a motion-picture image lookgood? Technically, projected images can bedefined by a number of different parameterssuch as brightness, contrast, color space, colordepth, resolution, device refresh rate and,for motion pictures, temporal resolution. Thesubjective evaluation of an image as “good”or “poor” is based on the overall effect of allof these parameters working together.Nevertheless, resolution in particular isoften sold as the most important parameter.However, most people would agree that adim, washed-out image with poor colors, asan extreme example, wouldn’t look good nomatter how sharp. Everything else has to be inplace for resolution to make a difference. Notall projection technologies are equally adeptat getting everything right.Counting pixelsSince resolution has garnered so muchattention, let us examine it in detail. Resolutionis often equated with the pixel format ofthe display device, hence it is said that theresolution of a “2K” projector is 2048 x 1080while that of a “4K” projector is 4096 x 2160.However, a display’s pixel format is not thesame as its resolution. The pixel format tellsyou only how much information an image cancontain, whereas resolution tells you how muchof that information is actually conveyed to thescreen and can (potentially) be seen bythe audience.Resolution depends on both the projectiontechnology and on the capabilities of thehuman visual system (hereafter HVS) in atheater environment.With respect to the HVS, optometristscommonly use the Snellen eye chart (Figure 1)to determine approximate visual acuity basedon the finest line of text that can be read froma certain reference distance. Normal acuity isidentified as 20/20 (or 6/6 in metric), signifyingthat a specific line on the chart can be readat a distance of 20 feet (6 meters). The finestdetails of the letters on that line (e.g., thestrokes in the letter E) are one angular minuteof arc wide, or 1/60th of a degree. If the chartwere presented as a digital image usingdiscrete pixels, 20/20 vision would then equateto resolving 60 pixels per degree.However, “resolving” in this context meansonly that we can identify a letter made up ofpixels. To clearly see the shape of each pixel(whether it is a square or a round dot, forexample), or any separation or gap betweenadjacent pixels that may exist, would likelyrequire even greater visual acuity.This is important in the context of digitalcinema, since all currently practical projectiontechnologies use square pixels with a smallinter-pixel gap.Brightness is also a consideration. In theSnellen test the chart should be adequatelyilluminated, such that the minimum luminanceof the white background is 120 candelas persquare meter (cd/m²) [1].This ensures dilation of the eye’s pupil to about3mm, resulting in maximum acuity [2]. Thenominal luminance of peak white specified fordigital cinema images, on the other hand, isonly 48 cd/m², more commonly expressed as14 foot-lamberts (fL) [3]. Combined with theeffect of a dark surround, this results in a pupildilation in the range of 4-6mm [2]. Becauseoptical aberrations increase with lens aperture,this causes a roughly 30% drop in visual acuity,equivalent to 20/28 vision or about 42 pixelsper degree [4].However, some studies have suggested thatnormal visual acuity could be as low as 44pixels per degree, which in a darkened theaterenvironment becomes just 32 pixels perdegree [5].Note that the above numbers apply onlyto a predominantly white image containingblack text, like the Snellen chart. The averageluminance of most real images in a movie,which contain a mix of colors and gray levels,is far lower, resulting in pupil dilations at thelarge end of the range and hence, even lessacuity. Very dark scenes in particular maycause the HVS to approach or even enter the“mesopic” region, in which it is more sensitivethan normal to light, but with less colordiscrimination and with far less acuity.Looking sharpRegardless of what specific acuity numberapplies while watching a movie in a theater,it tells us only about the finest details wecan resolve. As it turns out, due to anotherproperty of the HVS the finest details donot contribute much to our perception ofsharpness. A relationship called the ContrastSensitivity Function (Figure 2) describes howdifferent spatial frequencies (the rate at whichlight and dark lines alternate) are seen withdifferent amounts of contrast. We see detailsmost clearly – that is, with the greatest1

digital cinema projectionFigure 1Typical Snellen Eye ChartFigure 2Contrast Sensitivity Functioncontrast – in a range between 4 and 9 linepairs line per degree, equivalent to between8 and 18 pixels per degree [6]. An image thatreproduces spatial frequencies in this rangewith high contrast will look sharp. Higherspatial frequencies in the image contributesignificantly less to that perception.To understand the affect of contrast sensitivity,we must examine seating distances in atheater. Despite different screen sizes,the distances of seats from the screen intoday’s stadium-seating venues vary over aremarkably similar range when expressedas multiples of screen height. The closestseats are usually a little less than one screenheight away, while the farthest are somewhatcloser than three screen heights (Figure 3).From one screen height away, the screenwill span 53 degrees of our vertical field ofview. Horizontally, it will be anywhere fromalmost 100 degrees to nearly 130 degrees,well beyond what most viewers prefer toexperience. Choosing the more demandingend of the range of spatial frequencies thatare important for sharpness – 18 pixels perdegree – results in a minimum requirementof 956 vertical pixels (pixel rows). In thehorizontal direction, a flat (1.85:1) picturewould require a minimum of 1769 pixels.(For scope, the requirements will dependon whether the screen is constant widthor constant height.) If the pixel-to-pixelcontrast is very high for the most importantspatial frequencies, an image with thesepixel dimensions or greater will look sharp.This conclusion is consistent with thesubjective experiences of countless theaterpatrons viewing DLP Cinema ® images overthe past several years. The 2K images fromthese projectors, which fit within a full-frameimage “container” of 2048 x 1080, do indeedlook sharp, even from the front row of thetheater. This wouldn’t be possible wereit not for the high performance of DLP ®technology itself, which provides the extremelyhigh pixel-to-pixel contrast required.I see pixelsWhat then of 4K? As we have seen, it isnot necessary for achieving sharp imagessince even 2K projection provides sufficientresolution, assuming the use of DLPtechnology. The primary advantage of 4K is,rather, the reduction it offers in the visibility ofpixels, an issue that is entirely different thanresolution. For certain images (the closingcredits of a movie, for example) viewers seatedin the first few rows of a theater can easilydetect the square shape of the pixels in a 2Kimage of characters as they roll up the screen.The closest seats have traditionally alwaysbeen the least desirable for most patrons,since viewing angles are overly wide and neckinclinations are severe. In the case of a 35mmpresentation, film grain is also much moreobvious. However, because of its randomnature film grain may be less objectionable toviewers than a static pixel structure.A 4K image has twice as many pixels in boththe horizontal and vertical directions as a 2Kimage, which results in four pixels in the spacea single pixel occupies in a 2K image. Sincethe pixels are half the size in each directionthey are proportionally harder to detect. Thisbecomes more important for wider screens,since the increased width of the theater placesmore seats closer to the screen.One argument made for 4K is simply that ithas far more pixels than 1080p high-definitiontelevisions. Why would patrons come to atheater to see almost the same number ofpixels they can watch at home? This onceagain incorrectly equates image qualityto a single parameter: pixel count, and itconveniently ignores the fact that digitalcinema images are far, far better in other, moreimportant respects than even the best homevideo formats (e.g., Blu-ray Disc).Compared to Blu-ray Disc, digital cinemaimages have a wider color gamut, fourtimes the color information, up to sixteentimes finer gray shade resolution and havebeen compressed far more lightly with acompression scheme that inherently createsless visible artifacts (JPEG 2000).Another essential difference between cinemaand home theater is that very few homes arelikely to have a 20-foot wide or larger screen(most are less than four feet wide). Theargument that sitting proportionally closerto a smaller screen is equivalent to viewing alarge screen in a cinema ignores the fact thatour inter-ocular distance does not change –we sense the screen’s real size and distance.2

Digital cinema projectionFigure 3Typical stadium seating auditorium where the front row is just under 1 screenheight from the screen and the last row around 3 screen heights from the screenProjection technologiesUltimately, the choice of projectiontechnology has the greatest affect onimage quality, as well as on other factorsimportant to theatrical exhibition, includingfunctionality, reliability and cost of ownership.Two projection technologies are used todayfor digital cinema: LCoS (Liquid Crystalon Silicon) and DLP. These are also usedcommonly for other projection applications,where they are joined by LCD (Liquid CrystalDisplay). LCD, however, has not measuredup to the demanding requirements ofdigital cinema and no LCD projectors arecurrently certified for playing Hollywoodstudio content. The same is true for alternatetechnologies such as GLV (Grating LightValve) and scanning laser projection, whichtoday see very limited use and are not yetcapable of the light levels required for digitalcinema at a price anywhere close to beingaffordable. DLP and LCOS projectors thatuse lasers as light sources are expected to bemore viable, however they will face some ofthe same issues discussed in this article.LCoS and DLP are fundamentally verydifferent technologies. LCoS works byimpressing a voltage proportional to adesired gray level across a very thin layerof liquid crystal material to control thepolarization of light generated from thelight source. In cinema applications the lightsource is nearly always a Xenon arc bubblelamp. In an LCoS device (Figure 4) theliquid crystal layer is sandwiched between atransparent glass electrode on one side andwhat is essentially an integrated circuit withan aluminum mirror as a top layer on theother. After passing once through theliquid crystal layer, the light reflects off themirror and passes through the layer again,going in the opposite direction (Figure 5).To create images, the device is organizedinto a rectangular array of pixels that areindividually addressed using a series ofrow and column electrodes that are hiddenbeneath the mirror layer.DLP is based on an entirely differentprinciple. Thereis no liquid crystal layer.Instead, light from the lamp directly hitsand reflects off an array of extremely tiny,aluminum mirrors on top of an integratedcircuit, one mirror for every pixel in theimage (Figure 6). Each mirror is individuallyaddressed, not by an analog voltageproportional to pixel intensity, but by asingle digital bit. In the “1” or “on” state,a mirror tilts over at an angle, directing thelight out the projector’s lens to the screen. Inthe “0” or “off” state, the mirror tilts in theopposite direction, directing the light to alight absorber (Figure 7). This produces peakwhite or pure black, respectively. To generategray levels between, the mirrors are flippedbetween the on and off states thousandsof times a second. At that rate the resultingindividual pulses of light seem to mergecompletely and we see only an average lightlevel proportional to the ratio between theon and off times.LCoS and DLP projectors for digital cinemaachieve color images by splitting white lightfrom a Xenon high-intensity discharge lampinto red, green and blue components anddirecting them to individual devices, one foreach component color. A prism assemblyoverlays the three resulting componentimages into a full-color image for projectiononto the screen.LCoS vs. DLPThe fundamental differences betweenLCoS and DLP directly affect the designof digital cinema projectors, leading tosignificant differences in some very importantperformance parameters. The first parameteraffected is optical efficiency. This primarilyaffects the lamp power required to achievethe image brightness specified by SMPTE(Society of Motion Picture and TelevisionEngineers) on a screen of a given size [3].Lower optical efficiency means higher power,which increases operating costs due tohigher electricity consumption and morefrequent lamp changes (as well as a higherunit cost on the lamp itself if a larger wattagelamp is required). It also puts a limit on thelargest screen size a projector can support,which becomes an even greater issue for 3D(more about this later).For example, based on publishedspecifications Sony’s SRX-R320 projector,which uses a proprietary version of LCoScalled SXRD, outputs 21,000 center lumensusing a 4.2 kW lamp [7]. The optical efficiencyis thus 5 lumens per watt. In comparison,the Christie ® CP2220 DLP Cinema projector,using a much less powerful 3.0 kW lamp, canoutput as much as 22,000 center lumens,which is an optical efficiency of 7.3 lumensper watt. This is nearly a 50% advantage forDLP technology. Measurements of actualprojectors operating in theaters suggest thetrue advantage may be closer to 100% infavor of the Christie CP2220.Uniformity across the image is anotherarea in which LCoS and DLP perform quitedifferently. This is directly due to the natureof the devices. LCoS is fundamentally ananalog technology, in which gray levels are3

digital cinema projectionFigure 4LCoS (Liquid Crystal on Silicon) deviceFigure 5Light path through an LCoS deviceproportional to a voltage. (LCoS is digitalonly in the sense that the image is composedof discrete pixels.) The voltage required fora given gray level depends primarily on theelectro-optical properties of the liquid crystallayer, which can vary across the device. Theseproperties are also affected by environmentalconditions, in particular temperature.The liquid crystal heats up as it inevitablyabsorbs some of the extremely high intensitylight passing through it in a digital cinemaapplication. This can cause significant nonuniformitiesand color shifts in the image.Unlike LCoS, DLP is a truly digital technology.Pixels are binary; they are either on or off.Gray levels are achieved by varying the timingbetween on and off. Not only is this highlyreproducible, the timing required for a givengray level is not sensitive to temperatureor other environmental conditions. Hence,gray shades and colors are inherently stableand uniform across the device. Achievingsimilar performance with LCoS is challenging.Careful, on-site calibration appears to berequired, which should be done periodically.Sony offers an optional, CCD-camera-basedsystem that automates the process.It is noteworthy that DLP Cinema technologywas recognized by the Academy Board ofGovernors who bestowed the A.M.P.A.S. ®Scientific and Engineering Award* in 2009 forcolor accurate digital intermediate previewsof motion pictures.Reliability is another important issue.After many years of operation, DLP hasan admirable track record. For example,Christie’s DLP Cinema projectors have anuptime ratio of 99.999% as determined bynetwork monitoring of thousands of installedprojectors over a period of several years [8].The reliability of LCoS projectors is harderto establish, since Sony has not publishedreliability data (as of this writing).Perhaps this is simply because far fewer siteshave been operating long enough to gathermeaningful statistics. However, the problemsSony has experienced with consumer SXRDproducts may be cause for concern [9].One of the realities of Hollywood approveddigital cinema today is that there are threemanufacturers of DLP Cinema projectors,but only a single manufacturer of LCoSprojectors, resulting in more choices onthe DLP side. The DLP Cinema brand alsoassures a measure of interchangeabilityand a high degree of interoperability withother equipment in the theater. The latestgeneration of DLP Cinema projectorscontinues this philosophy, allowing thirdpartyvendors to design Image Media Blockproducts that install inside the projectors,all based on a common specification. Sony’ssolution, in contrast, is entirely proprietary.Figure 6DLP deviceFigure 7Light path to a hinged pixel on a DMD (Digital micromirror device)*Recipients of the Academy Plaque are D. Scott Dewald, Greg Pettitt, Brad Walker and Bill Werner.4

DiGital Cinema ProJeCtionHow it Works3D technologiesStereoscopic 3D is a major catalyst fordigital cinema. Attempts to bring it totheaters in the early 1950’s and again in theearly 80’s were thwarted by the practicallimitations of film projection. Today, digitalcinema technology has allowed a true 3Drenaissance, making it more practical forgeneral theater exhibition than ever before.You have many choices for adding 3D to aDLP Cinema projector, with current offeringsfrom Dolby, MasterImage, RealD, XpanD andPanavision. All of them share the techniqueof rapidly alternating between left-eye andright-eye images. They differ in how theyensure that each of those images gets to thecorrect eye.The RealD and MasterImage systems employdevices unique to each that, when placedin front of the projection lens, change thepolarization of light between the left-eyeand right-eye images. Viewers wear passive(non-powered) glasses that direct onepolarization state to the left eye and theother polarization state to the right eye.In contrast, the Dolby and Panavisionsystems employ a color filter wheel insidethe projector that changes the color gamutbetween left- and right-eye images. Thepassive glasses viewers wear are essentiallysophisticated color filters.The XpanD system differs in that it does notchange the images leaving the projectors.Rather, the glasses act like shutters, activelyswitching between the left and right eyes,alternately blocking the view of one whileopening the view of the other.The choice of one system over the other maydepend on how practical implementationdifferences affect your business. The RealDand MasterImage systems require silverscreens, while Dolby, Panavision and XpanDdo not. (However, since all 3D systems benefitfrom a higher gain screen than normal, anew screen may be beneficial in any case.)The glasses used by RealD and MasterImageare inexpensive enough to be given away topatrons. The Dolby, Panavision and XpanDglasses are relatively expensive, so they aretypically collected and washed between uses.All of the 3D systems for single DLP Cinemaprojectors switch between left-eye andright-eye images at 144 frames per second(Figure 8), a technique known as “tripleflash.” Sony’s SXRD is not fast enough todo this, despite being one of the fastestimplementations of LCoS available. 1Consequently, none of the systemsdescribed above will work with currentSXRD cinema projectors.One technique that does work, however, isto display the left-eye and right-eye imagestogether in the same image, one positionedabove the other (known as “over/under”). Acomplicated optical adapter then projectsand overlays the two images onto the screen,where they are viewed with RealD polarizingglasses (Figure 9). This approach leads to anumber of compromises.The first compromise is to light output.Despite heroic efforts to recover light thatwould otherwise be thrown away, a 3D imageappears through the glasses to be, at best,only 18% as bright as a 2D image from thesame projector on the same screen, basedon Sony’s own published numbers. RealD’sXL system for DLP Cinema projectors, incomparison, is at least 50% more efficient [11].Coupled with the superior optical efficiencyof DLP Cinema projectors, which can outputover 30,000 lumens, this allows DLP Cinemato support much larger 3D screens than LCoStechnology based projectors from Sony.The second compromise is to resolution.Paradoxically, the 3D image from a 4K SXRDprojector could easily appear less sharp thana 3D image from a 2K DLP Cinema projector.Each eye’s image starts out in a 2K formatfrom the server. However, flat (1.85:1) imagesneed to be resized to fit on the LCoS devicefor proper display through the 3D opticalsystem – in practice, scope (2.39:1) imagesare also resized for another reason. Resizingcan degrade the image, especially when itreduces the pixel count as happens in the1.85:1 case. The complex optics that projectleft-eye and right-eye images using separatelenses and overlay them on the screen canfurther degrade the image, if extreme care isnot taken during alignment.The 3D adapter will also greatly impact2D presentations if it is not removed for2D showings. Since alignment is so tricky,there is a natural temptation to leavethe adapter in place and simply removethe polarizers, which effectively turns a4K projector into a poor 2K projectorwith much reduced brightness.1 Triple flash frames change at a rate of one every 7ms. For an acceptable level of crosstalk between left-eye and right-eye images the device must respond much faster.Currently, SXRD projectors require nearly 2.5ms to change the state of a pixel, which is more than 33% of the triple-flash frame time and hence unsuitable [10].5

How it WorksInputOne Projector(Field-Sequential)Double Flash1/24 secLRLRL R L R24 fps48 fps96 fpsSXRDpanelsRLPrism unit Rotary lensLRProjection lensSeparationPrism3D filterDiGital Cinema ProJeCtionTriple FlashL R L L R L144 fpsProjection lensScreenFigure 8“Triple Flash” 3DFigure 9Sony SXRD “Over / Under” 3D Optical SystemConclusionWhich digital cinema projection technology will best serve theProjection lensneeds of your business and the expectations of your customers? 3D filterMaking that determination requires examining all aspects ofperformance R with respect to imageLquality, functionality, reliability,SXRD and cost of ownership.SeparationpanelsPrismLRFor more information, Prism unit Rotary please lens contact Christieat christiemarketing@christiedigital.comProjection lensalen KoebelScreenAs a senior design quality specialist in Christie’s Global Qualitydepartment, Mr. Alen Koebel is responsible for developing andpromoting world-class design practices and procedures withinChristie. Alen’s role involves reviewing product designs, developingtest and measurement methodologies and promoting the principlesof lean product development.Figure 10Flat left-eye and right-eye images must be resized for theover/under format.Alen’s product design experience ranges from early, all-analog CRTprojectors to high-powered, all-digital DLP Cinema projectors. Alenhas been directly involved in the design of many of the company’smajor product lines including a role as a system architect for theindustry-leading CP2000 family of DLP Cinema projectors that startedthe on-going conversion of cinemas to digital projection.Alen is a member of the Society for Information Display. He is alsoa Contributing Editor for Widescreen Review magazine, a wellrespectedpublication for the home-theater market.References[1][2][3][4][5][6][7][8][9][10][11]BS 4274-1:2003, Test charts for clinical determination of distance visual acuity —Specification, British Standards Institute (2003)Reeves, Prentice. The Response of the Average Pupil to Various Intensities ofLight. Journal of the Optical Society of America. 4, 35-43 (1920)SMPTE 431-1-2006, D-Cinema Quality - Screen Luminance Level, Chromaticity,and Uniformity. Society of Motion Picture and Television Engineers (2006)Fulton, James T., Processes in Biological Vision, Vision Concepts (2009)Schubin, Mark. High & Why. Videography, (October, 2004).Cowan, Matt. Digital Cinema Resolution – Current Situation and FutureRequirements. Entertainment Technology Consultants (2002).Product brochure: SRX-R32 4K Digital Cinema Projector, LMT-300 Media Block,STM-100 Theater Management System. DI-0189 (MK10626V1) Sony ElectronicsInc. (2009)Christie Digital Systems. Christie Releases Digital Cinema Reliability Data:99.999% Proven Uptime [Press release]. (March 30, 2009).United States District Court, Southern District of New York, Case 1:06-cv-05173-RPP. Sony SXRD Rear Projection Television Class Action Litigation. (2008)SXRD 4K Projection Whitepaper, DI-0099A, Second Edition, V3.11,Sony Electronics Inc. (January 2008)Robinson, Andrew. Screen Selection for Digital Projection.Cinema Technology. (June 2009)6

Corporate officesChristie Digital Systems USA, IncUSA – Cypressph: 714 236 8610Christie Digital Systems Canada Inc.Canada – Kitchenerph: 519 744 8005Independent salesconsultant officesItalyph: +39 (0) 2 9902 1161South Africaph: +27 (0) 317 671 347Worldwide officesUnited Kingdomph: +44 (0) 118 977 8000Germanyph: +49 2161 664540Franceph: +33 (0) 1 41 21 44 04Spainph: +34 91 633 9990Eastern Europe andRussian Federationph: +36 (0) 1 47 48 100United Arab Emiratesph: +971 (0) 4 299 7575Indiaph: (080) 41468940Singaporeph: +65 6877 8737China (Shanghai)ph: +86 21 6278 7708China (Beijing)ph: +86 10 6561 0240Japan (Tokyo)ph: 81 3 3599 7481Korea (Seoul)ph: +82 2 702 1601For the most current specification information, please visit www.christiedigital.comCopyright 2011 Christie Digital Systems USA, Inc. All rights reserved. All brand names and product names are trademarks, registered trademarksor tradenames of their respective holders. Christie Digital Systems Canada Inc.’s management system is registered to ISO 9001 and ISO 14001.Performance specifications are typical. Due to constant research, specifications are subject to change without notice.Printed in Canada on recycled paper. 3088 Oct 11

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