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F.A.Q.

F.A.Q. 

What is LED display screen? 
There are two types of LED screen: conventional, using discrete LEDs, and surface mounted device (SMD) panels. Most outdoor screens and some indoor screens are built around discrete LEDs, also known as individually mounted LEDs. A cluster of red, green, and blue diodes is driven together to form a full-color pixel, usually square in shape. These pixels are spaced evenly apart and are measured from center to center for absolute pixel resolution. The largest LED display in the world is over 1,500 foot (457.2 m) long and is located in Las Vegas, Nevada covering the Fremont Street Experience.

Most indoor screens on the market are built using SMD technology—a trend that is now extending to the outdoor market. An SMD pixel consists of red, green, and blue diodes mounted on a chipset, which is then mounted on the driver PC board. The individual diodes are smaller than a pinhead and are set very close together. The difference is that the maximum viewing distance is reduced by 25% from the discrete diode screen with the same resolution.

Indoor use generally requires a screen that is based on SMD technology and has a minimum brightness of 600 candelas per square meter (cd/m², sometimes informally called nits). This will usually be more than sufficient for corporate and retail applications, but under high ambient-brightness conditions, higher brightness may be required for visibility. Fashion and auto shows are two examples of high-brightness stage lighting that may require higher LED brightness. Conversely, when a screen may appear in a shot on a television show, the requirement will often be for lower brightness levels with lower color temperatures (common displays have a white point of 6500 to 9000 K, which is much bluer than the common lighting on a television production set).

For outdoor use, at least 2,000 cd/m² is required for most situations, whereas higher-brightness types of up to 5,000 cd/m² cope even better with direct sunlight on the screen. (The brightness of LED screen can be reduced from the designed maximum, if required.)

Suitable locations for large display screen are identified by factors such as line of sight, local authority planning requirements (if the installation is to become semi-permanent), vehicular access (trucks carrying the screen, truck-mounted screens, or cranes), cable runs for power and video (accounting for both distance and health and safety requirements), power, suitability of the ground for the location of the screen (if there are no pipes, shallow drains, caves, or tunnels that may not be able to support heavy loads), and overhead obstructions.

Where can be application for LED in lighting field? 
Grow lights composed of LEDs are more efficient, both because LEDs produce more lumens per watt than other alternatives, and also because they can be tuned to the specific wavelengths plants can make the most use of.

Light bulbs

Lanterns

Streetlights

Large scale video displays

Architectural lighting like led wall washer, led rope light, led tube light, led panel light and so on...

Light source for machine vision systems, requiring bright, focused, homogeneous and possibly strobed illumination.

Motorcycle and Bicycle lights

Flashlights, including some mechanically powered models.

Emergency vehicle lighting

Backlighting for LCD televisions and displays. The availability of LEDs in specific colors (RGB) enables a full-spectrum light source which expands the color gamut by as much as 45%.

Stage lights using banks of LED's as replacement for incandescent bulbs. LED's produce less heat so LED stage lighting is cheaper to operate and reduces the risk of fire considerably.

LED-based Christmas lights have been available since 2002, but are only now beginning to gain in popularity and acceptance due to their higher initial purchase cost when compared to similar incandescent-based Christmas lights. For example, as of 2006, a set of 50 incandescent lights might cost US$2, while a similar set of 50 LED lights might cost US$10. The purchase cost can be even higher for single-color sets of LED lights with rare or recently-introduced colors, such as purple, pink or white. Regardless of the higher initial purchase price, the total cost of ownership for LED Christmas lights would eventually be lower than the TCO for similar incandescent Christmas lights since the LED requires much less power to output the same amount of light as a similar incandescent bulb. More to the point, LEDs have practically unlimited life and are hard-wired rather than using unreliable sockets as do replaceable bulbs. So a set of LED lights can be expected to outlive many incandescent sets, and without any maintenance.

What is the useful life of LEDs? 
When you buy a package of light bulbs, you’ll see a number on the box that says something like “Life: 1125 hours.” This is defined as the time it takes for 50% of test samples to burn out. But there’s really not a standard definition like this for LEDs—they gradually decrease in brightness instead of “burning out.” The number 100,000 hours (about 11 years) gets mentioned a lot. Since an LED is a diode, even after it ceases to produce light, it will still use power.

So it becomes more useful to discuss “lumen maintenance” with LEDs. Manufacturers often publish a curve of light output vs. time, which describes the LED’s lumen maintenance profile. Indicator LEDs in clear epoxy packages can drop to 80% light output within a few thousand hours due to yellowing of the package material. The degradation occurs due to heat generated in the junction. So lumen maintenance is an important spec for a designer building arrays of indicator LEDs for illumination. Improvements in packaging and heat sinking are improving the lumen maintenance characteristics of illumination-grade LEDs. But this is a significant area of difference between arrays of inexpensive indicator LEDs and illumination-grade LEDs. You may start out with the same number of lumens, but the indicator types will typically degrade more rapidly.

What is SMD type of LEDs? 
Surface mount devices (SMD) are more common than ever thanks to miniaturization. LEDs inside disk drives, modems, and small personal electronic devices are more often SMD devices these days. Most of the size of a 5mm or 3mm indicator LED is the epoxy package–the actual LED junction is quite small. With SMD LEDs, you don’t get much extra packaging, just the glowing bits. Without the epoxy package focusing the beam, they tend to have wide viewing angles, and are often used with light pipe in a device.

SMD LEDs are available in four sizes, which are designated 1206, 0805, 0603 and 0402 . 0402 is the smallest, with overall package size of 1.0 mm x 0.5 mm x 0.45 mm (L x W x H). Then 0603 at 1.6 mm x 0.8 mm x 0.6 mm. 0805 are slightly larger, at 2.0 mm x 1.25 mm x 0.8 mm. 1206 are the big brothers of the family, at 3.2mm x 1.5 mm x 1.1 mm.

Electrically, these aren’t very different from the LEDs you’re used to. Both of these are rated for a typical forward current of 35 mA, with a typical forward voltage of 3.6V (4.0V max). Where they stand well apart from the crowd is optically–these are much wider view angle at 140°. The little guy is rated 200 mcd and the 0805 is rated 120. That may seem low if you’re used to specs on 5mm LEDs, but remember that luminous intensity in millicandelas changes as the view angle, so these have a comparable overall flux.

Handling these is difficult. They come in a tape reel packaging designed for automated assembly. Solder reflow is the usual way to connect these to your circuit board, but for home hobbyists with steady hands, a fine-tipped soldering iron may get the job done for you. Be quick–five seconds at 260°C can cook them.

What are the electrical characteristics of LEDs? 
It’s useful to think about two main types of LEDs—the familiar indicator LEDs that come in 5mm and 3mm epoxy packages, and “illumination-grade” LEDs, which are high-output devices, designed for lighting.

A typical indicator LED has a forward voltage rating between 2 and 4 Volts of DC. You may see maximum ratings above that. A typical drive current for indicator LEDs, even high-brightness ones, is 20 milliamperes (mA). From this you can see an indicator LED dissipates a modest amount of power—a few tens of milliwatts compared to the few tens of Watts a familiar incandescent bulb uses. In other words, the power used by an indicator LED is one thousandth of that used by a familiar light bulb.

Arrays are constructed to take advantage of this low power consumption. A series string of ten blue LEDs will take a 33 VDC forward voltage to light, but still only draw 20 mA of current from the source. So the supporting wiring can be less expensive for an LED array compared to a light bulb (which may draw an Ampere of current—fifty times as much as the LED). For parallel arrays, the current combine. So you can drive 10 blue LEDs in parallel from a 3.3V source, but the current drawn will be 200 mA. This flexibility in array construction is part of what makes LEDs very popular in mobile, battery-powered devices. The designer can arrange LEDs to take best advantage of the power source that’s available.

Illumination-grade LEDs have comparable forward voltages to indicator LEDs. This is a reflection of the fact that the junction material is the main determinant of the forward voltage. But the junctions in illumination-grade LEDs are typically larger, and can draw more current, and dissipate more power (while producing more light). A Luxeon Star LED has a drive current of 350 mA, and dissipates about 1 Watt of power.

Another important LED spec is maximum reverse voltage. A diode conducts current when a forward voltage is applied, but will not conduct if a reverse voltage is applied, up to a point. Reverse voltages in excess of the maximum can cause the diode to fail.

What is Inside an LED? 
LED's are special diodes that emit light when connected in a circuit. They are frequently used as "pilot" lights in electronic appliances to indicate whether the circuit is closed or not. A a clear (or often colored) epoxy case enclosed the heart of an LED, the semi-conductor chip.

The two wires extending below the LED epoxy enclosure, or the "bulb" indicate how the LED should be connected into a circuit. The negative side of an LED lead is indicated in two ways: 1) by the flat side of the bulb, and 2) by the shorter of the two wires extending from the LED. The negative lead should be connected to the negative terminal of a battery. LED's operate at relative low voltages between about 1 and 4 volts, and draw currents between about 10 and 40 milliamperes. Voltages and currents substantially above these values can melt a LED chip.

The most important part of a light emitting diode (LED) is the semi-conductor chip located in the center of the bulb as shown at the right. The chip has two regions separated by a junction. The p region is dominated by positive electric charges, and the n region is dominated by negative electric charges. The junction acts as a barrier to the flow of electrons between the p and the n regions. Only when sufficient voltage is applied to the semi-conductor chip, can the current flow, and the electrons cross the junction into the p region.

In the absence of a large enough electric potential difference (voltage) across the LED leads, the junction presents an electric potential barrier to the flow of electrons.





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