LEDs are still popular and improving after all these years: Part 1
This article discusses a 35-year-old display technology that itself has rapidly changed - the LED. This overview covers the origins of LEDs, the traditional applications, and how improvements in the technology have stimulated new applications.
Source of information: Maxim Integrated Products, Inc. & Dallas Semiconductor Corp.
Introduction
In recent years, articles have focused on new display technologies. Articles have covered the explosion of TFT color LCD panels of ever-increasing size into laptops and flat screen monitors; PDP (plasma display panels) for high definition TV CRT replacement; polymer LED (PLED) or organic LED (OLED) displays for the small color displays in games, cell phones, and PDAs.
A Brief History of LEDs
Commercial research into LED technology started in the early 1962s, notably at Bell Labs, Hewlett-Packard, IBM, Monsanto, and RCA. Work on gallium arsenide phosphide (GaAsP) led to the introduction of the first commercial 655nm red LEDs in 1968, by H-P and Monsanto. In 1971 H-P released the 5300A 500MHz portable frequency counter using a GaAsP LED display. LED displays flourished in the early 1970s as numeric displays in pocket calculators by HP, Texas Instruments, Sinclair and others. For a short time, LEDs appeared in digital watches, but were soon replaced by LCDs. Meanwhile, LEDs replaced incandescent and neon lamps as status indicators and became the standard numeric and alphanumeric display choice for instrumentation.
The LED's hottest competition in the 1970s and 1980s for consumer goods came from vacuum fluorescent displays (VFDs), whose bright blue-green display offered high intensity and high contrast when viewed through a green or blue filter. VFDs were first developed by ISE Electronic Corporation in 1967. ISE, often known by the division name of Noritake, together with Futaba and NEC, offered display tubes from the late 1960s and early 1970s, starting with simple single digit displays used in rapidly growing desktop calculator market.
Multi-digit display tubes appeared soon, reducing manufacturing cost, and these are possibly best remembered for their appearance in the popular Casio pocket calculators. Later, Samsung started making tubes for their own consumption for use in consumer goods. In 1993, NEC sold their complete manufacturing line to ZEC in China, and between them Futaba, ISE, Samsung, and ZEC produce around 95% of the world's VFD tubes production.
In the 1980s and onward, monochrome LCDs competed strongly with LEDs and VFDs for consumer devices, instrumentation, and automotive panels. LCDs have the advantage of lowest power and easily customization, and became the obvious choice for battery operated applications. Although LCDs don't emit light, there are many applications where ambient light can be guaranteed. Alternatively, the light from a couple of green, orange, or yellow LEDs can be diffused and spread behind a small (10 square centimeter) LCD with an opaque plastic molding, to provide a cheap and pleasant backlight.
Who Manufactures LEDs?
The worldwide production of LEDs is now around 4 billion units a month. According to ITIS (Industrial Technology Information Service) of Taiwan, Taiwan now produces around half the world's demand from its over 30 LED manufacturers, with Japan and USA being the next most productive. Ten years ago, Japan was the lead producer, and Taiwan's output was a little over 10% of the world's demand. Most LED manufacturers are actually assemblers and packagers, buying wafers or dice from foundries in Japan, USA, and (more recently) Taiwan.
The C.I.E., Lumens and Candelas
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| Fig. 1 Color spectrum wavelength in nanometers | Fig. 2 Photopic chromaticity diagram |
It's probably helpful to include in a discussion of display technology a short tutorial of radiometric and photometric theory. Radiometry is the measurement of radiant energy at all wavelengths (visible and invisible), while photometry is the measurement of apparent brightness to the human eye. The human eye “sees” the range of light wavelengths from 380 nm to 740 nm as the familiar color spectrum (Fig. 1).
The Commission Internationale de l' Eclairage (CIE) formalized standards for the measurement of light, and the response of the human eye or “standard observer”, back in the 1930s. These standards characterized the variation in eye response over the entire visible range under a variety of lighting conditions, such as daylight and night. The CIE also defined the primary colors.
- Red - 700 nm
- Green - 546,1 nm
- Blue - 435,8 nm
These standards and definitions have been controversial, and other standards exist. The points of interest for displays are that the human eye response peaks roughly at green 555 nm, is sensitive to yellow, falls off sharply towards blue at 400 nm, and also towards red at 700 nm.
This can be seen in the 1931 photopic (daylight) chromaticity diagram, which is shown in a simplified form in Fig. 2. The curve for scotopic (night-adapted) is quite different, peaking at about 512 nm.
Radiant light intensity (all wavelengths) is measured in lumens. The lumen is defined such that 683 lumens of light is provided by 1 watt of monochromatic radiation at a wavelength of 555 nm. Luminous intensity, in candelas (cd), results from the application of the CIE color response to the radiant flux and provides the measurement for the visible portion of a light source. Display intensity, therefore, is described in cd or mcd to indicate the light output useful to the observer.
