The development history, basic parameters and principles of LED

LED Development History

The incandescent light bulb is arguably the most important invention of the early 20th century. This period has been called the "Second Industrial Revolution," and the incandescent light bulb changed the lives of ordinary people to a great extent. Oil lamps, arc lamps, and gas lamps had all existed, but none had as much of an impact as the incandescent light bulb.

Ordinary incandescent lamps had a short life (less than 1000 hours), low efficiency (less than 5% in some lamps) and a low color temperature (2700-3100K). These "defects" encouraged more research, which by 1959 led to an improved incandescent lamp - the halogen lamp. This new lamp was filled with a gas and a halogen such as iodine or bromine. The halogen increased the life of the tungsten filament and prevented the evaporated tungsten from depositing on the inner surface of the bulb. Brighter and smaller, the halogen lamp became the perfect light source for reading.

Halogen is widely used in car headlights and brake lights, and higher brightness makes driving safer. In 1991, Philips developed the xenon lamp, which has 3 times the brightness and 2 times the energy efficiency. Xenon lamp is a high-intensity discharge (HID) lamp, which now includes many types, such as sodium lamp, mercury vapor, etc. and metal halide lamp.

Today's incandescent lamps are brighter, have better color, last longer, and are smaller in size. But the oil crisis of the 1980s sparked a focus on energy efficiency and the environment. Artificial light sources were identified as an area where efficiency improvements were needed, and a program dedicated to finding a "fourth generation" of light sources was launched.

Energy Efficient Lighting (EEL)

Once again, General Electric was the inventor of a new lighting technology: the compact fluorescent lamp (CFL). The CFL is a small integrated package that includes a base, a tube, and a ballast.

Fluorescent lamp ballasts were originally inductive and operated at electrical line frequency. However, light flicker was easily noticeable, uncomfortable for some people, and reportedly caused migraines and epileptic seizures in some people. CFLs use electronic ballasts that operate at a much higher frequency, around 20 KHz, eliminating the flicker problem.

The color emitted by fluorescent lamps was another problem that needed to be solved. Philips solved the problem by creating green, blue and red phosphors. When mixed in the right proportions, they produce artificial light that is very similar to natural conditions. These new three-primary phosphors also have the advantage of energy saving, which has promoted their widespread adoption.

Modern traditional lighting includes incandescent lamps (including halogen lamps), HID and compact fluorescent lamps. Although engineers and designers continue to improve them, some inherent problems still exist: they contain harmful materials such as mercury; they radiate electromagnetic interference and consume high energy.

New technologies

There is another new lighting technology that has smaller size and lower energy consumption, higher efficiency, longer life, faster switching speed and negligible environmental impact. This technology is high- power light-emitting diodes (LEDs), and many manufacturers are now competing for market share. Philips Lumileds, OSRAM, GE, Cree , Avago, Optek and Dialight Lu mid rives are the main players. These companies are investing heavily in traditional lighting applications. Many countries, including EU countries, Australia and Canada, have proposed to stop using incandescent lamps by 2010.

Thanks to the flexibility of high-power LEDs , a wide range of new lighting applications are emerging. Endoscopes in healthcare, decorative lighting in architecture, headlights and brake lights on cars, signal lights in traffic and mining are just a few examples.

Incandescent lamps have illuminated our world for 130 years, but they will soon become history. The prediction was correct, and white LEDs were fully developed in 2010. LEDs, especially high-power LEDs, herald a new lighting revolution.

Analysis of basic characteristic parameters of LED

Light intensity is defined as the luminous flux emitted per unit solid angle , with the unit being candela ( candela , cd). Generally speaking, a light source will radiate its luminous flux in different directions with different intensities. The radiant intensity of visible light emitted per unit solid angle in a specific direction is called light intensity.

Chromaticity

The human eye's perception of color is an intricate process. In order to quantify the description of color, the International Commission on Illumination (CIE) recorded the visual sensation caused by the human eye to radiant energy of different wavelengths based on the visual experiment of standard observers, and calculated the color matching function of the three primary colors of red, green and blue. After mathematical conversion, the so-called CIE1931ColorMatchingFunction (x((), y((), z(())) was obtained. Based on this color matching function, several color measurement definitions were subsequently developed, allowing people to describe and use color.

According to the CIE1931 color matching function, the stimulation value of the human eye to visible light is expressed as XYZ, and the x, y values ​​are converted into CIE1931 (x, y) chromaticity coordinates through the following formula. Through this unified standard, the description of color can be quantified and controlled.

x,y: CIE1931 chromaticity coordinates (ChromaticityCoordinates)

However, since the color gamut constructed by (x,y) chromaticity coordinates is non-uniform, it is difficult to quantify color differences. Therefore, in 1976, CIE converted the CIE1931 chromaticity coordinates to form a color gamut that is close to a uniform chromaticity space, allowing color differences to be quantified. That is, CIE1976 UCS (Uniform Chromaticity Scale) chromaticity coordinates, represented by (u'', v').

Dominant wavelength (λD)

It is also one of the methods to express color. After obtaining the chromaticity coordinates (x, y) of the device under test, mark it on the CIE chromaticity coordinate diagram (as shown below), connect the E light source chromaticity point (chromaticity coordinates (x, y) = (0.333, 0.333)) with the point and extend the connection line. The wavelength value where this extension line intersects with the spectrum track (horseshoe) is called the dominant wavelength of the device under test. However, it should be noted that under this marking method, the same dominant wavelength will represent multiple different chromaticity points, so it is more meaningful when the chromaticity point of the device under test is adjacent to the spectrum track, and white light LEDs cannot describe their color characteristics in this way.

Purity

It is an auxiliary representation when describing color with dominant wavelength. It is defined as the percentage of the straight-line distance between the chromaticity coordinates of the DUT and the chromaticity coordinates of the E light source and the chromaticity coordinates of the spectral locus (Spectral Locus) of the dominant wavelength of the DUT. The higher the purity, the closer the chromaticity coordinates of the DUT are to the spectral color of the dominant wavelength. Therefore, the higher the purity of the DUT, the more suitable it is for describing its color characteristics with the dominant wavelength. LED is an example.

Color Temperature

When the radiant energy distribution of a light source is the same as the radiant energy distribution of a standard black body (BlackBodyRadiator) at a certain absolute temperature, the chromaticity of the light source is the same as the chromaticity of the black body radiation. At this time, the chromaticity of the light source is expressed by the corresponding absolute temperature, which is called the color temperature (ColorTemperature). The chromaticity presented by the black body radiation at each temperature can be marked on the chromaticity diagram as a curve, which is called the Planckian Locus. The higher the temperature of the standard black body, the more blue stimulation the light radiated by it produces to the human eye, and the red stimulation component is relatively reduced. However, in actual measurement, no light source has the same radiant energy distribution as a black body. In other words, the chromaticity of the light source to be measured usually does not fall on the Planckian Locus. Therefore, the chromaticity coordinates of the light source to be measured are calculated to be closest to a certain coordinate point on the Planck locus. The black body temperature of this point is defined as the correlated color temperature (CCT) of the light source , which is usually calculated using the CIE1960UCS (u,v) chromaticity diagram and described with the color difference △uv. It should be noted that this representation method is only meaningful when the chromaticity of the light source is close to the Planck locus, so for LED measurement, it is only applicable to the color description of white light LEDs.

LED light emitting principle

Light Emitting Diodes, commonly called LEDs, are just tiny light bulbs. But unlike common incandescent bulbs, LEDs don't have filaments, and they don't get particularly hot. Instead, they emit light simply by the movement of electrons in semiconductor materials. Because LEDs don't have filaments to burn out, they last longer. And the small plastic bulbs of LEDs make them more durable, plus LEDs can fit more easily into current electronic circuits. The light-emitting process of traditional incandescent bulbs involves generating a lot of heat, which is a complete waste of energy. LEDs generate very little heat, and relatively speaking, the more electricity that is used to directly emit light, the less energy is needed.

Light is a form of energy that can be released by atoms. It is made up of many tiny particle-like bundles that have energy and momentum but no mass. These particles are called photons , and are the most basic unit of light. Photons are released when electrons move. In atoms, electrons move in orbits around the atom. Electrons have different energies in different orbits. Generally speaking, electrons with greater energy move in orbits farther away from the nucleus. When an electron jumps from a lower orbit to a higher orbit, the energy level increases, and conversely, when it falls from a higher orbit to a lower orbit, the electron releases energy. The energy is released in the form of photons. Higher energy drops release higher energy photons, which are characterized by their high frequency.

A free electron falls from the P-type layer through the diode into an empty electron hole. This involves falling from the conduction band to a lower orbital, so the electron releases energy in the form of a photon. This happens in any diode, you just see the photons when the diode is made of certain materials. In the atoms of a standard silicon diode, for example, the atoms are arranged in such a way that when the electron falls over a relatively short distance, it is invisible to the human eye. As a result, the electron frequency is so low that it cannot be seen by the human eye.

In visible light LEDs, such as those used in digital displays and clocks , the size of the gap determines the frequency of the photons, or in other words, the color of the light. While all diodes emit light, most are not very efficient. In ordinary diodes, the semiconductor material itself absorbs a lot of the light energy and ends up absorbing it. Light-emitting diodes are covered by a plastic bulb that concentrates the light in a specific direction.

The price of semiconductor components has dropped significantly in the past 10 years. I believe that in the future, light-emitting diodes will be a more cost-effective lighting option with wider applications.