OLED means Organic Light Emitting Diode. It works in similar way than (semiconductor) LED. Both need positive and negative charge carriers to generate electrical current and finally generate light.
It is a surface light source based on organic material layers. LED, on the other hand, is a point light source that is based on semiconductor materials. But the operation is based on the same principle.
OLED as Surface Light
The large-area LED modules, that are covered by opal diffuser, are basically similar light sources as OLED panels. Without this diffuser, these LED modules also are point light sources because of small SMD LED diodes they have on their surface.
OLED panels are surface light sources by nature. They give uniform light.
The benefits OLED:
Its light has spectral power distribution very close to sunlight.
Color rendering index (CRI) of 90.
Produces no glare
Produces very little heat(<35°C),
Doesn’t produce any UV and therefore it doesn’t cause blue light hazard risk.
Panels are thin and lightweight.
Simple light: panels don’t need many accessories unlike LED, such as heat sinks, diffusers, or other optics. It only needs the power source and the light source itself.
Spectrum of OLED
Structure of OLED
The OLED structure consists of layers. These layers have different purposes.
There are basically THREE different kinds of layers that have some purpose. Of course, you need anode and cathode terminals to bring electricity from outside world to the panel. As an example, LG Display uses Aluminium as cathode material and ITO (indium tin oxide) as their anode material.
Electrical current flow guidance: Electron Transport Layer (ETL), Hole Transport Layer (HTL) and Hole Injection Layer (HIL). These layers are used to transport charge carriers in optimal way to the light generation layer, EML. But also, they have to be optically suitable for light generated in the EML layer so that as much light as possible is extracted from the panel.
Third type of layer: Encapsulation. The encapsulation layer is used to protect inner optically active layers from any outside harm that could deteriorate the operation of the panel.
This kind of set of layers is called a stack.On top of the stack is the encapsulation layer.
For example, LG Display uses two-stack structure for 3000K and 4000K OLED panels and three-stack structure for 2700K panels. Because there are more stacks in 2700K version, the overall voltage over the panel is a bit higher.
One major problem with the organic materials is that they are very sensitive to oxygen and moisture. This means that OLED panels need to be protected – as even a single water or oxygen molecule can harm the panel.
The encapsulation layer also protects from minor physical impacts. If this encapsulation layer deteriorates it will affect the optical layers. Usually strong glass is used for rigid OLED panels. But flexible panel is gaining more and more popularity. Flexible panels use plastic.
The major drawbacks of OLED panels are:
Easy to break
At the moment, most panels use glass substrates. These substrates are very fragile and are easy to break when not handled with care. This will improve in future as technology develops and plastic substrates will gradually replace glass.
You can’t use the panels in temperature of under 0 degrees of Celsius. This will obviously place some constraints for the use.
It is very probable that the cold endurance will get better in the future as the technology develops.
OLED is still very young technology and it can’t produce very large amounts of light. It also loses to LED in luminous efficacy.
You can use several different dimming options to dim LED Lighting. What are the possibilities and what dimming should you look from a LED driver? I’m going to answer these questions in this blog post by going through the different systems.
The goal is to give you the basic understanding of the dimming methods available at moment.
I am grouping the dimming methods in two main groups: analogue and digital.
When you want to control lighting, you have to know some basic issues of your lighting fixtures:
Are your fixtures dimmable? If yes, what is the dimming method which works together with your fixtures
If your fixtures are non-dimmable, then you can only have on/off – function.
Analogue dimming covers all dimming systems that don’t transform the dimming signal into bits and controls the lighting in analogue manner.
Phase dimming systems dim the lights by altering the supply voltage.
Leading & trailing edge dimming
Before LEDs, we used to dim halogen lamps with wall dimmers. We can still use these kinds of dimmers. But dimmer, driver and LED-module must be compatible with each other.
This type of control is accomplished without any need for an additional control wire. It involves connecting a dimmer in series between one of the mains wire and the equipment.
The dimmer cuts part of the mains voltage sinusoidal waveform to a greater or lesser extent in order to dim luminous flux even from 1% to 100% (this value depends on dimmer and driver).
Depending on how the driver makes the mains voltage cut, it is possible to distinguish between two types of dimming:
Trailing edge dimming
Dimming cut-off in the wave on its ascending side, from the beginning (phase cut-off at ignition). This is traditionally used in halogen lamps supplied through electromagnetic transformers.
Dimming by cut-off in the wave on its descending side, from the end cutting backwards (phase cut-off at switch off). And this way of dimming causes less interferences than leading-edge dimming.
There are dimmers and equipment that support both types of dimming, and others that support only one type.
Leading & Trailing-edge dimming LC
Leading-edge dimming L
Trailing-edge dimming C
The 1-10V system enables dimming of the luminous flux from around 1…10% to 100%. This is done by sending an analogue signal to the equipment over an additional, two-wire control line. These control wires have positive and negative polarities respectively and that must be kept in mind when wiring up the system.
The analogue signal has a direct voltage value of 1V to 10V. 1V or short-circuiting the fixture’s input control gives the minimum light level. While 10V or leaving the input control circuit open gives out the maximum light level.
International standard, IEC 60929, defines the regulation curve. The regulation curve represents the relationship between the control line voltage and the luminous flux. It reflects a practically linear relationship in the range of 3V to 10V.
To get a response adapted to that of the human eye it is possible to use logarithmically controlled potentiometers.
Regulation curve by IEC 60929
These in light fixtures generate power control with 1-10V dimming. Driver supplies a current to the controller through equipment control terminals. The controller current must be from 10µA to 2mA. The maximum control line current is obtained with a voltage of 1V and the minimum with a voltage of 10V.
This dimming system is unidirectional, i.e. the information flows in one direction, from the controller to the light fixture. The latter generates no feedback to control. This means that this system can’t be controlled by a software. Groups have to be created by wiring. This system can be integrated into building control systems.
The voltage drop in the control line wiring limits its length. Therefore, the maximum distance is limited by the number of control gears connected. The latter establishes the current per line and the cable diameter used.
Touch Control Push Button (analogue but can be connected to digital systems)
Touch Control is a system that enables the simple and economic dimming of luminous flux. It uses the mains voltage as a control signal, applying it with a standard push button on a control line, without any need for specific controllers. The Touch Control system enables you to carry out the basic functions of a regulation system with a power-free pushbutton. Depending on how long the button is pressed it is possible to switch the light on or off or dim it. Switching the light on or off is done by short, sharp pressing or “click”. If the button is pressed for a long time it is possible to dim the luminous flux between the maximum and minimum levels alternately.
This is a unidirectional interface, i.e. information flows in one direction. The equipment does not generate any type of feedback, so it can’t be controlled with a software. Groups have to be created by wiring. This system cannot be integrated into building control systems.
The length of the wiring and the number of equipment that can be connected, are theoretically unlimited. But in, asynchronism may occur during switching on and dimming, at distances longer than 25 meters, and with a larger number of fixtures connected. Owing to its characteristics, the use of this dimming method is recommended for individual offices, small meeting rooms or bedrooms, landings and small spaces in general.
Digital dimming covers all dimming systems that transform the dimming signal into bits and controls the lighting in digital format.
DALI Regulation (digital)
As revealed by the meaning of its acronym, Digital Addressable Lighting Interface, DALI is a digital and addressable communication interface for lighting systems.
This is an international standard system in accordance with IEC 62386, which ensures compatibility and interchangeability between different manufacturers’ equipment marked with the following logo: DALI controller
It is a bi-directional dimming interface with a master-slave structure. The information flows from a controller, which operates as the master, to the control gears that only operate as slaves. The latter carries out the orders or responds to the information requests received.
Digital signals are transmitted over a bus or two-wire control wire. These control wires can be negatively and positively polarized, though the majority control gears are designed polarity free to make connection indifferent.
You don’t need especially shielded cables. It is possible to wire the power line and DALI bus together with a standard five-wire cable.
Unlike other systems, you don’t need to create wiring groups. Therefore all the pieces of fixtures are connected in parallel to the bus. Without bearing in mind the grouping of these, simply avoiding a closed ring or loop topology.
You don’t require mechanical relays to switch the lighting on or off, given that this is done orders sent along the control line. You don’t need are bus termination resistors either.
Consequently, the DALI interfaces offer wiring simplicity in addition to great flexibility when it comes to designing the lighting installation.
The maximum voltage drop along the control line must not exceed 2V with the maximum bus current of 250mA. Therefore, the maximum wiring distance allowed depends on the cable cross-section, but it must never exceed 300m in any case.
After wiring, the DALI lighting system is configured with the software. You can create up to 16 different scenarios, addressing the equipment individually up to a maximum of 64 addresses. This can be made with groups up to a maximum of 16, or simultaneously by means of a “broadcast” order. You can change the configuration at any time without any need for re-wiring.
The DALI system has a logarithmic regulation curve adjusted to human eye sensitivity, defined in the international standard, IEC 62386. The possible regulation range is set at from 0.1% to 100%. The driver manufacturer determines the minimum.
DALI Regulation Curve by IEC 62386
With the software, you can change the “fade rate”. “Fade rate”is the time needed to go from one light level to another(fade time) and the speed of the change.
The DALI system lies in the fringe between the complex and costly but powerful ones; control systems for buildings that offer total functionality and the most simple and economic regulation systems, for example, the 1-10V one.
You can use this interface in simple applications independently, to control a luminaire or a small room. You can also use it in high-level applications such as being integrated by gateways into building smart control systems.
These are the most common systems you can use to dim LED. There are a lot of different dimming systems for different driver manufacturers. I can’t cover all of those in a single blog post. I will be writing a different post about wireless dimming options.
If you have anything you would like to know, you can always contact me firstname.lastname@example.org .
We see more and more light sources that supposed to be exactly the same color temperature, but actually appear different to human eye. So why the same color temperatures look different?
When people talk about color temperature, they are usually talking about correlated color temperature instead (CCT). There is a difference between these two.
Color temperature (CT)
Color temperature (CT) defines what is the exact spot of the light source is on the planckian locus line.
This line in pictured in the below image as the black line in the middle.
So if there are two light sources, that have a color temperature of 4000K, they both look exactly the same as they both are on the same spot.
Correlated color temperature (CCT)
Correlated color temperature is used when the light source is off from the planckian locus. If CT defines the exact point on locus, then CCT defines the perpendicular line which runs directly through that exact point. So if a light source is off from the locus, then CCT is the CT point which is closest on the locust.
So for example if a light source has a CCT of 4000K, that means that it can be on any point on the line that runs through the 4000K point on the locus.
You can see these lines on the image.
Typically a light sources color temperature is announced as CCT. So if it is said that two light sources have a CCT of 4000K, this means that they are on the same line that runs through the 4000K spot, but may, in fact, look totally different.
Usually, light source’s chromaticity is defined in diagram as chromaticity coordinates. In this diagram, you can’t determine that CCT is the shortest distance to the locus. You can see the chromaticity diagram in the image below.
So when you have three luminaires which have the same 4000K CCT, you can have three totally different colored lights.
If the light has a greenish white light, that will mean that the chromaticity coordinate is above the planckian locus.
If the light has a purple tone, then chromaticity coordinates are below the locus.
If the light is normal white light, then the chromaticity coordinates are on the locus or at least very close to it.
So please remember that staring at the CCT doesn’t always tell you everything. If you use two different light sources with the same CCT, you should always check the coordinates and see if these two are actually the same color.
In my previous blog post, I wrote about the general development of the LED. Focus was in the development of LED component. How it was transformed from a laboratory experiment to a mass production of commercial product. Different colors of LEDs were produced by applying different semiconductor layers on certain substrate materials.
Still, there was one obstacle to overcome. LEDs emitting blue color couldn’t be produced with reliability good enough for mass production. In this post I tell how LEDs with blue light were developed. And how it finally made possible of creating the white LED. Blue LED was the base for the current white LED revolution. This invention was awarded with the Nobel prize in Physics in September 2014.
Although LED components could produce red, green and yellow light, blue was unreachable. The main reason for early stage difficulties related to quality of materials. Materials like zinc selenide (ZnSe) and silicon carbide (SiC) produced inefficient light emission. So they were not suitable to use. Part of this was because of the quantum mechanical feature called indirect bandgap. The indirect bandgap allows only small part of the energy go to light production. Most of it turns to heat. In direct bandgap materials, the energy distribution is opposite. A greater part of the supplied energy goes to the light production. This enables more efficient light-emitting components.
One solution was gallium nitride (GaN). First experiments started already on the end of 1950s at Philips Research Laboratories. But, at that time growing larger crystals was difficult. There was also another problem with GaN at that time. Growing semiconductor structures having p-n junction for diode operation was not possible. The researchers concentrated on gallium phosphide (GaP) material instead.
The next solution on the line was to use the growing method Hybride Vapour Phase Epitaxy (HVPE). Yet, there were many problems with the method. For example the presence of hydrogen passivated p-type doping. So forming good enough p-n junctions was impossible. Also, surface roughness was not controllable enough. So there were two main reasons for lack of progress. One being material growth problems and other problem in semiconductor doping, especially p-type doping.
In the 1970s, scientists developed new growing methods: MBE and MOVPE. After the development of these methods, the material quality problems could be solved. This was thanks to good enough manufacturing techniques or growing methods. Also material stess/strain problems could be solved with new inventions on growing temperatures. Isamu Akasaki and a PhD student Hiroshi Amano were conducting research on GaN in Nagoya University. At the same time Shuji Nakamura was doing research on the same material in Nichia Chemical Corporation.
Both groups used a so-called spacer layer between the sapphire and the rest of the semiconductor structure. The difference between two research groups was the spacer layer material. Akasaki and Amano used aluminium nitride (AlN) while Nakamura’s groups used GaN. Another crucial thing was the growing temperature. The Spacer layer was grown at much lower temperature than the rest of the layers in semiconductor structure.
After solving the biggest problem, two more existed:
How to get a proper p-n junction, by solving p-type doping issue, to produce good diode performance.
How to get better light emission efficiency to develop more efficient blue light emitting LED diode.
The solution for the first challenge above was found by accident. While studying the zinc-doped GaN with a scanning electron microscope. Akasaki and Amono noticed that the material emitted more light This meant that in p-n junction p-type doping was higher thus enabling more efficient light emission. The same effect was also noticed when GaN was doped with magnesium.
The effect was later explained by Shuji Nakamura. The role of hydrogen in passivating p-type doping mentioned earlier in this post was essential. Electron irradiation in scanning tunnel microscope prevents this passivation of hydrogen. This leads to activation of p-type dopants. The same effect can also be achieved with thermal treatment, also known as thermal annealing.
Then there was a target for better light emission efficiency. How to collect electrons and holes, into small volume to generate efficient light. In light generation, a process called electron-hole recombination, generates light of certain wavelength. In this post we deal with blue light generation. Blue light was the last obstacle that had to be won to finally create a white light LED.
The solution to efficient light production was to use so-called double heterostructures. The special case for these is quantum well structure that is used for example in manufacturing of semiconductor lasers. The structure means that there are two interfaces with two different semiconductor materials. They form a “sandwich” structure. Usually, the middle layer has a smaller band gap than the layers above and below it. This makes it possible to collect electrons and holes into the middle layer. This enables more efficient recombination and more efficient light generation. In the case of blue LED the sandwich structure example is p-type AlGaN/InGaN/n-type AlGaN. Thus there is:
p-n junction to create diode structure electrically
double heterostructure to create suitable band gap structure to enable efficient light generation
This way both the problems in material mechanical quality and optical quality had been solved. Efficient blue LED was able to being produced. These advancements led later to the production of blue laser diodes.
Soon after the invention of blue LED, white LED was developed.
There are two ways to produce a white light LED:
combining red, green and blue LEDs to create white light
use blue LED with phosphor material that converts blue light to white light by adding red and green regions into the spectrum of light
These advancements in manufacturing processes and semiconductor structures were huge. This made possible further advancements in creating more and more powerful LEDs.
Isamu Akasaki, Hiroshi Amano and Shuji Nakamura won the Nobel Physics Prize in 2014 for “the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources”. Additionally, Shuji Nakamura has won the Millennium Technology Prize on 2006 for his contribution in developing blue and white LED light.
LED – The replacement for every light source we possess? Thanks to the tightened regulations and more energy efficient approach, LED has started to replace all the traditional lighting. But what do those three letters hold inside? What actually IS LED?
Literally, it is a light-emitting diode. Technically it is PN-junction. Functionally it is a light source.
What actually happens in a functional sense for lighting? By driving the diode in forward biased mode light can be produced if materials are selected in a proper way. Basically, this means that materials have to be selected so, that the energy driven from power supply is transferred mostly to light emission, not to heat. More and more efficient structures have been developed to produce light more efficiently by using optically different material layers, which keep the light inside those layers.
In electrical sense, PN-diode consists of positively doped (p-type) and negatively doped (n-typed) semiconductor layers. When power supply’s positive pole is on p-type and negative on n-type, the supply voltage is forward biased. At certain voltage the diode starts to give remarkable output current, this voltage is called threshold voltage. In another words, the PN-diode conducts current if it is connected forward biased to power supply. If the power supply’s positive pole is on n-type and negative on p-type, the diode does not conduct current. Then it is connected to reverse biased power supply. This means that it is in OFF state. If the reverse bias voltage grows too large, the diode will damage permanently.
Generation of white light
There are basically two ways to generate white light. One is to use individual LEDs consisting of three colors (red, green, blue) and then mix those colors to form white light. Another way is to use a phosphor material to convert monochromatic light from a blue LED to broad spectrum white light.
Let’s take a look at the generation of white light with the help of phosphor in more detail. In these kinds of LEDs, blue emitting LEDs are coated with phosphors of different compositions to form white light. Color temperature, which measures how white colors experienced, depends on the dominant wavelength of the blue LED and the composition of the phosphor.
Citizen SMD and COB LEDs
The measures for LED
Besides color temperature (CCT), other important measure for white LED is color rendering index (CRI). It is sometimes also called Ra index. CRI is a measure of how well the white (LED) light source can reproduce the colors of various objects in comparison with an “ideal” or natural light source. Maximum value of CRI is 100. Current LED generations reach already CRI>90, even CRI 97. There is also new discussion on color rendering challenging this old way of thinking. Is CRI the best way to measure the “quality” of light? Is it accurate enough and does it really tell anything? But that’s a topic for another post.
In addition to Color temperature and CRI, other important measures for general lighting LEDs are:
Lumen output tells the amount of lumens the LED produces (luminous flux).
Luminous efficacy tells the amount of lumens you get per watt fed to LED.
Tells you what voltage must be applied over PN-junction to get the LED conducting current.
Tells you what current you need to feed the LED with.
To get the picture, here are the same measures of a traditional incandescent lamp:
CCT: ~ 2700K-3000K
Lm Output: ~ 600-700lm
Efficacy: ~ 9lm/W -16lm/W
One important characteristic for white lighting LED is Tj-temperature. The letter ‘j’ refers to junction. So basically Tj-temperature is the temperature of the PN-junction. Absolute maximum temperature for Tj-temperature is around 150 Celcius degrees. Already at that temperature LED lifetime drastically reduces. Tj-temperature can be calculated from Tc-temperature (temperature of the cathode). Tj-temperature depends on how well the heat is conducted away from the diode. All lifetime estimations are based on the junction temperature. Tj is all that matters when LED diode operation is in question.
So, that concludes the main details of how LED works. If you have any comments or questions, don’t hesitate to leave your comment below.
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