Lasers

ARTICLE

LASERS:
The Next Wave in Large-Screen Displays?

by Peter H. Putman, CTS

One of the benefits of writing about large-screen electronic display technology is that you get a chance once in a while to look into the "crystal ball". To be sure, I have my hands full keeping up with all of the new projectors, monitors, interfaces, screens, and distribution/routing equipment rolling out into the marketplace each month. It's always a welcome break to read about - or see first-hand - a new way to create and display electronic images.

In December 1998, I made the trek to Portsmouth, New Hampshire to visit of the offices of COLOR - the COrporation for Laser Optics Research. COLOR has been around since 1987, and was originally formed to develop and market laser imaging engines for everything from still graphics to full-motion video. Their first patent was granted in 1988 for developing an acousto-optic modulation system, to be used with pulsed lasers for showing video images.

Since 1996, COLOR has developed and installed several ColorVision large-screen display systems in locations as diverse as an indoor arena in Providence, RI, in the baggage area at Manchester (NH) airport and at the Museum of Science in Boston. All of these displays show both graphics and video on screens ranging from 7.5' x 10' in size to 9' x 12'.

So - what's the big deal with using lasers? It all has to do with efficiency. Lasers (unlike projector lamps) are coherent light sources, meaning their photons don't disperse as they move away from the source. A beam of laser light is essentially the same diameter 1 mile distant from its source as it is 1 foot distant. On the other hand, a beam of light from a projection lamp would be so dispersed at a distance of one mile that you'd be hard-pressed to see it or measure it. Even the beam from an electron gun in a vacuum tube will diffuse and disperse over a long path.

With a coherent source, less energy is required to produce a given intensity, or image brightness. That's primarily due to the response of our eyes, which respond to certain colors more strongly than others. Although we can see a wide spectrum of green colors, there is one wavelength in particular (600 nanometers) at which we have peak sensitivity. The same holds true for red (550 nanometers) and blue (445 nanometers). In effect, our eyes perform some of the needed amplification!

Now, we've got three pure color sources that are optimized for our optical "bandpass filters". If each laser can be somehow modulated to trace an image, a fairly efficient projection system results. COLOR does this by using a piezoelectric crystal and modulating it with acoustically-coupled energy. The shape of the crystal changes ever-so-slightly in response to the changing amplitude-modulated (yes, AM!) signal, deflecting the laser beam accordingly.

With a single laser, a raster-scanned image can be easily reproduced with a 30 Hz (and even 60 Hz) picture refresh. By modulating and converging red, green, and blue lasers, a full-color image is produced. Of course, laser artifacts such as "speckle" (an apparent graininess to the image) must be filtered out to make the final image more pleasing to the eye. There's also the question of getting enough energy from blue lasers to match the levels of the red and green units, not to mention putting all of this stuff into a workable chassis and powering it!

But the folks at COLOR have done just that, and are achieving pretty good results with projected video. Granted, the laser/power supply rack is large, so ColorVision displays are best-suited for installs where there is a little bit of real estate, such as inside a hanging scoreboard. But optics aren't as much of an issue as they would be with a regular projector, since the laser beam stays converged over an infinite distance. This eliminates a lot of focus and depth-of-field problems, and also makes it possible to project on uneven or curved surfaces.

Most importantly, lasers are resolution-independent imaging devices. Their spot size is small enough to trace all the current computer and video resolutions, not to mention both 1280x720 and 1920x1080 HDTV. The key is that unlike a CRT projection engine, beam spot size is totally independent from beam intensity. Also, intensity is largely independent of projection distance! Think of a super-powered, lensless CRT projection system and you've got the picture.

There are other ways to harness laser light for electronic imaging. Bob Martinsen, COLOR's director of optical engineering, also showed me a prototype 3-color projector using LCD panels. Instead of using amplitude modulation to deflect the laser beams, the LCD panel became the modulating surface. The lasers were now used as "pure" RGB sources, following a rather complex optical path before being diffused (like conventional projected light) and focused onto three individual 1024x768 polysilicon panels.

The resulting images then passed through a combining prism/light integrator and through a lens to a nearby screen, showing text patterns and images from the 1998 INFOCOMM® Projection Shoot-Out®. (This has to be the world's largest "desktop" projection system!) What I saw appeared to be a typical image from an LCD projector, except that there was no overall color cast or tint caused by using a metal-halide projection lamp.

Optical system efficiency was very high, too. The final image measured around 500 ANSI lumens in brightness, but there wasn't much more than that to begin with! Contrast this with a standard desktop projector, where the projection lamp must generate up to ten times the light actually measured on the screen to overcome scattering and refraction losses. If the light doesn't scatter or refract, light integrators and condensers can be tossed out. Starting with RGB light sources further does away with dichroic filters and mirrors, making for a very efficient projection system.

Of course, there are a few catches. I mentioned size earlier, which is being resolved by using diode laser sources. These are considerably smaller and will make it possible to design a 3-laser projection system about the size of a current-model light-valve projector. The other catch is a bit of a head-scratcher - the narrow spectral response of the lasers results in images that don't have the beautiful whites of xenon lamps, or even three-gun CRT projectors.

While the laser RGB images do produce a clean white, there are other color impurities that make up "white" in our eyes. Dichroic filters aren't quite as narrow-banded as - the red dichroic in a projector may pass everything from violet to warm yellows, while the green dichroic will also pick up some yellows and even blue shades like aqua. The blue dichroics will pass everything from green/blue shades to violet, letting a little red in. (CRT phosphors are also somewhat broad-banded.) Like harmonics, these blends of colors produce a "white" that's good enough for our eyes.

By concentrating its energy along a narrow wavelength, the spectral purity of a given laser color is very high. This means the re-combination of red, green, and blue images may lack "warmth", simply because shades of yellow, amber, orange, etc. are missing. Of course, the same argument is used by audiophiles who think digital sound is "too pure" and also lacks "warmth", probably due to the lack of harmonics previously heard on tube and older solid-state equipment.

The solution may lie in specially-coated screen surfaces, or in optical coatings that could add in the "missing" color spectra after the red, green, and blue laser-generated images have been reconverged. Despite this, the principles demonstrated in both the acousto-coupled modulation and three-color additive projection systems are sound, and will begin showing up in more large-screen displays in the coming months.

This article currently appears in the February 1999 Issue of Sound & Video Contractor.

©1999 Peter H. Putman / Primedia Intertec


						ARTICLE
					LASERS: The Next Wave in Large-Screen Displays?


					by Peter H. Putman, CTS One of the benefits of writing about large-screen electronic display technology is that you get a chance once in a while to look into the "crystal ball". To be sure, I have my hands full keeping up with all of the new projectors, monitors, interfaces, screens, and distribution/routing equipment rolling out into the marketplace each month. It's always a welcome break to read about - or see first-hand - a new way to create and display electronic images. In December 1998, I made the trek to Portsmouth, New Hampshire to visit of the offices of COLOR - the COrporation for Laser Optics Research. COLOR has been around since 1987, and was originally formed to develop and market laser imaging engines for everything from still graphics to full-motion video. Their first patent was granted in 1988 for developing an acousto-optic modulation system, to be used with pulsed lasers for showing video images. Since 1996, COLOR has developed and installed several ColorVision large-screen display systems in locations as diverse as an indoor arena in Providence, RI, in the baggage area at Manchester (NH) airport and at the Museum of Science in Boston. All of these displays show both graphics and video on screens ranging from 7.5' x 10' in size to 9' x 12'. So - what's the big deal with using lasers? It all has to do with efficiency. Lasers (unlike projector lamps) are coherent light sources, meaning their photons don't disperse as they move away from the source. A beam of laser light is essentially the same diameter 1 mile distant from its source as it is 1 foot distant. On the other hand, a beam of light from a projection lamp would be so dispersed at a distance of one mile that you'd be hard-pressed to see it or measure it. Even the beam from an electron gun in a vacuum tube will diffuse and disperse over a long path. With a coherent source, less energy is required to produce a given intensity, or image brightness. That's primarily due to the response of our eyes, which respond to certain colors more strongly than others. Although we can see a wide spectrum of green colors, there is one wavelength in particular (600 nanometers) at which we have peak sensitivity. The same holds true for red (550 nanometers) and blue (445 nanometers). In effect, our eyes perform some of the needed amplification! Now, we've got three pure color sources that are optimized for our optical "bandpass filters". If each laser can be somehow modulated to trace an image, a fairly efficient projection system results. COLOR does this by using a piezoelectric crystal and modulating it with acoustically-coupled energy. The shape of the crystal changes ever-so-slightly in response to the changing amplitude-modulated (yes, AM!) signal, deflecting the laser beam accordingly. With a single laser, a raster-scanned image can be easily reproduced with a 30 Hz (and even 60 Hz) picture refresh. By modulating and converging red, green, and blue lasers, a full-color image is produced. Of course, laser artifacts such as "speckle" (an apparent graininess to the image) must be filtered out to make the final image more pleasing to the eye. There's also the question of getting enough energy from blue lasers to match the levels of the red and green units, not to mention putting all of this stuff into a workable chassis and powering it! But the folks at COLOR have done just that, and are achieving pretty good results with projected video. Granted, the laser/power supply rack is large, so ColorVision displays are best-suited for installs where there is a little bit of real estate, such as inside a hanging scoreboard. But optics aren't as much of an issue as they would be with a regular projector, since the laser beam stays converged over an infinite distance. This eliminates a lot of focus and depth-of-field problems, and also makes it possible to project on uneven or curved surfaces. Most importantly, lasers are resolution-independent imaging devices. Their spot size is small enough to trace all the current computer and video resolutions, not to mention both 1280x720 and 1920x1080 HDTV. The key is that unlike a CRT projection engine, beam spot size is totally independent from beam intensity. Also, intensity is largely independent of projection distance! Think of a super-powered, lensless CRT projection system and you've got the picture. There are other ways to harness laser light for electronic imaging. Bob Martinsen, COLOR's director of optical engineering, also showed me a prototype 3-color projector using LCD panels. Instead of using amplitude modulation to deflect the laser beams, the LCD panel became the modulating surface. The lasers were now used as "pure" RGB sources, following a rather complex optical path before being diffused (like conventional projected light) and focused onto three individual 1024x768 polysilicon panels. The resulting images then passed through a combining prism/light integrator and through a lens to a nearby screen, showing text patterns and images from the 1998 INFOCOMM® Projection Shoot-Out®. (This has to be the world's largest "desktop" projection system!) What I saw appeared to be a typical image from an LCD projector, except that there was no overall color cast or tint caused by using a metal-halide projection lamp. Optical system efficiency was very high, too. The final image measured around 500 ANSI lumens in brightness, but there wasn't much more than that to begin with! Contrast this with a standard desktop projector, where the projection lamp must generate up to ten times the light actually measured on the screen to overcome scattering and refraction losses. If the light doesn't scatter or refract, light integrators and condensers can be tossed out. Starting with RGB light sources further does away with dichroic filters and mirrors, making for a very efficient projection system. Of course, there are a few catches. I mentioned size earlier, which is being resolved by using diode laser sources. These are considerably smaller and will make it possible to design a 3-laser projection system about the size of a current-model light-valve projector. The other catch is a bit of a head-scratcher - the narrow spectral response of the lasers results in images that don't have the beautiful whites of xenon lamps, or even three-gun CRT projectors. While the laser RGB images do produce a clean white, there are other color impurities that make up "white" in our eyes. Dichroic filters aren't quite as narrow-banded as - the red dichroic in a projector may pass everything from violet to warm yellows, while the green dichroic will also pick up some yellows and even blue shades like aqua. The blue dichroics will pass everything from green/blue shades to violet, letting a little red in. (CRT phosphors are also somewhat broad-banded.) Like harmonics, these blends of colors produce a "white" that's good enough for our eyes. By concentrating its energy along a narrow wavelength, the spectral purity of a given laser color is very high. This means the re-combination of red, green, and blue images may lack "warmth", simply because shades of yellow, amber, orange, etc. are missing. Of course, the same argument is used by audiophiles who think digital sound is "too pure" and also lacks "warmth", probably due to the lack of harmonics previously heard on tube and older solid-state equipment. The solution may lie in specially-coated screen surfaces, or in optical coatings that could add in the "missing" color spectra after the red, green, and blue laser-generated images have been reconverged. Despite this, the principles demonstrated in both the acousto-coupled modulation and three-color additive projection systems are sound, and will begin showing up in more large-screen displays in the coming months. This article currently appears in the February 1999 Issue of Sound & Video Contractor. ©1999 Peter H. Putman / Primedia Intertec