Light-Emitting Diode, LED
Light source that produces light as a result of recombination of positively charged holes and negatively charged electrons at the junction of inorganic, solid-state, p- and n-type semiconductor material.
Solid-State LED Light Sources
LEDs are solid-state radiators where the light is created inside solid-state material [2, 3, 4]. Common semiconductor diode chips, used today in so many electrical circuits, all use much the same technology. The light-radiating diode versions are called light-emitting diodes or LEDs. The name LED is commonly used for light-emitting diodes made of inorganic semiconductor material; they are point light sources. Light-emitting diodes made of organic semiconductor material are referred to as OLEDs; they are planar light sources. A separate chapter deals with OLEDs.
Principle of Solid-State Radiation
Like gas discharge lamps, solid-state lamps cannot function when they are operated direct from the mains supply voltage. Solid-state light sources are low-voltage rectifiers that allow current to pass in one direction only. This means that the AC mains supply has to be transformed to low voltage and then rectified into a DC supply. Small fluctuations in supply voltage cause large variations in current that can damage the light source. High-power LEDs need an electronic driver to obtain a constant current characteristic.
Principle of White LEDs
Good-quality white light, which is especially important when it comes to providing good color rendering, is obtained by using a blue LED chip in combination with fluorescent material that converts much of the blue light into light of different wavelengths spread over almost the whole visible spectrum. In LED technology it is customary to call such fluorescent materials, phosphors: hence white LEDs based on this principle are called “white-phosphor LEDs.” By mixing different phosphors in different proportions, white LEDs producing different tints of white light with different color-rendering capabilities can be made.
Materials and Construction
Semiconductor LED chip
Electrodes and bond wires
Phosphors (for white LEDs)
Semiconductor Chip Material
Shape of the Chip
In many cases, the chip is placed in a reflector cup which, because of its shape, helps to direct the light in an upwards direction. Highly reflective material is used: for example, metal or ceramic material.
The silicon lens on top of the LED chip serves as protection for the chip. More importantly, it helps in increasing the light extraction from the chip and as such is essential for a high lumen efficacy of the LED. This is because by introducing a lens medium between the chip and the air with a refractive index value between that of air and that of the chip material, the angle over which light can escape from the chip increases.
Electrodes and Bond Wires
In order to be able to apply power to the chip, the p and n parts of the chip have metal contacts called electrodes. Bond wires connect the electrodes with the electrical connections. They are usually gold wires. Since the electrodes intercept light leaving the chip, the dimensioning of the electrodes and bond wires, especially on the side of the main light-escape route, is one of the factors that determines the light efficiency of the LED.
As mentioned above, the most important method for producing white light with LEDs is by applying a phosphor coating to a blue-light LED that converts part of the blue light into longer-wavelength, green, yellow, and red light. Different compositions of different phosphors are used to produce white light of different color tints. Since the basis of the process is the blue light of the chip, the final efficacy will become higher as more blue is kept in the light. However, this implies a high color temperature (cool-white light) and relatively poor color rendering. If, with a different phosphor composition, more blue light is converted, the color quality will improve at the expense of a somewhat-lower efficacy.
Phosphors Applied Direct to the Chip
LED Cluster Modules
Temperature of the Chip Junction
It has already been mentioned that with rising temperature of the p-n chip junction, the performance of LEDs decreases: particularly the light output and lifetime. The performance data are usually specified for a junction temperature (Tj) of 25 °C. However, under normal operating conditions, a junction temperature of 60–90 °C is easily obtained. Depending on LED type, the lumen output falls to 60–90 % when the junction temperature increases from 25 to 80 °C. Amber and red LEDs are the most sensitive to changes in junction temperature, and blue LEDs the least.
The mass production of LEDs results in LEDs of the same type varying in color, light output, and voltage. In order to ensure that LEDs nevertheless conform to specification, LED manufacturers use a process called binning in their production process. At the end of the manufacturing process, LED properties are measured and LEDs are subsequently sorted into subclasses or “bins” of defined properties. As far as the color quality is concerned, the tolerances in these definitions (based on MacAdam ellipses) are such that visible differences in color between LEDs from the same bin are minimized. As the definition of a bin does not change with time, the same quality is also assured from production run to production run.
With the advancement of knowledge concerning LED materials and the mass-production process, it may be expected that binning will ultimately no longer be required (“binning-free LEDs”).
The energy balance of an LED is much easier to specify than that of conventional light sources. This is because no energy is radiated in the UV and infrared region of the spectrum, which means that the energy balance comprises only visible radiant energy and heat energy. Today, white LEDs transform 20–30 % of the input power into visible light and the remaining part into heat. The light percentage comes close to that of fluorescent lamps and will soon supersede it.
System Luminous Efficacy
Sometimes, luminous efficacies are specified for the bare chip. It is evident that the ancillary devices described in the previous sections, which are essential for a proper functioning of LEDs, absorb light. The only realistic thing to do, therefore, is to specify luminous efficacy (and light output as well) for the total LED package. As with most conventional lamps, the luminous efficacy of LEDs is dependent on the power of the LED and on the color quality of the light it produces. Higher-power LEDs have higher efficacies, while those with better color rendering have lower efficacies. Today cool-white LEDs are commercially available in efficacies up to some 150 lm/W and warm-white LEDs with color-rendering indices larger than 80 in efficacies of around 130 lm/W (all lm/W values include driver losses). Retrofit LED bulbs with warm-white light (around 2,700 K) and color-rendering index better than 80 are available in efficacies up to 80 lm/W. Here, too, cooler-white versions are slightly more efficient.
As discussed at the beginning of this chapter, in coming years it may be expected to see further important improvements in luminous efficacies for white LEDs – even up to slightly more than 200 lm/W.
Today, single LEDs exist in lumen packages varying from a few lumen (indicator lamps) to more than 1,000 lm. In the latter case, severe screening is called for to restrict glare, because so much light comes from such a very small light-emitting surface. By mounting multi-LEDs on a printed circuit board, LED modules can be achieved with much larger lumen packages.
In the case of high-performance LEDs, it takes a very long time before they actually fail – usually considerably more than 50,000 h. Before that time, however, their lumen depreciation is so great that the LED is no longer giving sufficient light for most applications. Therefore, for LED lifetime specifications, the length of time that it takes to reach a certain percentage of its initial light value is used. Based on a depreciation value of 70 %, lifetime values of between 35,000 and 50,000 h are common for high-performance LEDs. LED bulbs, with their limited space for handling heat, have a lifetime of some 25,000–35,000 h (25–35 times longer than an incandescent lamp).
The electric current passing through the chip’s junction, and the heat generated in it, degrades the chip material and is so responsible for light depreciation. Decoloration of the housing and yellowing of the primary lens may be further reasons for lumen depreciation. In white-phosphor LEDs, chemical degradation of the phosphor material also causes lumen depreciation. As mentioned above in the section on LED lifetime, for high-performance LEDs 30 % lumen depreciation is reached at around 35,000–50,000 h.
Run-Up and Reignition
LEDs give their full light output immediately after switch on and after reignition.
LEDs can be dimmed by simple pulse-width modulation down to 5 % of full light. Not all retrofit LED lamps can be dimmed on normal, commercially available dimmers. Special retrofit LED lamps that are designed to be dimmed on such dimmers are so specified on their packaging.
As has already been mentioned several times, limitation of the junction temperature of the LED chip is essential for the proper functioning of LEDs in terms of lumen output, lumen efficacy, lamp life, and even color properties. In high-temperature environments the products perform worse, while at low temperature they perform better. The actual influence of the junction temperature is different for the different types of LEDs. In extreme temperature environments, therefore, relevant information for a particular product has to be obtained from the manufacturer.
Mains Voltage Variations
LED drivers are designed to drive LEDS on constant current. In this way the influence of mains voltage variations is not an issue.
UV and IR Component
LEDs radiate only visible radiation. There is no ultraviolet or infrared radiation.
Single LEDs (Fig. 16).
Multiple LEDs on flat or three-dimensional PCB boards (Fig. 17).
LED modules (or LED engines) with secondary optics and with or without built-in driver that can be used in the same way as lamps. Interfaces are standardized for interchangeability (Fig. 18).
Multiple LEDs on strings (Fig. 19).
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