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The Many Colors of White LED
By Louis M Brill
As white light LEDs make their way into homes and businesses, so is their shining presence progressively showing up in signage as an illuminated source for channel letters, edge lighting and in a few cases, small displays. As white LEDs increase in brightness, power and cost efficiency, they continue to fill these sign lighting niches with an increasing demand from sign designers and fabricators.
The presence of white is a many splendor thing that appears in several forms in a wide range of colors from "warm" incandescent white to a "cooler" florescent white, depending on its application. Not only does white appear as a solid color, but it can also present itself as a subjective color, mixed from a blend of red, green and blue. Color mixing LEDs are used specifically for full color and video LED boards, where white is a blended color. This article focuses only on the spectral range of white as a pure color with applications for sign illumination.
In the early years of white LEDs (late-1990s) its initial appearance was slightly "murky" as much of the LED white light had color aberrations with fringes of pink or blue embedded within the white light, making it less desirable as a light source. In today's LED manufacturing community, white LEDs have come a long way in terms of brightness, uniformity of illumination, color efficiency and reduced cost per lumen.
"The overall efficacy of white light LEDs has more than doubled within the last two years and has done so with comparable price declines, making it a more affordable lighting product for secondary manufacturing usage," says Bob Steele, director of optoelectronics, Strategies Unlimited, (Mountain View CA) and conference chair for an annual High Brightness (HB) LED conference held every first quarter in the Bay Area.
Steele noted that white light LEDs are now in steady demand and are being directed to a number of application markets (mobile communications, general illumination, automobile instrument panels, and signage). Currently there is enough demand for white LEDs that more than 50% of the overall consummation of high brightness LEDs is accounted for by white. Of that demand for white LEDs, signage has become a niche market with the greatest percentage of white light LEDs going toward channel letter illumination (both front and rear lit), and a smaller use (at this time) for display case illumination.
Lumens/watt (lpw) is the standard used by the lighting industry to measure the conversion of electrical energy to light. As a reference, conventional incandescent light bulbs are typically in the 10 to 20 lumens per watt range, while compact fluorescent lamps range from 50 to 60 lumens per watt. Steele observed that several years ago the standard measure for LED brightness was about 30 lpw. By 2006, the efficacy of white light LEDs has more than doubled, as noted in a recent press release by Cree Lighting (Durham, NC), an advanced semiconductor company who recently announced breakthrough performance results achieved in their standard white 7090 Power LED. XLamp.™ In that presentation, Cree was able to demonstrate a typical luminous flux of 80 lumens corresponding to 70 lumens per watt at 350 mA. This represents a 43 percent increase in brightness compared with the maximum luminous flux of their white LEDs currently in production.
As for fabricating white LEDs there are several ways of creating white, but the most popular approach involves using a blue LED as a basic light source that is illuminated against a yellow phosphor, which in turn creates a 'fluorescing' white or what is known as a 'phosphor-converted' LED. In terms of packaging LEDs, once the 'die' is created (this is the equivalent to a light bulb filament), the LED can be packaged in one of several ways including a surface mounted device (SMD) or is encapsulated in a clear epoxy whose end point acts as a lens to direct the light as a focused beam.
To delineate a specific chromatic gamut,, colors are measured in a temperature range known as Kelvin. The spread of white within this part of the spectrum has a very wide range from the low end of 2800o K which is in the range of a warm, 'manila' colored white to the high end of the white scale which tops out at 9000 o K where it is seen as a glacial 'blue-white.' The sweet spot of white is about 6500 o K, which is considered the standard for a fully saturated white. The many colors of white are defined by the various light sources used in indoor/outdoor settings such as the office (the cool "bluish" white from fluorescent lamps), the home (incandescent "yellowish" white), industrial (brilliant "blue-white" of mercury vapor and metal halide lamps), and street settings ("yellow-white" of high-pressure sodium street lamps).
In terms of white light product usage, Gordon Routledge, marketing director and founder of LumiDrives (Knavesborough, North Yorkshire, England) noted, "when it comes to white, everyone's an expert, and they'll tell you what works for them and how much more "real" they want it to be as a replacement to their existing incandescent and florescent lighting.”
The creation of LEDs can be described in many ways and in the interest of properly following its fabrication process, we refer to its manufacturing order of creation (MOC) as follows: Level I (manufacturing of the die), Level II (transferring the die into a component chip), Level III (turning the chip into an LED lighting fixture), and Level IV (using the LED lighting fixture as a sign illumination source). In the interest of understanding white light LEDs, various LED companies from different levels of the MOC have been interviewed to shine some light on where LED white lights are going within the sign market arena.
Nichia (a level I and a level II company) sees the application of signage as a growing market starting with channel letters, which has already been a vast consumer of red LEDs. Now that white light LEDs have significantly improved as a lighting source, they too are being applied in greater numbers to the signage community.
Currently Nichia offers at least 30 or more LED package styles with units designed to satisfy different market applications including cell phone back lighting, LCD television back lighting, general illumination, and automotive exterior and interior lighting and signage. To satisfy that demand, Nichia offers at least four of its LED module packages to the sign community as follows:
* Rigel ˝ W
* Power Rigel 1 watt
Osram Opto Semiconductors
Osram fabricates the basic LED component and develops the light source as a Level I, II, and III manufacturer. Osram has been working with white LEDs since 2000, says Brian Terao, Senior Product Marketing Manager of LED Products, "Our LED creation process utilizes the blue LED with yellow phosphor to produce a solid state white light and we have an ongoing program to continually improve our white light which allows us to address different kinds of applications, particularly signage."
The company's product portfolio offers a variety of LED light sources which allows a sign designer to choose specific types of LEDs which would best fit their application. The different packages allow the sign designer different levels of performance to address the varying needs and targets of various sign project applications, both in form factor and in luminance value:
* Power TOPLED (LW E6SG)
* Advanced Power TOPLED (LW G6SP),
* Golden DRAGON version 1 (LW W5SG)
* Golden DRAGON version 2 (LW W5SM)
Osram as an international LED lighting distributor has found that between the United States and Europe, each region has its own sensibilities about what ranges of white light they want for their signage and lighting needs. "At this time, we have found the key market growth in signage is in channel letter illumination, followed by back lighting for display cases", notes Terao.
"White light LEDs have benefited from significant technical and performance improvements since their earlier days," says SloanLEDs marketing Director, Ron Wallace. "In their early years, white light LEDs were probably consumed (mostly with channel letters) in only about 5% of the total use of LEDs, which was pretty much dominated by red LEDs. In today's lighting marketplace, white light LEDs have been greatly improved with decreased cost per unit, increased efficiency of brightness and a longer product life (now quoted at about 50,000 hrs of constant use). Because of these overall improvements, white light LEDs have become one of our top categories for sign illumination."
As for how much of the range of the white color spectrum SloanLED has focused on,Wallace noted, "in the interest of simplicity and consistency, we offer one shade of white at 6500 K which we feel is the 'sweet spot' of white. This creates more of a standard and less confusion for sign designers in trying to match an LED white to the white of their logo brand. Because the LED white light is better, it's even easier now to back light and illuminate a full color sign."
SloanLED provides sign designers and fabricators with a number of different LED packages allowing it to be applied to all kinds of sign applications.
* Great White
* White Short
* Mini White
* LED strip;
LED T12 Lighting Units
The T12 LED white lighting fixture is a new product being tested as a back lighting fixture for street signs in several cities around the United States. The company is pleased enough with the product that it will soon offer a four-foot and a six-foot white LED lighting fixture to expand its presence as a fluorescent tube competitor in that part of the signage lighting market.
FloodLED Flood Light
The significance of our consummation of incandescent, florescent and HID white light is in front of our eyes every day. We see it in our homes and offices, on our desks and ceilings, and in the streets on signs and lamp posts. While the light is bright, at what cost in terms of high wattage consumption and inefficient power usage?
Christopher James, marketing manager for Cree Lighting (Durham, NC), dramatically summarized LEDs emergence as an evolved lighting source" "The real game is how LEDs will replace the light bulb." He said this isn't a question of "if." but "when." The timing, he said relies on technological improvements and market acceptance by both the business community and the public. Thus the biggest challenge for LED providers is to convince consumers that properly designed LED products are, or will be an affordable solution to existing incandescent and/or florescent lighting products.
The tide of white light LEDs is following a path already carved out by monochromatic (particularly red and amber) LEDs. Witness the ongoing changes in automotive brake lighting systems, in traffic lights and in our own world of signage as LEDs have become more and more a permanent part of seeing signs in their best light. As noted in a recent LED conference, “The battle for white LEDs is going to be fought application by application and lumen by lumen." Amen to that.
Louis M. Brill is a journalist and consultant for high-tech entertainment and media communications. He can reached at (415) 664-0694 or firstname.lastname@example.org
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