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The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of the Society of Motion Picture and Television Engineers (SMPTE), and its printing and distribution does not constitute an endorsement of views which may be expressed. This technical presentation is subject to a formal peer-review process by the SMPTE Board of Editors, upon completion of the conference. Citation of this work should state that it is a SMPTE meeting paper. EXAMPLE: Author's Last Name, Initials. 2011. Title of Presentation, Meeting name and location.: SMPTE. For information about securing permission to reprint or reproduce a technical presentation, please contact SMPTE at [email protected] or 914-761-1100 (3 Barker Ave., White Plains, NY 10601).
SMPTE Meeting Presentation
High Performance Polarization-Based 3D and 2D Presentation (Part I)
Gary D Sharp, Ph.D, RealD, 5335 Sterling Drive, Boulder Colorado, 80301, [email protected]
Miller H Schuck, Ph.D, RealD, 5335 Sterling Drive, Boulder Colorado, 80301, [email protected]
Dave Coleman, Ph.D, RealD, 5335 Sterling Drive, Boulder Colorado, 80301, [email protected]
Kevin Curtis, Ph.D, RealD, 5335 Sterling Drive, Boulder Colorado, 80301, [email protected]
Scott Gilman, RealD, 5335 Sterling Drive, Boulder Colorado, 80301, [email protected]
Erik Arend, RealD, 5335 Sterling Drive, Boulder Colorado, 80301, [email protected]
Written for presentation at the SMPTE Technical Conference 2012
Abstract. Each digital cinema 3D delivery system has a characteristic efficiency. The 3D-efficiency gives the relative luminance of 3D presentation compared to 2D presentation. In the absence of careful attention to design, component efficiencies, and maintenance, the result can be a peak luminance well below 4.5 fL, even on average sized screens. In addition, aspects of a 3D system can determine the quality of 2D presentation. Given the large installed base and immediate need for higher brightness images, methods for increasing brightness and image quality are sought which enhance existing projector platforms. This paper evaluates loss mechanisms in 3D projection systems and shows that high-brightness 3D is feasible using lamp-based illumination. These solutions can be implemented using single-projector sequential 3D, for conventional sized screens (averaging 40'), as well as premium large format screens in excess of 60'.
.
Keywords. 3D, digital cinema, projection, DLP, polarization, 3D efficiency
2
Introduction
In 2005, RealD participated in a 100-screen showing of Disney’s “Chicken-Little” in 3D using digital cinema projectors. Since then, we’ve experienced a rapid build-out of the digital projection platform. Today, there are approximately 80,000 digital cinema projectors installed, over 43,000 of which are 3D enabled using various technologies. The annual 3D movie slate is approximately 40 feature films. While great strides have been made in producing compelling 3D content and increasing the performance of the delivery systems, there remains work to be done. On the delivery side, customers are particularly demanding higher brightness 3D. This realistically must be accomplished without replacing recently-purchased projectors, or incurring massive retrofit costs. Equally important, a consistently bright 2D/3D experience requires adhering to lamp-life criteria and regular equipment maintenance schedules. Finally, it will ultimately be necessary for 3D enabled auditoria to accommodate 2D presentation without compromising quality.
This paper analyzes the performance of various DLP projector based 3D delivery systems that utilize polarization. Solutions are offered that enhance the 3D presentation, particularly as it pertains to higher brightness. A metric is used for capturing the specific impact of 3D on luminance: the 3D-efficiency. The 3D-efficiency eliminates several system variables by assuming that an auditorium is configured to meet 2D brightness requirements. The 3D-efficiency further allows a direct brightness comparison between each of the present 3D delivery systems, assuming a common screen gain.
There are numerous factors contributing to the result of a standard cinema brightness measurement (e.g. SMPTE RP94-2000[1]) including: the native lumen output of the projector, the spectral content of that output, port-glass transmission, screen size, geometric-efficiency (screen aspect and keystone correction), screen scatter and absorbtion characteristics, viewing location, and (2D) color balance. In a 3D presentation there are additional factors to consider, including; efficiency of the 3D encoding mechanism, temporal efficiency (for single projector), efficiency of 3D decoding mechanism (i.e. the eyewear), and 3D color-balance.
The 3D-Efficiency
A useful metric that isolates the 3D contribution is the 3D-efficiency given by
where the 3D measurement is made with the entire delivery system in place (and in operation), and both measurements are made along the identical optical path, but at somewhat arbitrary screen position and viewing location1. Note that the above assumes that no 3D-delivery components remain in the light path for 2D presentation. The 3D-efficiency is beneficial, for example, in giving a direct calculation of 3D luminance from a 2D measurement
where the above calculation assumes that there is no “3D-switch” (e.g. an increase in lamp current or a change in lamp type for the 3D mode). Apart from the color balance contribution
1 Polarization based 3D systems typically have some roll-off in the bright state as the throw ratio increases. For
this analysis, we assume measurements are made under the same throw ratio conditions.
3
that links the projector and 3D system, the 3D-efficiency is independent of the details of the projection system, screen, theatre geometry, and measurement setup. However, it is directly applicable to site-specific performance estimates, such as during the design phase of an installation.
The 3D-efficiency also enables a somewhat fundamental comparison between the effectiveness of 3D delivery systems in the use of projector lumens. Direct comparisons are especially appropriate between systems that require a polarization preserving screen, or at least have a similar screen gain requirement. In the following analysis, we consider 3D efficiency for each polarizing 3D technology, with special emphasis on PLF (premium large format) presentation.
Example: Two 3D Systems for PLF
An accurate measurement of 3D efficiency is done at the system level, incorporating the color balance required to meet the DCI white specification. The color balance efficiency is the result of interplay between the native output spectrum of the projector and the chromatic losses of the 3D delivery system. In the following, we analyze the color balance, throughput and accuracy of two dual-projector systems: the circular-polarizing RealD XL-DP and a conventional linear-polarizer (LP) dual-projector configuration.
Light emerging from the DLP projector is randomly polarized, and in a conventional linearly polarized (LP) configuration, more than half of the light is absorbed by the input linear polarizer. Such 3D systems require dye-type linear polarizers to withstand the thermal load from absorption. Dye-type polarizers have lower transmission than (iodine) display polarizers. Photopic efficiency due to internal-absorption of a high-quality dye-type polarizer is approximately 40%, with lower transmission in the blue. Dye-type LP’s are generally laminated between glass plates with broadband AR coatings.2 Paired LP’s (one for the left eye, one for the right eye) are used for dual-projector installations.
The XL-DP is a polarization converting system placed at the output of the projection lens. The XL-DP consists of a tilted polarizing beam-splitter (PBS) which separates the linear polarized eigenstates into two optical paths. The p-polarized light is transmitted through the PBS. The s-polarized light is reflected by the PBS, reflected (again) by a mirror, and directed toward the screen on a path parallel to the p-polarized light. An achromatic polarization rotator follows the mirror in the s-path and transforms the s-polarized light to the p-state, creating a common polarization in each path. Magnification optics in the p-path compensate for the additional length of the s-path, giving accurate image overlay at the screen. Because the polarizing efficiency (PE) of the PBS is fairly high, only a low-PE high-transmission iodine “clean up” polarizer is needed. The photopic transmission of the XL-DP averages 87%. The two paths then pass through identical quarter-wave plates, creating identical circularly polarized output states. The non-etendue preserving nature of the projection screen allows superposition of images at the screen without loss. Paired XL-DP’s (one for the left eye, one for the right eye) are likewise required for dual-projector installations.
In order to accurately compare the XL-DP and LP systems for 3D efficiency, a test rig was constructed to measure the transmission of the XL-DP and LP over the visible spectrum. The test rig consists of a lamp focused into a homogenizing lightpipe, followed by a projection lens to project the image of the lightpipe toward a screen. An integrating sphere follows the projection lens and the XL-DP and LP systems are inserted between the projection lens and sphere. An
2 The dye-type LP system under test was implemented with Polatechno SHC polarizer, B270 glass plates and R
<0.5% AR coatings over the visible spectrum.
4
iris was placed at the lightpipe output to limit the size of light bundle entering the sphere. For the XL-DP, circular polarization (CP) eyewear was used to optically decode the bright state image3. In the case of the LP system, LP eyewear was used to decode the image.
The light inside the sphere was measured with a PhotoResearch PR-650 spectroradiometer. The PR-650 has a spectral resolution of 4nm, and measurements were made over 380-780nm. Measurements were made of the unaltered projection system, then with the XL-DP and CP eyewear inserted, and finally with the dye-type LP and LP eyewear inserted. The XL-DP and LP system transmission were determined by dividing the resulting altered spectra by the unaltered projection system spectra.
The results are depicted in Figure 1. The upper blue curve is the transmission of the XL-DP and CP eyewear, while the lower red curve is the transmission of the dye-stuff LP and LP eyewear. The advantages of polarization conversion technology are evident in the overall transmission levels in the two plots. Both systems show roll-off in the deep blue portion of the spectrum (due to polarizer and glass absorption). The XL-DP shows a roll-off in the red portion of the spectrum, attributable to dispersion of the combined retardation of the XL-DP and cinema eyewear retarder films. This transmission data is used to calculate the color-balanced throughput and color accuracy of the two systems.
Figure 1 Measured transmission: XL-DP and CP eyewear (upper blue curve), dye-type polarizer and LP eyewear (lower red curve).
DLP-based digital cinema projectors split the xenon lamp spectrum into three (RGB) color bands. The spectra of the color bands vary somewhat between manufacturers, model, and projector unit due to design choices and manufacturing sensitivities. The unaltered combination of red, green and blue spectra at full code value is termed native white.
The DCI has specified a color-balanced white chromaticity (x,y) = (0.314, 0.351) in 2D at the
screen, as well as the minimum enclosed color gamut[3]. Color accuracy is specified as +/- 4E. SMPTE RP 431-2:2011 has specified tolerances significantly larger than the original DCI
3 Note the upper and lower optical paths of the XL-DP were measured separately, then summed to produce a
single transmission plot.
5
specification[2] (x,y of +/-0.010 and +/-0.020 for red and green respectively, and x of +0.010/-
0.030 and y of +0.020/-0.040 for blue).
Native spectra from three digital cinema projectors were measured (we label them A, B and C). For each of the native spectra, we calculate the color-balancing coefficients, color correction efficiency and color accuracy at each primary for a 2D presentation. We use the CIE 1931 color-matching functions for all color related calculations. The color-balancing coefficients, k’i, are determined via SMPTE RP 176[4] and RP 177[5] formulae and result in a color-balanced
luminous flux
where R , G and B are integrated luminous flux out of the red, green and blue channels and k’r, k’g and k’b are the resulting red, green and blue coefficients (0 < k’i < 1).
The color correction efficiency, , is defined by[6]
where is the color-balanced luminous flux out of the projector, and is the uncorrected native flux.
Once the white-point color balancing has been carried out, a separate color-balancing step is performed for each primary. The color accuracy of each resulting primary is determined by
recording the maximum value of x and y with reference to the DCI-specified primaries.
We next apply the XL-DP and LP system transmission spectra to the native projector spectra, and calculate the same quantities for the 3D systems. Throughputs for the two color-balanced 3D systems are then compared to the color-balanced 2D system, giving the 3D-efficiency.4
Figure 2 depicts the native (solid curves) and color-balanced (dashed) primary spectra of a digital cinema projector. Typically, digital cinema projectors have slight excesses of blue and red, which are reduced to produce the required color-balanced white. The dashed plots show the lowered blue and red outputs. Note that the green spectra remain the same (k’g = 1.0), maximizing the color-balanced output.
Figure 3 depicts the spectral outputs of a sample projector with the XL-DP and LP system inserted. The upper curves depict the XL-DP results. The solid lines are the uncorrected spectra, that is, the native projector spectra modified by the transmission of the XL-DP system. The upper dashed curves are the color-balanced spectra of the XL-DP. We can see in this case that the XL-DP’s half-wave roll-off in red limits the color-balancing, and the green and blue spectral outputs are reduced to produce the correct white point.
The lower curves in Figure 3 are results for the LP system. The solid lines are the uncorrected spectra, while the dashed lines are the color-balanced spectra. We can see in this case that the blue and red spectra are reduced to produce the correct white point.
4 For single projector systems, a duty cycle factor representing the left/right switching and required projector dark
time (for 3D modulator settling) is also required in the throughput calculation.
6
400 450 500 550 600 650 7000
0.2
0.4
0.6
0.8
1
1.2
Projector Native, Color-balanced spectra
wavelength (nm)
A.U
.
Figure 2 Projector A: Primary spectra, native (solid line) and color-balanced (dashed).
Table 1 is a summary of the results for three projectors (A, B, C) and the XL-DP and LP systems. Given the spectral output variations from projector to projector, we see resulting variations in the color-balancing coefficients, color-correction efficiencies, throughputs, and color accuracies. Color corrected 3D throughput (compared to color-corrected 2D throughput) or “3D efficiency” for the XL-DP ranges from 0.689-0.736, compared to 0.324-0.350 for the LP system. Color accuracy for the XL-DP has a maximum of 0.010, while the LP system has a maximum of 0.012 (both systems well within SMPTE requirements).
The results show the color-balanced throughput of the XL-DP (with CP eyewear) system to be twice as high as the color-balanced throughput of the dye-type LP (with LP eyewear) system. This doubling of efficiency in the XL-DP is utilized to enable larger and brighter screens than competing LP systems.
As an example, we consider a theater that is properly configured for projecting a 2D image at the DCI standard brightness of 14fL using one projector in a dual-projector configuration (a common practice). If an XL-DP system is then utilized for 3D (one projector for each eye), the resulting image brightness is (14fL) x (0.7) = 10fL per eye. If a dye-type LP system is utilized for 3D (again, one projector for each eye), the resulting image brightness is (14fL) x (0.35) = 5fL per eye. In this case, the XL-DP system produces a 3D image very close to the DCI 2D standard of 14 +/- 3 fL, without any changes to the lamp type or lamp current. The LP system must change the lamp, or lamp current settings to improve its brightness.
We also consider the reverse situation. Suppose we configure our 3D system to run at the minimum 11fL in the DCI 2D specification. For an XL-DP system, this implies the 2D presentation (without lamp changes) will be 11fL/0.7 = 16fL, well within the DCI requirements. For an LP system, the 2D presentation (without lamp changes) will be 11fL/0.35 = 31fL, well outside of the DCI specification. Given many exhibitors are choosing to use high power lamps and running them at minimum current to extend the lamp’s lifetime, there is no headroom for reducing the lamp current in the LP system, requiring a lamp change for DCI compliant 2D presentation in these cases. The XL-DP allows for 2D and 3D presentation at compliant brightnesses without lamp current or type changes, even in the “no headroom” case.
7
400 450 500 550 600 650 7000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1XL/CP system, LP/LP system: native & color-balanced spectra
wavelength (nm)
A.U
.
Figure 3 Projector A: Primary spectra for XL-DP and CP eyewear (upper curves) and dye-type LP and LP eyewear (lower curves). Uncorrected spectra are solid lines. Color-balanced spectra are dashed.
Table 1 Summary of color and throughput analyses for dual systems.
A B C A B C A B C
R 0.988 0.905 0.893 1 0.971 0.895 0.981 0.903 0.825
G 1 1 1 0.946 1 0.939 1 1 0.931
B 0.754 0.702 0.89 0.859 0.843 1 0.917 0.851 1
0.979 0.953 0.968 0.951 0.981 0.932 0.99 0.966 0.908
N/A N/A N/A 0.696 0.736 0.689 0.349 0.35 0.324
R 6 4 3 7 4 4 5 3 3
G 4 11 2 3 10 2 4 12 2
B 0 0 0 0 7 0 0 8 0
x,y max
(X .001)
2D Projector XL-DP and CP eyewear Dye LP and LP eyewear
k'
nCC
3D Efficiency
Additional examples: Single projector implementations
The same analysis was completed on several single-projector systems, including shutter glass systems, spinning disc polarizers, ZScreens and active-switching XL’s. In each case, the systems were driven to their high transmission states. Dark times for each system were extracted from OEM projector macros and included in the throughput calculations.
Again, the benefits of polarization conversion are evident in the XL’s 3D efficiency results (Table
2). For an active XL, a 14fL 2D setup will produce a (14fL) x (0.28) = 4fL 3D image without a lamp change. For the other systems, the 3D image drops to 2fL without a lamp change.
8
Table 2 Summary of color and throughput analyses for single projector systems.
Active XL & Zscreen & Spinning wheel Cinema Active
2D Projector CP eyewear CP eyewear & CP eyewear eyewear
Projector A A A A A
R 0.988 1 1 1 1
G 1 0.839 0.881 0.883 0.96
B 0.754 0.856 0.849 0.815 0.921
0.979 0.871 0.903 0.901 0.966
N/A 0.258 0.128 0.13 0.139
R 6 8 7 7 7
G 4 1 2 1 4
B 0 3 2 0 0
Projector B B B B B
R 0.905 1 1 1 0.962
G 1 0.912 0.959 0.961 1
B 0.702 0.862 0.855 0.822 0.891
0.953 0.925 0.959 0.957 0.983
N/A 0.281 0.14 0.143 0.145
R 4 5 5 5 4
G 11 8 9 8 11
B 0 13 10 9 8
Projector C C C C C
R 0.893 0.897 0.905 0.942 0.833
G 1 0.838 0.887 0.924 0.889
B 0.89 1 1 1 1
0.968 0.86 0.897 0.932 0.881
N/A 0.257 0.129 0.137 0.128
R 3 5 4 4 4
G 2 2 2 3 1
B 0 1 0 0 0
x,y max
(X .001)
k'
nCC
3D Efficiency
x,y max
(X .001)
k'
nCC
3D Efficiency
x,y max
(X .001)
k'
nCC
3D Efficiency
Summary
We’ve determined the 3D-efficiency for several single-projector and dual-projector 3D projection systems, taking into account modulator type, eyewear, and color-balancing. The difference in 3D-efficiency between projectors illustrates the impact of native spectra on 3D color-balance. The advantages of highly efficient polarization conversion are evident in the results, particularly in the changeover from 2D to 3D presentation. Owing to high 3D-efficiency, a dual-projector XL-DP is shown to produce DCI compliant luminance in both 2D and 3D presentation with no change in lamp current.
References 1. SMPTE RP 94:2000 Gain Determination of Front Projection Screens (R2007). 2. SMPTE RP 431-2:2011 D-Cinema Quality - Reference Projector and Environment. 3. Digital Cinema Specification. Version 1.2, March 7, 2008. Digital Cinema Initiatives,
LLC. 4. SMPTE RP 176:1997 (Archived 2010) Derivation of Reference Signals for Television
Camera Color Evaluation (R2003) 5. SMPTE RP 177:1993 (Archived 2010) Derivation of Basic Television Color Equations
(R2002) 6. E. Stupp and M. Brennesholtz. Projection Displays. Wiley & Sons (1999).
7. R. W. G. Hunt. Measuring Color. Fountain Press (1998).
The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of the Society of Motion Picture and Television Engineers (SMPTE), and its printing and distribution does not constitute an endorsement of views which may be expressed. This technical presentation is subject to a formal peer-review process by the SMPTE Board of Editors, upon completion of the conference. Citation of this work should state that it is a SMPTE meeting paper. EXAMPLE: Author's Last Name, Initials. 2011. Title of Presentation, Meeting name and location.: SMPTE. For information about securing permission to reprint or reproduce a technical presentation, please contact SMPTE at [email protected] or 914-761-1100 (3 Barker Ave., White Plains, NY 10601).
SMPTE Meeting Presentation
High Performance Polarization-Based 3D and 2D Presentation (Part II)
Gary D Sharp, Ph.D, RealD, 5335 Sterling Drive, Boulder Colorado, 80301, [email protected]
Miller H Schuck, Ph.D, RealD, 5335 Sterling Drive, Boulder Colorado, 80301, [email protected]
Dave Coleman, Ph.D, RealD, 5335 Sterling Drive, Boulder Colorado, 80301, [email protected]
Kevin Curtis, Ph.D, RealD, 5335 Sterling Drive, Boulder Colorado, 80301, [email protected]
Scott Gilman, RealD, 5335 Sterling Drive, Boulder Colorado, 80301, [email protected]
Erik Arend, RealD, 5335 Sterling Drive, Boulder Colorado, 80301, [email protected]
Written for presentation at the SMPTE Technical Conference 2012
Abstract. Each digital-cinema 3D delivery systems has a characteristic efficiency. The 3D-efficiency gives the relative luminance of 3D presentation compared to 2D presentation. In the absence of careful attention to design, component efficiencies, and maintenance, the result can be a peak luminance well below 4.5 fL, even on average sized screens. In addition, aspects of a 3D system can determine the quality of 2D presentation. Given the large installed base, methods for increasing brightness and image quality are sought which leverage existing projector platforms. This paper evaluates loss mechanisms in 3D projection systems and shows that high-brightness 3D is feasible using lamp-based illumination. These solutions can be implemented using single-projector sequential 3D, for conventional sized screens (averaging 40'), as well as premium large format screens in excess of 60'.
Keywords. 3D, digital cinema, projection, XL, XL-DP, polarization, ZScreen, 3D-efficiency
2
Introduction
When analyzing the brightness of 3D-enabled projection systems, it is convenient to separate those factors specific to the 3D delivery, and those common to both 2D and 3D delivery.
The 3D-efficiency, given by
is a specific characteristic of the 3D delivery equipment. Apart from coupling through color balance considerations, the 3D-efficiency is independent of other aspects of the system. Extremely high 3D-efficiencies are only feasible with configurations that virtually eliminate encoding/decoding losses (e.g. polarization and sequential), though in practice, insertion-losses (imperfect polarizers, reflections, etc.) account for significant additional loss. So in general, there is a significant discrepancy between 2D and 3D luminance that must be managed to insure high quality presentation.
In the absence of a “3D-switch” in projector output lumens, 3D and 2D brightness are directly related by the 3D-efficiency. For instance, a well-maintained single-projector system that delivers a 14fL 2D presentation can consistently produce a 4.0 fL 3D presentation with a (sequential) RealD XL system. In order to further increase 3D luminance, say to 6 fL, the projector must produce a 2D luminance of 22.6 fL.
The push to achieve acceptable 3D lumininance on a system with low 3D-efficiency produces one of two outcomes; either the 3D brightness is unattainable due to projector limitations, or the 2D is far too bright. For instance, a first-generation 3D system (with a 3D-efficiency of 13%), requires 46 fL 2D to generate a 6 fL 3D image.
A 3D-switch can in principle be used to bridge the gap (e.g. by over-sizing the lamp and operating at reduced current for 2D presentation). However, exhibitors may already be using this technique to extend lamp life for constant-brightness, which can leave little or no headroom for implementing a 3D-switch.
Given current projector output limitations, methods for increasing 3D luminance include; (1) further increasing the projector output lumens through brighter light sources (e.g. lasers); (2) increasing the efficiency of system components, and; (3) increasing the screen gain. In the event that a desired 3D luminance is somehow attainable, it then becomes necessary to consider the impact of the 3D-efficiency on the 2D presentation. The possibilities for 2D presentation include attenuating the projector output (which may include leaving 3D encoding equipment in place), providing a version that is color-timed for a higher brightness, or (again) implementing a 3D-switch.
Laser Illumination
Projector manufacturers are actively engaged in demonstrating laser-based illumination which, among other potential benefits, can increase 3D brightness1. As discussed above, lamps can produce very bright 2D presentations, and moderately bright 3D presentation on average (~40ft) sized screens. However, for larger screens even more light is needed. One approach is to simply increase the wattage of the lamp, which neglects the role that etendue plays in projection. Etendue is a geometric quantity that describes the light carrying capacity of an optical system2. Projection systems have an etendue that is defined by the panel size and projection lens f-number. Lamps have an etendue that is determined by the arc size and solid
3
angle of emission. Typically, as the arc lamp power is increased, the etendue of the arc is also increased. The lamp light captured and used by the projection system is proportional to the ratio of the two etendue values. As the lamp etendue increases beyond the projection system etendue, the projection system efficiency drops and less light is delivered to the screen. Currently, lamps designed to match the etendue of digital projectors are limited to around 6-7kW of power.
Another factor affecting projection system luminous output is thermal loading. As more optical power is put into the optical path of the projector, the temperature of the display chip is increased due to absorption of light. The DLP chip has a specified temperature range (temperature difference between top and back of the panel) that can be support by the chip and not invalidate the warranty. The allowable power levels are determined by the light absorption (which depends upon the power spectral density of the source) and the efficiency of the projector cooling system. Cinema projectors use liquid cooling of the DLP panels for high power operation.
Lasers are being developed to increase the brightness of projection system as they have very low etendue and high power (i.e. high brightness). Since the laser etendue is very small compared to the projection system etendue, multiple lasers can be optically combined to increase the brightness of the projection system. Using typical laser wavelengths (approximately 465nm, 532nm, and 635nm), cinema projector prototypes with efficient cooling systems have demonstrated outputs of >60,000 lumens, where current xenon lamp projectors produce >30,000 lumens of color balanced lumens. Thus lasers can increase the brightness of the system by a factor of two.
Lamp output power changes significantly as the lamps age. This effect can be mitigated by operating the lamp in a constant output power mode, a feature that is offered in cinema projectors. However, constant output power mode requires the lamp to be initially set to a lower wattage than the rated maximum, such that the lamp current can be increased to maintain a constant power over the lifetime. Lasers have the potential to output more consistent light levels, since they tend to be very stable over the lifetime of the source. The potential for a practical implementation of the 3D-switch is another compelling aspect of laser illumination.
Efficiency Improvements
Incremental efficiency gains can be made by carefully scrutinizing the performance of each component, the projection system design, and if possible, the auditorium design. Individually these losses seem modest compared to polarization/sequential losses, but collectively they can have a very significant impact on luminance. The following are examples of elements that should be carefully considered:
An uncoated port window with absorptive glass (typically blue absorption is the issue) can have a transmission that is over 10% less than a broad-band AR coated product using water-white glass (e.g. Abrisa Technologies). Note that glass with low birefringence (retardation below 10 nm under operating thermal load) is necessary for polarization-based 3D delivery.
Higher 3D efficiency is possible by considering the interplay between 3D system chromatic losses and projector filtration. Given that 3D systems are relatively light starved, it may be preferable to tailor the projector native output spectrum so as to minimize the losses associated with 3D color balance. Our testing has shown that 3D color balance losses vary significantly between projector models/manufacturers, and can be quite high.
4
The geometrical loss is associated with over-fill of the screen (cropping), the relationship between native screen format and DLP aspect, and keystone correction. By selecting a flat-format screen, using a zoom-lens, and lens shift, these losses can be greatly mitigated.
Incremental increases in eyewear efficiency (which are currently most viable with premium products) include use of higher transmission polarizer (>42%), and possibly adding antireflection coatings. This can result in a 10% increase in transmission and the added benefit of a more relaxed visual experience through reduction in stray light.
Screens currently have approximately 5-7% loss due to perforations for acoustic transmission. We have demonstrated that acceptable mid/high frequency acoustic transmission can be achieved with 1-2% hole area by reducing the scale of the perforation pattern. This has the added benefits of eliminating resolvable holes, as well as moire that can result with digital projectors.
Screen Technology
The silver-screens in use today for polarization-based 3D systems have unique characteristics that we’ve invested considerable effort to understand. We have attempted to reduce screen performance to measurable (and ideally traceable) parameters wherever possible, though some characteristics are still best evaluated visually. The parameters we use to quantify screen performance include; (1) Peak Gain; (2) Half-Gain-Angle (HGA); (3) Total Integrated Scatter (TIS), and; (4) Stereo Contrast Ratio (SCR).
The gain is the reflectivity of the screen divided by that of a Lambertian reference. This is normally measured near normal incidence. The peak gain (usually termed the gain) is the gain measured in the specular (i.e. mirror) direction. Silver-screens have an angle dependent gain that decays to a small fraction of the peak gain at large angles. The HGA represents the viewing angle for which the luminance falls to 50% of the peak gain. The peak gain of the screen is coupled with the HGA, so increasing it typically involves decreasing the HGA. In practice, this increases the peak brightness of the image, while reducing the brightness in the corners (since angles map to positions on the screen). Note that while illuminance is fixed for all viewers by the projection geometry, the luminance is a function of viewer position, so the peak gain position is different for every viewer.
In order to increase peak gain while simultaneously maintaining (or increasing) brightness in the corners, the screen simply must become more efficient. We use total-integrated-scatter (TIS) as the metric for overall screen efficiency. This is given by illuminating the screen material near normal incidence, and collecting all of the light reflected back toward the audience. In the most general sense, scattering surfaces are characterized by the bi-directional reflectance distribution function (BRDF). The BRDF gives the differential reflectivity of the screen, which is a function of both illumination and observation directions.
Currently, we measure TIS over the full viewing hemisphere without regard for state of polarization (SOP). A further refinement would be to measure it to a representative viewing cutoff angle (eliminating any TIS benefit of light outside of the audience viewing locus), and including polarization (eliminating any TIS benefit of light in the depolarized term). The TIS values in Table 1 were measured by illuminating screen samples at 8° off-normal, with 532 nm laser light, and collecting all scattered light with an integrating sphere. The TIS is given by the ratio of this power to the power collected from a Lambertian reference (Spectralon).
A screen surface with higher TIS simply makes more light available to the audience without any additional projector lumens. Moreover, the scatter profile of a higher TIS surface can be tailored to the specific environment, ensuring that lumens are most efficiently steered to viewers given a
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particular geometry (projection/viewing and screen curvature). Engineering a high-TIS surface allows the the visual experience to be improved by increasing brightness near screen-center, increasing the brightness uniformity (i.e. broader HGA), or a combination of the two. For instance, doubling the TIS allows doubling the peak gain, without reducing the HGA. This has the equivalent benefit of adding a second projector at a relatively low cost. RealD has developed a high TIS polarization preserving screen material, which can be tailored for specific peak gain and HGA4. Table 1 shows TIS values for two commercially available silver-screens and three samples of RealD screen material.
Screen Sample Peak Gain HGA TIS
Silver (Vendor A) 2.0 23° 52%
Silver (Vendor B) 2.8 19° 57%
RealD (Sample 1) 4.1 22° 89%
RealD (Sample 2) 2.5 30° 89%
RealD (Sample 3) 1.8 45° 89%
Table 1. Performance measured from various polarization preserving screen samples.
Figure 1 shows simulation results of peak gain versus HGA for different values of TIS. Note that this result is not completely general, since it corresponds to a specific gain profile function.
Figure 1. peak gain versus HGA for different values of TIS.
Consider a system designed to deliver a 2D luminance, , using a screen with gain, G. If a 3D
luminance, is required, this can be accomplished by increasing the screen gain to
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For example, a system delivering 14 fL 2D on a standard silver screen with G=2, produces 6 fL 3D with a RealD XL system by adjusting the peak gain to G’=3.2. A screen surface with a TIS of 89% can produce this peak gain, while additionally increasing the HGA from 22° to approximately 26°.
The technology we use to create surfaces with higher TIS (beyond the scope of this paper) also enables other performance advantages. These include broader HGA, increased stereo-contrast-ratio (SCR), and improved visual appearance (for both 2D and 3D). The latter refers in particular to the visual appearance near the specular direction (sometimes referred to as the “hot-spot”).
The desired HGA of the scatter profile is somewhat dependent upon the performance objecives (e.g. cinema versus a color-grading suite), geometry of the projection/viewing/screen-curvature, and overall luminance challenges. While a Lambertian screen provides the most uniform luminance for all viewers and viewing angles, it is not ideal from an experience point of view if the result is low overall brightness. Moreover, Lambertian surfaces spill significant light onto surrounding surfaces, which reduces contrast. Conversely, extreme gain and narrow HGA produces an image with unacceptable taper in brightness (or shading). Further research is required for determining optimum gain profiles, based on the overall visual impression of screen brightness at each location3.
Figure 2 shows a luminance profile from two screen samples measured with a Radiant Imaging camera. Both (uncurved) samples are of identical size and were illuminated and observed under identical conditions. The throw ratio was 1.4 and the measurement was made with the camera centered and at a half-throw distance. Fig 2a corresponds to a commercially available silver screen with a 1.8 gain and a 24° HGA. Fig 2b corresponds to a RealD screen sample with a 1.3
gain and a 38° HGA. Note that the latter has a reduced TIS value, but is useful in illustrating the
benefits of broad HGA on the spatial distribution of luminance.
It is important to note that the visual appearance of a gain-screen hot-spot is not a fundamental consequence of a narrow HGA (i.e. the shading). Indeed, a white screen and a silver-screen of similar gain can have a very different appearance when viewed in the specular direction. A typical white screen provides a wealth of scattering diversity (sub-wavelength surface/bulk scatter) resulting in a visually featureless appearance, with the unfortunate consequence that polarization is not preserved. A silver-screen can preserve polarization, though there tends to be reduced scattering robustness that can produce inhomogeneous appearance and macroscopic texture. The actual visual appearance in the hot-spot is the result of the feature size/morphology of the reflective material, the statistical distribution of the scatterers (which can vary spatially), and the impact of scattering statistics on the illuminating source. That is, in the absence of sufficient scattering diversity, the effect of residual spatial coherence can be evident even with lamp illumination (i.e. speckle). However, we have demonstrated that many hot-spot issues can be effectively eliminated by carefully engineering screen configurations/surfaces, resulting in a pleasing matte appearance while preserving polarization.
The stereo contrast ratio (SCR) is a measure of the effectiveness of a 3D system in avoiding cross-talk between eyes. It is normally measured at a screen height viewing distance, center aisle, with a light meter aimed at the location of peak luminance. One of the eyewear lenses is placed over the meter and, in a sequential system, the projector modulates between a full-white and black field. By reversing the phase, the meter measures both the on-state and off-states, with the ratio giving the SCR. In a dual-projector system, the measurement is made by alternately blocking the light from each projector displaying a full-white frame.
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The SCR is a system-level measurement, including the contributions of every component along the optical path that affects the encoding/decoding of stereo imagery. At present, the contribution of the screen tends to dominate because it is so difficult to control polarization with a scattering surface. Current silver-screens have surfaces that are stastical in nature, which creates a coupling between HGA and SCR. SCR tends to decrease as HGA is increased, where for instance, a 100:1 SCR requirement (for circular polarization) currently limits the HGA to 20-25°. However we have recently demonstrated engineered surfaces in which >500:1 SCR is possible with >45° HGA.
Figure 2 Luminance maps of (a) a silver screen, and (b) the RealD screen. The screens are 24’ wide, flat, with a throw-ratio of 1.4
It should be noted that the SCR is angle dependent, with the hot-spot measurement giving the highest result. The ghost term (in the denominator) represents the depolarization component, which tends to be uniform in angle space. As such, the SCR tends to decay in a manner that tracks the gain profile (e.g. the SCR falls to roughly 50% of the peak value at the HGA).
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Conclusions
This paper has considered several technologies for improving the brightness of 3D images in the cinema. We have measured the enhancement factors associated with several 3D-delivery technologies, the RealD screen, and recently demonstrated laser illumination, which are summarized in Table 2. Much can be accomplished with existing (lamp-based) projectors to increase 3D brightness by exploiting technologies that increase system efficiency. Efficiency gains have the benefit that they provide more lumens to the audience without increasing operating cost. In a PLF environment, for instance, we propose a dual projector platform (eliminating sequential loss), the XL-DP (nearly eliminating polarization loss), and (in the near future), a new high TIS screen. The latter will both enhance peak brightness (and/or improve brightness uniformity), provide a matte appearance (significantly improving both 2D and 3D image quality), and boost SCR. Collectively, these technologies represent a practical path for achieving high brightness presentation of 3D with currently installed projectors. When laser illumination becomes available, an additional 2x multiplier will be achieved since these technologies are all complementary.
Technology 3D Multiplier
2D Multiplier
Status
Benchmark (e.g. ZScreen)
1.0x NA In Market
XL 2.0X NA In Market
XL-DP 2.7X NA In Market
RealD Screen 1.7X 1.7X Prototype Phase (<1 year)
Dual Projector 2.0X 2.0X (with warping)
In Market
Laser 2.0X 2.0X Prototype Phase (<5 years)
Table 2. Efficiency factors for enhancing 3D brightness.
Table 2 summarizes the technology improvements that are available benchmarked versus a ZScreen (13.2% efficient). These factors are multiplicative, so choosing the RealD XL and RealD Screen results in a 2.0 x 1.7 = 3.4x improvement, while a PLF system can have a 2.7x1.7x2=9.2x brightness advantage.
In addition to the above, several methods for achieving incremental efficiency gains were discussed which impact both 2D and 3D. If these considerations are taken into account, it can have tremendous value in terms of efficient use of projector lumens.
References
1. IBC 2012 Christie Laser Projector demonstration of 63,000 lumens.
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2. W.T. Welford and R. Winston, “High Collection Nonimaging Optics”, Academic Press, San Diego, 1989.
3. Martin Richards and Dave Schnuelle, The Effective Gain of a Projection Screen in an Auditorium”, SMPTE Mot. Imag. J., Vol 119, no 7, 62-67, 2010.
4. David A Coleman and Gary D Sharp, US Patent No. 7,898,734, March 1, 2011.
