how to create barcode in vb.net 2008 The Basics on LEDs in Software

Creation ANSI/AIM Code 128 in Software The Basics on LEDs

1.13 The Basics on LEDs
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How does an LED work The following outlines some basic principles of an LED. White LEDs need 3.6 V DC and use approximately 30 milliamps of current; a power dissipation of 100 milliwatts (mW). The positive power is applied to one side of an LED semiconductor through a lead often called an anode, and another lead called a whisker. The other side of the semiconductor is attached to the top of the anvil. This is the negative power lead, which is called the cathode. It is the chemical makeup of the LED semiconductor that determines the color of the light the LED produces.
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The epoxy resin enclosure has three functions. It is designed to allow the most light to escape from the semiconductor, it focuses the light and protects the LED semiconductor from the elements. The entire LED unit is totally embedded in an epoxy. This is what make LEDs virtually indestructible. There are no loose or moving parts within the solid epoxy enclosure. Thus, a light-emitting diode (LED) is essentially a PN junction semiconductor diode that emits light when current is applied. It is a solid-state device that controls current without heated filaments and is therefore highly reliable. LED performance is based on a few primary characteristics:
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Monochromatic LEDs
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LEDs are highly monochromatic, emitting a pure color in a narrow frequency range. The color emitted from an LED is identified by a peak wavelength (Lpk) and measured in nanometers (nm). See Fig. 1.11. Peak wavelength is a function of the LED chip material. Although process variations are 10 nm, the 565 to 600 nm wavelength spectral region is where the sensitivity level of the human eye is highest. Therefore, it is easier to perceive color variations in yellow and amber LEDs than other colors. LEDs are made from gallium-based crystals that contain one or more additional materials (such as phosphorous) to produce a distinct color. Different LED chip technologies emit light in specific regions of the visible light spectrum and produce different intensity levels. See Table 1.3.
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550 Wavelength (nm)
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FIGURE 1.11
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An LED peak wavelength (Lpk). (See also color insert.)
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Standard Brightness LED Color Red Orange Amber Yellow Green Blue High Brightness
Chip lpk Iv Viewing Chip lpk Iv3 Viewing Material (nm) (mcd) Angle Material (nm) (mcd) Angle GaAsP/ GaP GaAsP/ GaP GaAsP/ GaP GaP GaP 635 605 583 570 565 120 90 100 160 140 35 30 35 30 24 AS AlInGaP AS AlInGaP AS AlInGaP GaN GaN GaN 635 609 592 520 495 465 900 30 1,300 30 1,300 30
1,200 45 2,000 30 325 45
Turquoise
TABLE 1.3 Comparison of Chip Technologies for Wide-Angle Non-Diffused LEDs
(See also color insert.)
LED White Light
When light from all parts of the visible spectrum overlap one another (Fig. 1.12), the additive mixture of colors appears white. However, the eye does not require a mixture of all the colors of the spectrum to perceive white light. Primary colors from the upper, middle, and lower parts of the spectrum (red, green, and blue), when combined, appear white. To achieve this combination with LEDs requires a sophisticated electrooptical design to control the blend and diffusion of colors. Variations in LED color and intensity further complicate this process. Presently, it is possible to produce white light with a single LED using a phosphor layer (Yttrium Aluminum Garnet) on the surface of a blue (Gallium Nitride) chip. Although this technology produces various hues, white LEDs may be appropriate to illuminate opaque lenses or backlight legends. However, using colored LEDs to illuminate similarly colored lenses produces better visibility and overall appearance.
FIGURE 1.12 The structure of an LED to produce a mixture of light: white light.
Photometer Iv (mcd) LED
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LED Intensity
LED light output varies with the type of chip, encapsulation, efficiency of individual wafer lots, and other variables. Several LED manufacturers use terms such as superbright and ultrabright to describe LED intensity. Such terminology is entirely subjective since there is no industry standard for LED brightness. The amount of light emitted from an LED is quantified by a singlepoint on-axis luminous intensity value (Iv). LED intensity is specified in terms of millicandela (mcd). This on-axis measurement is not comparable to mean spherical candlepower (MSCP) values used to quantify the light produced by incandescent lamps. Luminous intensity is roughly proportional to the amount of current (If) supplied to the LED. The greater the current, the higher the intensity. Of course, design limits exist. Generally, LEDs are designed to operate at 20 milliamps (mA). However, the operating current must be reduced relative to the amount of heat in the application. For example, six-chip LEDs produce more heat than single-chip LEDs because they incorporate multiple wire bonds and junction points that are affected more by thermal stress than single-chip LEDs. Similarly, LEDs designed to operate at higher design voltages are subject to greater heat. LEDs are designed to provide lifelong operation because of optimal design currents, considering heat dissipation and other degradation factors.
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