LASERS: the next wave?
Feb 1, 1999 12:00 PM,
Peter H. Putman
A benefit of writing about large-screen electronic display technology isthat I sometimes get a chance to look into the crystal ball. To be sure, Ihave my hands full keeping up with all of the new projectors, monitors,interfaces, screens and distribution/routing equipment rolling out into themarketplace each month. On the other hand, it is always a welcome break toread about-or see firsthand-a new way to create and display electronicimages.
In December 1998, I left for Portsmouth, NH, to visit the offices ofCOLOR-the COrporation for Laser Optics Research. COLOR was originallyformed in 1987 to develop and market laser imaging engines for everythingfrom still graphics to full-motion video. Its first patent was granted in1988 for developing an acousto-optic modulation system to be used withpulsed lasers for showing video images.
Since 1996, COLOR has developed and installed several ColorVisionlarge-screen display systems in locations as diverse as an indoor arena inProvidence, RI, the baggage area at Manchester, NH, airport and at theMuseum of Science in Boston. These displays show both graphics and video onscreens ranging from 7.59×109 (2.3 m x 3 m) to 99×129 (2.7 m x 3.7 m)
What is the big deal with lasers? It all has to do with efficiency. Lasers,unlike projector lamps, are coherent light sources; their photons do notdisperse as they move away from the source. A beam of laser light isessentially the same diameter 1 mile (1.6 km) from its source as it is 1foot (305 mm). On the other hand, a beam of light from a projection lampwould be so dispersed at 1 mile that it would be difficult to see ormeasure. Even the beam from an electron gun in a vacuum tube will diffuseand disperse over a long path.
With a coherent source, less energy is required to produce a givenintensity or image brightness. That is primarily due to the response of oureyes, which respond to certain colors more strongly than others. Althoughwe can see a wide spectrum of green colors, there is one wavelength inparticular (550 nm) at which we have peak sensitivity. The same holds truefor red (600 nm) and blue (445 nm). Essentially, our eyes perform some ofthe needed amplification.
Now, we have three pure color sources optimized for our optical bandpassfilters. If each laser can be modulated to trace an image, a fairlyefficient projection system results. COLOR accomplishes this by using apiezoelectric crystal and modulating it with acoustically coupled energy.The shape of the crystal changes slightly in response to the changingamplitude-modulated signal (yes, AM), deflecting the laser beam accordingly.
With a single laser, a raster-scanned image can be reproduced with a 30 Hz(or even 60 Hz) picture refresh. Modulating and converging red, green andblue lasers yields a full-color image. Of course, such laser artifacts asspeckle (an apparent graininess to the image) must be filtered out to makethe final image more pleasing to the eye. There is also the question ofgetting enough energy from blue lasers to match the levels of the red andgreen units, not to mention putting all of this stuff into a workablechassis and powering it.
The folks at COLOR have done just that, and they are achieving good resultswith projected video. Granted, the laser/power supply rack is large, soColorVision displays are best suited for installs where there is a littlebit of real estate, such as inside a hanging scoreboard. On the other hand,optics are not as much of an issue as they would be with a regularprojector because the laser beam stays converged over an infinite distance.This eliminates a lot of focus and depth-of-field problems, and also makesit possible to project on uneven or curved surfaces.
Most importantly, lasers are resolution-independent imaging devices. Theirspot size is small enough to trace all the current computer and videoresolutions, not tomention both 1,280×720 and 1,920×1080 HDTV. The key isthat unlike a CRT projection engine, beam spot size is totally independentfrom beam intensity. Also, intensity is largely independent of projectiondistance. Think of a super-powered, lensless CRT projection system.
There are other ways to harness laser light for electronic imaging. BobMartinsen, COLOR’s director of optical engineering, also showed me aprototype three-color projector using LCD panels. Instead of usingamplitude modulation to deflect the laser beams, the LCD panel became themodulating surface. The lasers were now used as pure RGB sources, followinga rather complex optical path before being diffused (like conventionalprojected light) and focused onto three individual 1,024×768 polysiliconpanels.
The resulting images then passed through a combining prism/light integratorand through a lens to a nearby screen, showing text patterns and imagesfrom the 1998 INFOCOMM Projection Shoot-Out. This has to be the world’slargest desktop projection system. What I saw appeared to be a typicalimage from an LCD projector, except that there was no overall color cast ortint caused by using a metal-halide projection lamp.
Optical system efficiency was high, too. The final image measured around500 ANSI lumens in brightness, but there was not much more than that tobegin with. Contrast this with a standard desktop projector where theprojection lamp must generate up to 10x the light actually measured on thescreen to overcome scattering and refraction losses. If the light does notscatter or refract, light integrators and condensers can be tossed out.Starting with RGB light sources further does away with dichroic filters andmirrors, making for an efficient projection system.
There are a few catches. I mentioned size earlier, which is being resolvedby using diode laser sources. These are considerably smaller and will makeit possible to design a three-laser projection system about the size of acurrent lightvalve projector. The other catch is a bit more puzzling-thenarrow spectral response of the lasers results in images that do not havethe beautiful whites of xenon lamps or three-gun CRT projectors.
Although the laser RGB images do produce a clean white, there are othercolor impurities that make up white in our eyes. Dichroic filters are notquite as narrow-banded. The red dichroic in a projector may pass everythingfrom violet to warm yellows, while the green dichroic will also pick upsome yellows and even blue shades, like aqua. The blue dichroics will passeverything from green/blue shades to violet, letting a little red in. CRTphosphors are also somewhat broad-banded. In a manner similar to audioharmonics, these blends of colors produce a white that is good enough forour eyes.
By concentrating its energy along a narrow wavelength, the spectral purityof a given laser color is high. This means the recombination of red, greenand blue images may lack warmth simply because shades of yellow, amber,orange and other colors are missing. Of course, the same argument is usedby audiophiles who think digital sound is too pure and also lacks warmth,probably due to the lack of harmonics previously heard on tube and oldersolid-state equipment.
The solution may lie in specially coated screen surfaces or opticalcoatings that could add the missing color spectra after the red, green andblue laser-generated images have been reconverged. Despite this, theprinciples demonstrated in both the acousto-coupled modulation andthree-color additive projection systems are sound, and they will beginshowing up in more large-screen displays during the coming months.