1.1 LED working principle
As the name implies, a light-emitting diode (LED) is a semiconductor device that emits light of a specific wavelength (color). Like other semiconductor chips, the semiconductor chip of the LED (the actual light-emitting unit of the LED) is also packaged in plastic or ceramic. Of course, there can be one in a package or multiple chips. When the LED is in forward conduction (open), the electrons recombine with the holes while releasing energy in the form of photons (as shown in Figure 1.1.1). This effect is often referred to as electroluminescence.
Figure 1.1.1 When an LED is excited, electrons and holes recombine, and at the same time, energy is released as photons of a specific wavelength (color).
LED technology, a commonly used term in lighting applications, is called solid state lighting (SSL). This is because the principle of illumination is distinguished from incandescent lamps (luminescence is achieved by thermal radiation in the visible spectrum), and the technique referred to by solid-state illumination is achieved by solid state electroluminescence.
How white LEDs work
The most common method is to use a single color LED (mostly a blue LED with indium gallium arsenide process) to achieve white light with different colors of phosphors. The corresponding LED is called a fluorescent white LED. High brightness LED (HB LED) is excited. Part of the blue light is converted to yellow light by the fluorescent layer, and the other part passes directly through the fluorescent layer in a blue light manner. Eventually, the mixture of blue and yellow light forms white light.
Figure 1.1.2 a): Internal structure of a common phosphor-based bright white LED
Figure 1.1.2 b): Blue photons generated by the combination of holes and electrons
Figure 1.1.2 c): part of the blue light passes directly through the phosphor layer, and the other part is converted to yellow light as it passes through the phosphor layer
Figure 1.1.2 d): Blue and yellow parts are mixed together to get white light
In the spectral analysis of phosphor-based white LEDs, we can clearly see the blue portion directly excited by the LED and the relatively broad spectral distribution of the yellow portion excited by the phosphor.
Figure 1.1.3 The combination of blue and yellow light gives white light, which is confirmed by Newtonian dispersion experiments.
2. LED optics
Optics is a branch of physics that studies the properties and behavior of light, including the interaction of light with objects, the construction of light instruments and photometric instruments.
A luminaire is a device used to change the light distribution of a source, diffuse light, or change the spectral composition. This is accomplished by using optical components (reflectors, diffusers, lenses, etc.) and accessories designed for a particular source. (sockets, leads, starters, ballasts, etc.). The luminaire also contains sections for securing and protecting the light source and wiring accessories.
Figure 2.1.1: Simplified LED luminaire functional combination
The main function of the optical element is to change the light flux intensity distribution of the source and/or to diffuse the light and change its spectral composition. Optical components of different geometries can produce different light intensity distribution curves (LIDC).
The purpose of using optical components:
Change the distribution of the source's luminous flux, modulate it or break it up; reduce the brightness within a certain angle that the observer can feel â€“ limit glare; change the spectrum emitted by the source â€“ filter. Light intensity distribution curve - LIDC
The light intensity of the approximate point source is measured in all directions, and the vector is marked in the space centered on the light source, and then the end points of the vectors are connected to obtain the brightness surface of the plane (Note: This is a 3D surface) ). In calculations, it is usually only necessary to know the numerical distribution of several specific sections in the 3D surface, which are usually through the center of the source. In this way, we get the light intensity distribution curve in polar coordinates.
Figure 2.1.2: Standard (EN13032-1) section of the light intensity distribution curve.
The LIDC is usually displayed on a plane that passes through the center of the light source or fixture. The most common beam surface is C-Î³ (note: the C plane we often say), whose axis is perpendicular to the main exit surface of the luminaire.
When the light intensity distribution curve is produced, the light intensity value is uniformly expressed by the luminous flux of the light source of 1000 lm. This is to make the light intensity distribution curve of the lamp not affected by the luminous flux of the light source used. Lighting space requires different light distribution curves to meet the criteria for specific applications or visual operations (see Figures 2.1.3 and 2.1.4). Below are some luminaires with different optics that can be adapted to a variety of needs.
Figure 2.1.3: Basic shape of the light intensity distribution curve
Figure 2.1.4: Basic direction of the light intensity distribution curve
Optical component efficiency - LOR (Luminaire Output Ratio)
The formula is as follows:
The efficiency of the optical component is equal to the ratio of the luminous flux of the luminaire to the total luminous flux of all sources.
A reflector is an optical element that controls the light of a source by reflection from a reflective material. Reflective materials are divided into specular, diffuse and mixed reflective materials.
There are two main types of reflectors: the first refers to four basic geometric conical reflectorsâ€”elliptical, ribbon, hyperbolic, and parabolic (Figure 2.2.1). The second refers to non-conical reflectors, such as square or asymmetrical, and their reflective surfaces are also basic geometric figures.
Figure 2.2.1: Four basic geometries of the reflector
Elliptical Reflector - If the light source is placed at the focus of the elliptical reflector, the beam will be reflected to the other focus of the imaginary ellipse. Such reflectors are often used in medium-wide and wide-light distribution lamps.
Strip reflectors - These reflectors are formed by connecting the centers of the circles to different arcs on the outside. The advantage of this type of reflector is that it accurately projects the light to the desired location, but the geometry of the reflector is very sensitive to manufacturing variations.
Hyperbolic reflectors - produce medium-width light distribution and wide light distribution.
Parabolic reflector - produces a narrow light distribution. Such reflectors are used in relatively small areas that require high levels of illumination.
Figure 2.2.2: Various reflectors for LED light sources
Polycrystalline Reflector - The reflector contains a large number of small surfaces with different angles of rotation for the reflector focus design to ensure a better flux distribution in the desired direction.
Figure 2.2.3: Polyplanar reflector: ensures better flux distribution in the desired direction
The shading angle indicates the angle at which the light source is blocked by the reflector inside the luminaire. The shading angle is the angle between the horizontal plane and the edge of the reflector and the end of the light source (Figure 2.2.4). The shading angle is defined as follows:
h: the distance from the light-emitting surface of the light source to the horizontal plane of the reflector light exit
R: radius of the reflector light exit
r: light source radius
Figure 2.2.4: Shading angle
Figure 2.2.5: End position of various light sources
Figure 2.2.5 shows the illuminating surfaces of different light sources. For example, the illuminated surface of a transparent incandescent bulb is the end of the other side of the filament relative to the viewer.
The diffuser scatters light as it passes through it. Diffuse light can also be obtained by diffuse reflection of light on a white surface. Based on the principle of diffusion, the diffusers are divided into the following types: milky white, Gaussian and prismatic diffusers. (as shown in Figure 2.3.1)
Figure 2.3.1: Basic type of diffusion mechanism
A diffuser with uniform dispersion penetration characteristics (milk-white type) can uniformly diffuse light from the light source to various directions without revealing the shape of the light source. A diffuser with a hybrid penetration characteristic (Gaussian or prismatic) changes the luminous flux distribution to a specific direction, not only does not reveal the shape of the light source, but also reshapes the light intensity distribution curve.
A milky white diffuser - a milky white diffuser - produces a cosine intensity distribution curve by passing light through a common diffusing material containing uniformly distributed scattering particles.
Gaussian diffuser - produces a Gaussian light intensity distribution curve. Light passes through a fine structured surface like a blasted surface and is scattered in different directions.
Figure 2.3.2: Comparison of a milky white diffuser with a Gaussian diffuser
Prismatic diffusers - a combination of microprism diffusers that are fundamentally refractive lenses. According to the law of refraction, geometric configurations such as pyramids, hexagons, domes, and triangles can be used to create the desired intensity distribution curve. They are commonly used in luminaires that require high illumination quality specifications (UGR-uniform glare value; average brightness of Lavg luminaires).
The following are examples of the most commonly used microprism diffusers:
A lens is an optical device with precise or nearly axial symmetry that allows light to penetrate and refract to converge or diverge.
Figure 2.4.1: Two basic types of lenses - convex (convergence) lenses
Figure 2.4.1: Two basic types of lenses - concave (diverging) lenses
A single lens contains an optical component. A composite lens includes a column of coaxial single lenses. Using a multi-lens combination can reduce more aberrations than using a single lens. The lens is mostly made of glass or transparent plastic.
Figure 2.4.2: Different types of lenses used by LED sources
2.5 Optical component materials
Different optical components require different optical materials. The reflectors are made of different surface treated aluminum and powder coated metal sheets. Transparent polycarbonate (PC), polystyrene, and polymethyl methacrylate (PMMA) are used for microprism type diffusers and lenses. .
Reflective Optical Materials - Materials with different properties are used to satisfy different types of reflections. Here are three basic types of reflection: specular, diffuse, and blended. The difference between different types of reflections is the ratio between specular and diffuse reflections.
Figure 2.5.1: Reflections of different types of materials
Aluminum - Because aluminum has an excellent reflectance, it is the most commonly used material in high quality reflectors. Anodized aluminum, polished aluminum, and aluminum sheets covered with multiple layers of silver are also used to achieve higher reflectivity and scratch resistance.
The surface treated steel sheet, which is sprayed with white powder into different shapes and structures, achieves the desired reflection. A special material (WHITE OPTIC 97) can be applied to the steel plate if high-efficiency Lambertian reflection is required.
Refractive optical materials:
PC - strong plasticity, thermoformable. Its refractive index is equal to 1.584.
PMMA - infrared light with a wavelength of 2.8 to 25 microns is permeable, and ultraviolet radiation with a wavelength shorter than 300 nm is not permeable. The reflectance is 1.49.
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