LED Technology

ARTICLE

EVERYTHING OLD IS NEW AGAIN
Even after newer technologies have faded, 30-year-old LED technology is still as bright as ever.

by Peter H. Putman, CTS

The history of large-screen display technology is littered with the remains of companies who were certain they had invented "the better mousetrap" from hybrid liquid crystal light valves (too unstable and complex) to laser CRT projection (which work great in the lab, but...) and analog micromirror imaging (not digital).

The struggle continues with existing technologies. In early January, an auction notice crossed my desk informing me on how I could participate in a web auction to bid on the assets of Colorado Micro Displays/Zight, a bankrupt manufacturer of liquid crystal on silicon (LCoS) imaging panels. Other LCoS fabricators continue to fight low yields and financial pressures.

So, what is the next big thing in large-venue imaging? Turns out that the biblical saying "the meek shall inherit the earth" isn't too far off. We need to look no further than the light-emitting diode (LED).

MIGHTY MIDGET

LEDs have been around for more than 30 years. This simple junction semiconductor emits continuous light when current flows through its junction at a low voltage. By itself, an LED is quite useful as a power saving indicator lamp. It's bright, has a fairly wide viewing angle, and can be arranged into matrices to display text, numerals, and even crude pictures.

LEDs also come in a variety of colors, including red, green, and blue -- the building blocks of any color imaging system. So how, exactly, can we incorporate them into a display capable of fast refresh rates -- fast enough to show SDTV and HDTV?

The answer is to think digital. By employing a pulse-width modulation (PWM) technique, we can switch diodes on and off fast enough to create grayscale values of R, G, and B. Because LEDs have a quick response time and no perceptible lag, PWM should and does work very well with LED imaging, making it possible for LEDs to follow the 30Hz or 60Hz refresh rates required by video. In fact, they can be refreshed several times faster, from 300Hz to 400Hz, if needed, to eliminate picture flicker and allow the display of rapid motion.

The next problem is light output. LEDs are certainly bright enough on our VTRs and cameras. But are they bright enough to use on an outdoor billboard? This task might seem impossible unless we think in terms of LED building blocks -- small matrices of LEDs that make up individual pixels. Such matrices would have at least a pair of green and red LEDs, plus a single blue LED. Or it might be expanded to have three each red and green LEDs, and a pair of blue diodes.

These individual matrices are a lot brighter than individual diodes, and make up a much larger pixel, one that appears very coarse to us as we view the LED display up close. The effect is like looking at a magazine photo with a magnifying glass. If you get close enough, all you see is the dot screen and not the image.

Because we have better alternatives for viewing electronic images up close, let's back off, say, 20 or 30ft., and take another look at our LED matrix. Now, it appears to be more a large pixel than a collection of dots. Stand even further back and all of a sudden the display takes on the characteristics of a large video screen. We don't see the LED matrix structures as much as we notice the images they form.

That's the key to using LED displays. They aren't intended to replace small-screen imaging. Rather, they are aimed at larger-than-life electronic displays, particularly those that must work under unforgiving ambient lighting. Typical applications would be at tradeshows, along highways, and in stadiums and arenas under full daylight or nighttime lighting.

What about color fidelity? It's better than you might think. The high refresh rate employed with LED displays allows for a wide color palette. Currently, the industry seems to be hovering around a 1.07 billion-color palette, which is defined as 1,024 colors per channel (or 1,024 shades each of red, green, and blue). This 10-bit color palette far exceeds that of any RGB imaging devices, with the exception of specialized electronic cinema projectors. And the displayable RGB color gamut is much larger than NTSC or HDTV gamuts.

THE NUTS AND BOLTS

LED displays are usually manufactured in tiles. Each tile might measure 40in. or so on a side and will contain rows of the pixel matrices described above. Low-voltage drivers for each pixel within the matrices are connected by a simple row-and-column system, similar to that used with plasma panels and older passive-matrix LCD displays. By simply applying a voltage on a given coordinate, the appropriate color LEDs within that pixel will fire.

The resulting panel is quite compact, similar to the individual tiles in a drop ceiling. Also, the tiles aren't very thick. A typical LED tile will have similar dimensions to a plasma monitor, so you can easily fly, hang, or flush-mount LED displays. If individual pixels happen to fail, they can be pushed out the front of the panel and replaced with another pixel. Weatherproof sealants around the pixel face keep the electronics high and dry under the harshest weather conditions.

The effective pixel pitch of a tiled display depends on the number of LEDs and their size, plus the diagonal measurement of each RGB diode grouping. They can resemble the face of playing dice. Smaller pitch sizes, such as 4mm, 6mm, and 8mm, are used for displays with close-up viewing, while larger-pitch 10mm, 15mm, 20mm, and even 40mm matrices are reserved for super-large screens with long viewing distances.

Brightness is proportionate to pitch. Panels sized 4mm will carry specifications of 700 to 1,000cd/m² (candelas per square meter), while 8mm panels can deliver from 1,000 to 1,200cd/m². If you are interested in more light output, you can get up to 6,000cd/m² from 25mm, 30mm, and 40mm pixel arrays. That makes for a blinding display and, certainly, one that's impossible to miss.

Achieving those brightness levels with tiled rear-projection cubes is possible, but requires more space and doesn't always work under high-ambient lighting. Front projectors are at an even bigger disadvantage -- light spilling on any imaging surfaces degrades contrast and raises black levels too high. Plus, projectors require a two-piece install with the screen, making the job more complex.

LED displays are always driven with progressive-scan signal sources, most often converted to SVGA (800x600) resolution. Scan converters and scalars are employed downstream from all video and RGB sources to provide constant output. So, all mixing, dissolve, wipe, and superimposition effects happen just as they would with interlaced and progressive-scan video.

LED displays are scalable. You can make them as big as you wish, and there have been some monster LED arrays constructed for public-display applications. One installation I visited a few years back was at PSINet Stadium in Baltimore, home of the Baltimore Ravens NFL team. This stadium featured a pair of 25ft. by 100ft. LED tiled displays (4:1 aspect ratio) at both ends of the stadium.

The Ravens media staff produced live coverage of games with a full production studio that would be the envy of any remote crew. Standard 480i cameras were run in 16x9 mode to provide the widescreen images, and a marked reticule in the viewfinder indicated the "safe" 4:1 area for action. Downstream scalars converted the main 480i feed to 600p to drive the LED walls. The display was quite impressive even in full daylight (more than 20,000lux).

LED installations usually incorporate some sort of ambient light sensors. There's no reason to run a panel at full power when ambient light levels are low. Super-bright LED displays also are hard on the eyes in darkened rooms. Similar technology can be found in plasma monitors, but they aren't capable of producing anything near the levels of an LED matrix without first going up in flames.

WHO'S GOT THEM?

The market for tiled LED displays has really taken off in the past five years. Some of the major players in LED include Barco, Lighthouse Technologies, Smartvision, and Electrosonic. The differences from one product to another aren't substantial (an LED is an LED, after all) and lie mostly in the size of the individual tiles and the number of LEDs in each pixel matrix.

Stadiums and arenas are obvious choices for LED displays, but LEDs also are gaining popularity in public concourses and malls. Themed retail stores are installing LED displays to pull shoppers in the door and keep them there. Live entertainment venues have hopped on the bandwagon with numerous touring musical acts using static and dynamic LED set pieces, including one that actually separated in half during the show.

Smartvision recently built a 9ft.x48ft., 15mm curved interactive LED display for the exterior of a new 110,000-sq.-ft. Toys R Us superstore, located on Broadway in New York City and occupying the facade of an existing movie marquee approximately 13ft. off the ground. The display is divided into two sections, the smaller section is 9ft.(high)x16ft.(wide) facing uptown and the larger is a curved 9ft.(high)x32ft.(wide) section facing downtown. Show & Tell Productions of Manhattan will be developing and producing programming software and content for this display.

According to Show & Tell's Phil Lenger, "We are very excited to be working with such a versatile display and intend to use it to its full potential to engage Toys R Us customers -- both inside and outside the store." Examples of interactive content include showcasing live events in the store, promoting special events, Toys R Us branding, toy trivia, movie trailers, and even a real time "stock ticker" of major toy stocks written in crayon. The Smartvision LED display is one of 20 major attractions contained in the 110,000-sq.-ft. flagship toy store.

The Toys R Us display is one of several curved, LED installations that Smartvision has completed recently. Perhaps the most famous is the Nasdaq Marketsite Tower display, which has a 45-ft. radius. Another is a convex and concave display for Paul Allen's Experience Music Project Museum in Seattle, which possesses an even tighter curve.

In 2001, Smartvision installed a 27ft.(high)x48ft.(wide) LED video display at Miller Park in Milwaukee, described as the most technologically advanced, aesthetically appealing ballpark in the world. The 270x484-resolution display contains 653,400 pixels using a 30mm dot pitch and is specified to deliver 5,000cd/m². According to Smartvision, the screen is bigger than a standard basketball court.

The Miler Park screen will be fed by a combination of SD video and SVGA graphics sources. "It's going to be a big part of the game presentation at Miller Park," said Michael Bucek, vice president of New Ballpark Development. "That's not something our fans are used to seeing. The clarity on this board is incredible. It's like watching television in somebody's house."

Barco will install a mix of seven indoor and outdoor LED video displays at the new Reliant Stadium in Houston, the first retractable-roof NFL football and rodeo stadium, in time for its September 2002 opening. The stadium will also host the Houston Livestock Show and Rodeo and the 2004 NFL Super Bowl. The stadium's highlight will be a pair 27ft.(high)x96ft.(wide) DLite 14mm LED displays for the north and south end zones. These displays, which are configured in an unusual 32:9 aspect ratio, will display two full-resolution, high-definition 16:9 aspect ratio images side-by-side, as well as a single 32:9 aspect ratio image.

In addition, Barco will hang four 7ft.x12ft. ILite LED displays to be located on the club-level areas of the stadium and one DLite 14 LED display in the Bud Plaza for pre- and post-event activities. Images will originate from an HDTV-capable production studio, feeding signals first as SD-SDI and then moving to HD-SDI with 10-bit signal processing. Sources will include Digital Betacam, HDCAM, and servers, and all image processing and scaling will be performed in a Barco D320 digitizer.

Steve Patterson, Houston Texans' executive vice president, says, "In replacing the Astrodome -- the eighth wonder of the world -- we felt we needed to create a video presentation that would 'wow' our patrons with 21st century technology the way the Astrodome impressed Houstonians in the 1960s. Barco's video displays and projectors will provide that 'wow' factor for us."

LED screens' ample brightness, wide viewing angle, and performance in less-than-ideal lighting make them a wise choice for large-venue displays, from trade shows to outdoor billboards to football arenas. It seems this 30-year-old technology has found its niche in the 21^st century.

Copyright 2002 Primedia Business Media / Peter H. Putman
This article appeared in the April 2002 issue of Video Systems.


						ARTICLE
					EVERYTHING OLD IS NEW AGAIN Even after newer technologies have faded, 30-year-old LED technology is still as bright as ever.


					by Peter H. Putman, CTS The history of large-screen display technology is littered with the remains of companies who were certain they had invented "the better mousetrap" from hybrid liquid crystal light valves (too unstable and complex) to laser CRT projection (which work great in the lab, but...) and analog micromirror imaging (not digital). The struggle continues with existing technologies. In early January, an auction notice crossed my desk informing me on how I could participate in a web auction to bid on the assets of Colorado Micro Displays/Zight, a bankrupt manufacturer of liquid crystal on silicon (LCoS) imaging panels. Other LCoS fabricators continue to fight low yields and financial pressures. So, what is the next big thing in large-venue imaging? Turns out that the biblical saying "the meek shall inherit the earth" isn't too far off. We need to look no further than the light-emitting diode (LED). MIGHTY MIDGET LEDs have been around for more than 30 years. This simple junction semiconductor emits continuous light when current flows through its junction at a low voltage. By itself, an LED is quite useful as a power saving indicator lamp. It's bright, has a fairly wide viewing angle, and can be arranged into matrices to display text, numerals, and even crude pictures. LEDs also come in a variety of colors, including red, green, and blue -- the building blocks of any color imaging system. So how, exactly, can we incorporate them into a display capable of fast refresh rates -- fast enough to show SDTV and HDTV? The answer is to think digital. By employing a pulse-width modulation (PWM) technique, we can switch diodes on and off fast enough to create grayscale values of R, G, and B. Because LEDs have a quick response time and no perceptible lag, PWM should and does work very well with LED imaging, making it possible for LEDs to follow the 30Hz or 60Hz refresh rates required by video. In fact, they can be refreshed several times faster, from 300Hz to 400Hz, if needed, to eliminate picture flicker and allow the display of rapid motion. The next problem is light output. LEDs are certainly bright enough on our VTRs and cameras. But are they bright enough to use on an outdoor billboard? This task might seem impossible unless we think in terms of LED building blocks -- small matrices of LEDs that make up individual pixels. Such matrices would have at least a pair of green and red LEDs, plus a single blue LED. Or it might be expanded to have three each red and green LEDs, and a pair of blue diodes. These individual matrices are a lot brighter than individual diodes, and make up a much larger pixel, one that appears very coarse to us as we view the LED display up close. The effect is like looking at a magazine photo with a magnifying glass. If you get close enough, all you see is the dot screen and not the image. Because we have better alternatives for viewing electronic images up close, let's back off, say, 20 or 30ft., and take another look at our LED matrix. Now, it appears to be more a large pixel than a collection of dots. Stand even further back and all of a sudden the display takes on the characteristics of a large video screen. We don't see the LED matrix structures as much as we notice the images they form. That's the key to using LED displays. They aren't intended to replace small-screen imaging. Rather, they are aimed at larger-than-life electronic displays, particularly those that must work under unforgiving ambient lighting. Typical applications would be at tradeshows, along highways, and in stadiums and arenas under full daylight or nighttime lighting. What about color fidelity? It's better than you might think. The high refresh rate employed with LED displays allows for a wide color palette. Currently, the industry seems to be hovering around a 1.07 billion-color palette, which is defined as 1,024 colors per channel (or 1,024 shades each of red, green, and blue). This 10-bit color palette far exceeds that of any RGB imaging devices, with the exception of specialized electronic cinema projectors. And the displayable RGB color gamut is much larger than NTSC or HDTV gamuts. THE NUTS AND BOLTS LED displays are usually manufactured in tiles. Each tile might measure 40in. or so on a side and will contain rows of the pixel matrices described above. Low-voltage drivers for each pixel within the matrices are connected by a simple row-and-column system, similar to that used with plasma panels and older passive-matrix LCD displays. By simply applying a voltage on a given coordinate, the appropriate color LEDs within that pixel will fire. The resulting panel is quite compact, similar to the individual tiles in a drop ceiling. Also, the tiles aren't very thick. A typical LED tile will have similar dimensions to a plasma monitor, so you can easily fly, hang, or flush-mount LED displays. If individual pixels happen to fail, they can be pushed out the front of the panel and replaced with another pixel. Weatherproof sealants around the pixel face keep the electronics high and dry under the harshest weather conditions. The effective pixel pitch of a tiled display depends on the number of LEDs and their size, plus the diagonal measurement of each RGB diode grouping. They can resemble the face of playing dice. Smaller pitch sizes, such as 4mm, 6mm, and 8mm, are used for displays with close-up viewing, while larger-pitch 10mm, 15mm, 20mm, and even 40mm matrices are reserved for super-large screens with long viewing distances. Brightness is proportionate to pitch. Panels sized 4mm will carry specifications of 700 to 1,000cd/m² (candelas per square meter), while 8mm panels can deliver from 1,000 to 1,200cd/m². If you are interested in more light output, you can get up to 6,000cd/m² from 25mm, 30mm, and 40mm pixel arrays. That makes for a blinding display and, certainly, one that's impossible to miss. Achieving those brightness levels with tiled rear-projection cubes is possible, but requires more space and doesn't always work under high-ambient lighting. Front projectors are at an even bigger disadvantage -- light spilling on any imaging surfaces degrades contrast and raises black levels too high. Plus, projectors require a two-piece install with the screen, making the job more complex. LED displays are always driven with progressive-scan signal sources, most often converted to SVGA (800x600) resolution. Scan converters and scalars are employed downstream from all video and RGB sources to provide constant output. So, all mixing, dissolve, wipe, and superimposition effects happen just as they would with interlaced and progressive-scan video. LED displays are scalable. You can make them as big as you wish, and there have been some monster LED arrays constructed for public-display applications. One installation I visited a few years back was at PSINet Stadium in Baltimore, home of the Baltimore Ravens NFL team. This stadium featured a pair of 25ft. by 100ft. LED tiled displays (4:1 aspect ratio) at both ends of the stadium. The Ravens media staff produced live coverage of games with a full production studio that would be the envy of any remote crew. Standard 480i cameras were run in 16x9 mode to provide the widescreen images, and a marked reticule in the viewfinder indicated the "safe" 4:1 area for action. Downstream scalars converted the main 480i feed to 600p to drive the LED walls. The display was quite impressive even in full daylight (more than 20,000lux). LED installations usually incorporate some sort of ambient light sensors. There's no reason to run a panel at full power when ambient light levels are low. Super-bright LED displays also are hard on the eyes in darkened rooms. Similar technology can be found in plasma monitors, but they aren't capable of producing anything near the levels of an LED matrix without first going up in flames. WHO'S GOT THEM? The market for tiled LED displays has really taken off in the past five years. Some of the major players in LED include Barco, Lighthouse Technologies, Smartvision, and Electrosonic. The differences from one product to another aren't substantial (an LED is an LED, after all) and lie mostly in the size of the individual tiles and the number of LEDs in each pixel matrix. Stadiums and arenas are obvious choices for LED displays, but LEDs also are gaining popularity in public concourses and malls. Themed retail stores are installing LED displays to pull shoppers in the door and keep them there. Live entertainment venues have hopped on the bandwagon with numerous touring musical acts using static and dynamic LED set pieces, including one that actually separated in half during the show. Smartvision recently built a 9ft.x48ft., 15mm curved interactive LED display for the exterior of a new 110,000-sq.-ft. Toys R Us superstore, located on Broadway in New York City and occupying the facade of an existing movie marquee approximately 13ft. off the ground. The display is divided into two sections, the smaller section is 9ft.(high)x16ft.(wide) facing uptown and the larger is a curved 9ft.(high)x32ft.(wide) section facing downtown. Show & Tell Productions of Manhattan will be developing and producing programming software and content for this display. According to Show & Tell's Phil Lenger, "We are very excited to be working with such a versatile display and intend to use it to its full potential to engage Toys R Us customers -- both inside and outside the store." Examples of interactive content include showcasing live events in the store, promoting special events, Toys R Us branding, toy trivia, movie trailers, and even a real time "stock ticker" of major toy stocks written in crayon. The Smartvision LED display is one of 20 major attractions contained in the 110,000-sq.-ft. flagship toy store. The Toys R Us display is one of several curved, LED installations that Smartvision has completed recently. Perhaps the most famous is the Nasdaq Marketsite Tower display, which has a 45-ft. radius. Another is a convex and concave display for Paul Allen's Experience Music Project Museum in Seattle, which possesses an even tighter curve. In 2001, Smartvision installed a 27ft.(high)x48ft.(wide) LED video display at Miller Park in Milwaukee, described as the most technologically advanced, aesthetically appealing ballpark in the world. The 270x484-resolution display contains 653,400 pixels using a 30mm dot pitch and is specified to deliver 5,000cd/m². According to Smartvision, the screen is bigger than a standard basketball court. The Miler Park screen will be fed by a combination of SD video and SVGA graphics sources. "It's going to be a big part of the game presentation at Miller Park," said Michael Bucek, vice president of New Ballpark Development. "That's not something our fans are used to seeing. The clarity on this board is incredible. It's like watching television in somebody's house." Barco will install a mix of seven indoor and outdoor LED video displays at the new Reliant Stadium in Houston, the first retractable-roof NFL football and rodeo stadium, in time for its September 2002 opening. The stadium will also host the Houston Livestock Show and Rodeo and the 2004 NFL Super Bowl. The stadium's highlight will be a pair 27ft.(high)x96ft.(wide) DLite 14mm LED displays for the north and south end zones. These displays, which are configured in an unusual 32:9 aspect ratio, will display two full-resolution, high-definition 16:9 aspect ratio images side-by-side, as well as a single 32:9 aspect ratio image. In addition, Barco will hang four 7ft.x12ft. ILite LED displays to be located on the club-level areas of the stadium and one DLite 14 LED display in the Bud Plaza for pre- and post-event activities. Images will originate from an HDTV-capable production studio, feeding signals first as SD-SDI and then moving to HD-SDI with 10-bit signal processing. Sources will include Digital Betacam, HDCAM, and servers, and all image processing and scaling will be performed in a Barco D320 digitizer. Steve Patterson, Houston Texans' executive vice president, says, "In replacing the Astrodome -- the eighth wonder of the world -- we felt we needed to create a video presentation that would 'wow' our patrons with 21st century technology the way the Astrodome impressed Houstonians in the 1960s. Barco's video displays and projectors will provide that 'wow' factor for us." LED screens' ample brightness, wide viewing angle, and performance in less-than-ideal lighting make them a wise choice for large-venue displays, from trade shows to outdoor billboards to football arenas. It seems this 30-year-old technology has found its niche in the 21^st century. Copyright 2002 Primedia Business Media / Peter H. Putman This article appeared in the April 2002 issue of Video Systems.