SMPTE's latest emerging-technology webinar focused on laser-illuminated projection, a subject of particular interest to AVS videophiles.
Last week, I attended a webinar entitled "Laser Science and Laser Illuminated Projection" hosted by SMPTE (Society of Motion Picture and Television Engineers) and presented by Bill Beck, whose official title is The Laser Guy at Barco, one of the major suppliers of digital-cinema projectors. Bill's presentation applied mostly to commercial cinema, but laser illumination is quickly finding its way into consumer products, so it's highly relevant to the future of home-theater projection.
Types of Laser-Illuminated Projectors
He started by stating that commercial laser-illuminated projectors are now available using one of two basic technologies. The most direct approach uses separate red, green, and blue laser diodes to illuminate three imagers, most commonly DLP chips, so it's no surprise that this technology is called RGB. Barco, Christie, NEC, and others make RGB laser-illuminated projectors.
The other technology uses blue laser diodes to provide the blue light and to excite a phosphor to emit yellow light, which is then separated into red and green; this is often called blue laser-pumped phosphor (BPP) or laser hybrid. Companies making BPP projectors include Christie, Digital Projection, Epson, NEC, and Sony, though some of these products are intended for the consumer market.
BPP projectors use blue lasers to provide blue light and to stimulate a phosphor to glow yellow. RGB projectors use separate red, green, and blue lasers.
One of the biggest differences between RGB and BPP projectors is light output—RGB models can reach 60,000 lumens or more, while commercial BPP designs max out at about 6000 lumens. (As we know from CEDIA, Epson's LS10000 consumer-oriented BPP model is rated at a maximum of 1500 lumens.)
Compared to the xenon lamps used in most current commercial projectors, lasers offer many advantages. One of the most important is longevity—lasers have an effective lifetime of 10,000 to 100,000 hours with little decline in brightness per hour of use. By contrast, xenon lamps must be replaced every 500 to 1000 hours or so.
RGB laser-illuminated projectors lose only 20% of their brightness over 30,000 hours of use, while BPP projectors lose that much brightness after about 8000 hours and reach half brightness after about 20,000 hours. Meanwhile, xenon lamps lose half their brightness after only 500-1000 hours, requiring up to 60 replacements during the lifetime of a single RGB laser engine.
Another advantage of lasers over xenon lamps is higher efficiency in converting electrical power to light output. Lasers can produce 5-6 lumens per watt from the wall, while xenon lamps produce 2-5 lumens per watt. Typically, laser projectors can reduce power consumption by 30-50% over lamp-based models.
Even so, RGB projectors can be as much as 2-3 times brighter than xenon lamp-based projectors, in part because lasers exhibit a low etendue, which means the light spreads out slowly from the source, whereas the light from a xenon lamp is emitted in all directions at once and must be focused. In addition, a xenon lamp creates light in a small area called the arc, which creates a bright spot, leading to image-uniformity problems that lasers don't have.
Current laser-illuminated projectors can achieve a full-on/full-off contrast ratio of 2300-3000:1, which is right in line with the DCI (Digital Cinema Initiative) spec of 2000:1. However, with redesigned projector optics, Beck says that a contrast ratio of 10,000:1 is possible. Of course, the content must be mastered with high dynamic range, and displaying such content on a laser-illuminated projector typically results in lower optical efficiency and more speckle.
Finally, lasers can provide a range color gamuts, though this capability is not without some problems. For one thing, lasers emit light in a very narrow range of wavelengths—essentially a single wavelength. This is great for the BT.2020 color gamut, whose primaries are single-wavelength, but this also results in speckling, an artifact that imparts a grainy look to the image—and not in a good, "film grain-like" way.
Another problem with narrowband primaries is called observer metameric failure. Metamerism is the matching of apparent colors with different spectral distributions; in video, any color within the gamut of the display, regardless of its spectral curve, can be matched with a unique mixture of red, green, and blue, which forms the basis of trichromatic colorimetry. Colors that match in this way are called metamers.
Humans perceive color thanks to light-sensitive cells called cones that coat the inner surface of the retina at the back of the eyeball. There are three types of cones, which are sensitive to different ranges of wavelengths that can roughly be categorized into red, green, and blue. The range of wavelengths for each type of cone is very broad, and different combinations of wavelengths can produce equivalent responses and thus the same color sensation.
However, the proportion of long, medium, and short wavelength-sensitive cones, the precise profile of light sensitivity in each type of cone, and other factors differ from one person to the next, so we all see slightly different colors. With broadband primaries, such as those from a lamp, these differences are obscured, and most of us see the image as having the same colors. But with narrowband primaries, such as those from lasers, differences in color perception become more pronounced, and different viewers will see different colors, an effect called observer metameric failure (OMF).
Here we see the 709, P3, and 2020 color gamuts and some notes about them as they pertain to laser-illuminated projectors.
Potential solutions to the speckle and OMF problems include using several different laser wavelengths for each primary and broadening the bandwidth of each primary. The red, green, and blue curves in each graph represent the sensitivities of the corresponding cones in the human eye. In this graphic, FWHM stands for "full width at half maximum," a measure of bandwidth.
When it comes to 3D, Christie, Barco, and others are working on so-called 6-primary (6P) designs in which slightly different wavelengths for red, green, and blue are used for each eye in conjunction with Dolby/Infitec passive-3D glasses. Barco puts both sets of lasers in one chassis and quickly alternates between them, while Christie uses dual projectors so that both sets of primaries can be on the screen simultaneously. Either way, Beck claims that image quality and brightness are much better than lamp-based color-separation or polarization systems.
Laser-illuminated projectors have come a long way in the commercial-cinema world, and that progress is bound to migrate to consumer products, as illustrated by the Epson LS10000. This technology offers many advantages and a few challenges compared with conventional lamps, but it seems to me that lasers have the potential to supplant lamps in the future. I look forward to following any developments in this particular corner of videophilia, and of course, I will report what I learn right here on AVS.
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