Glossary

This technical glossary is part of our personal journey to understanding the different scientific aspects of lighting. Whenever possible, we will try and put the focus on OLED-related topics as this is at the heart of our work. The purpose of the Glossary is to give you, the reader, a guide to your own journey to understanding specific terms and backgrounds. There is a tremendous amount of training and information material available and we found the challenge, not in lacking resources but rather in separating the wheat from chaff. We will therefore not copy or rewrite existing literature but point to sources we found useful amongst the variety to be found on the internet. The topics we will highlight here are things we found useful in order to get a better grasp on OLED technology and its features.

We do not claim to be infallible and encourage your suggestions for improvements, changes or additional topics to be covered.


Table of Content


Electrical Basics

If you want to build you own lamp using our OLED panels, there is no way around some electrical basics in order to light the panels up. You will come across terminology like ‘power supply’, ‘driver’, ‘constant current’ or ‘serial connection’ and depending on your background that might seem elementary or Greek to you. Either way, the topic is too complex to be dealt with in a short glossary post which is why we are suggesting two options here:

  1. If you are a novice in all electrical things and just want to do a nice design with an OLED, we will point you to all the things you need and provide you with a simple plug & play installation guide.
  2. For those of you with some background in driving LEDs or other electronics, we would encourage you to send us your questions regarding OLEDs. We will answer either individually or share the topic in the FAQs to the benefit of all users.

If you still want to learn some electronic basics without starting a degree course, you may begin your research here.

If you want to go deeper into electronics and actually understand how things work, we recommend enrolling into a proper course on electronics. You will need to dedicate time to it on a regular basis for several weeks in order to see some real progress so signing up for a course is a great way in helping yourself managing that challenge. Out of the multiple courses available online, we recommend:

Please note that the course offerings may change over time so best check out what is available.


Lighting Basics

Before going into detail about lighting-specific terms and problems, a basic understanding of visual light and its perception by the human eye is useful. For anyone new to the topic, we recommend the following two short videos by Dave Standard of Southern California Edison.


Colour

Colour is at the heart of every lighting product. Most people have an instinctive idea about colour and especially about the colour “quality” of a light source. However, in order to manufacture “high quality” light sources, a more scientific understanding and explanation of colour and its different aspects is required. For anyone new to the science of colour, we highly recommend Craig Blackwell’s short introduction into Colour Vision:


RGB-Light Source

An RGB-light source uses all three primary colours red, green and blue to create white light and many more colours.

When it comes to white light, there are generally two methods used which both create light appearing a s white to the human eye:

  1. An RGB-light source with a red, green and blue light emitter
  2. Blue light sources with a yellow filter (as used in white LEDs)

Both ways have their advantages and shortfalls; however it should be stressed that the light although being white in both cases will be ranked differently in terms of quality by the observer.


Chromaticity

Chromaticity is a model describing the quality of a colour without the influence of luminance. A good overview is given in the following Wikipedia article: en.wikipedia.org/wiki/Chromaticity.


Chromacity Diagram

A lot of technical specifications define the colour of a light source with a set of XY-coordinates. This system is necessary in order to define and manufacture artificial light sources with the required quality and variety.

The coordinates are based on a model created by the International Commission on Illumination (CIE) called the Chromaticity Diagram. More details can be found at Wikipedia.


Colour Temperature (CCT)

White light is commonly rated with the temperature of an ideal black body radiator and measured in Kelvin (K). To get a brief overview on CCT, we recommend following short video by Steve Standard:

CCT is widely used in OLED specifications as most OLED panels are made for general lighting applications, and therefore tuned to white light. In general terms, the following main categories are used:

  • 2700-3300K = warm white (i.e. the light of a candle)
  • 4000K = neutral white (i.e. a fluorescent lamp)
  • >5000K = cold white or daylight white (there is a lot of variety as daylight is not the same every day and every time of the day considering clouds, sun, summer, winter, …)

CCTs around 3000K are often used for home lighting applications (as a lot of people like a warm white) whereas neutral white is more common in commercial or office environments.

Please refer to Wikipedia for more detailed information on CCT.


Colour Rendering Index (CRI)

The CRI value of a light source is often referred to as a measure of how natural the light of an artificial light source is perceived – the higher the CRI, the better the quality of light. What the CRI figure actually does is compare a light source (i.e. an OLED panel) with a reference light source (i.e. day light for a Colour Temperature (CCT) > 5000K) under certain, specified criteria. To give an example, in two light sources where one is rated with a CRI of 80 and one with a CRI of 90, the latter will appear closer to natural daylight as the reference source than the former. However, trusting solely on CRI to determine a good quality light source can be disappointing – why?

As with any scientific concept, the CRI works within a set of specified criteria and one which is not well represented, for instance, is colour saturation. The human eye, on the other hand, has a natural preference for highly saturated colours. This means that an artificial light source with low CRI (<70) can actually be perceived as having better light quality than a higher CRI (<80) light source due to the fact that the former delivers a much higher level of colour saturation than the latter.

Wikipedia: https://en.wikipedia.org/wiki/Color_rendering_index


Colour Quality Scale (CQS)

CRI (Colour Rendering Index) as a metric to define colour quality does have some deficiencies, especially when it comes to LED and OLED light sources. The CQS developed by the National Institute of Standards and Technology (NIST) solves most of the problems are associated with CRI.

For a full overview over both metrics, we recommend the following video published by Olino.org:

Despite its advantages, CQS has still not been fully adapted by the international lighting community. As the problems with CRI still prevail, it is expected that a new metric for colour quality will be introduced in the near future.


Binning

Binning as a term widely used for LED products in particular and in the semiconductor industry in general. The term basically refers to a statistical classification of production output for a range of specified parameters. The main reason behind it is the sometimes huge degree of variation in yield within the semiconductor production process. Binning makes it possible to classify a production lot within a defined range and therefore much more adaptable for an end application. The main binning criteria used within the LED industry are:

  • Forward voltage
  • Colour
  • Luminous intensity

LED-Lampe
LED-Lampe mit fünf LEDs

Why is binning important? Taking the example of the torch below, LEDs are usually used as a group arranged within an application. If you have a number of LEDs closely placed together, the eye typically expects a uniform appearance of the array in terms of colour and brightness; variations are picked up by the eye making it less aesthetically attractive (we love to find the odd one out!). Binning helps to make a selection within a production lot and ensure that the LEDs in below torch, for instance, vary only within a tiny range not noticeable by the human eye.


Fab

This often used term refers to a semiconductor manufacturing plant, alternatively also referred to as foundry. For OLED manufacturing, it is common to label the fabs in generations according to the maximum substrate size they can manufacture.

Common fab generations used for OLED production are:

  • Gen 2: 370x470mm
  • Gen 3.5: 620x750mm (or six 14.1" panels)
  • Gen 4.5: 730x920mm
  • Gen 5: 1100x1300mm
  • Gen 5.5: 1300x1500mm (or six 32" panels)
  • Gen 8: 2160x2400mm
  • Gen 8.5: 2200x2500mm (or six 55" panels)

Currently, all leading manufacturers for OLED lighting panels use a Gen 2 fab (or below), however, upgrades to Gen 5 are in discussion for the coming years.


MacAdam Ellipse

MacAdam-Ellipse

One of the methods used to classify LEDs into colour bins (link to Binning) is the MacAdam ellipse, named after the American physicist David MacAdam. The basic idea behind this concept is to classify colour spaces according to the degree of how much the human eye can differentiate between them. The most commonly used MacAdam ellipse has 7 steps or circles within the CIE chromaticity diagram (link to Chromaticity). Two colour samples from within the inner 4 circles of the ellipse are barely distinguishable to the human eye. If they are even placed within the inner 2 circles, they are indistinguishable to the human observer.

Putting that into a practical context, LG Display for instance offers all OLED panels with a 4-step MacAdam binning as standard. This means within an display of several OLED panels, the colour differences would be barely distinguishable. On request, even a smaller MacAdam binning is possible, which would make the OLED panels completely indistinguishable in terms of colour variations.

Wikipedia: https://en.wikipedia.org/wiki/MacAdam_ellipse


Lifetime

OLED panels often specify a lifetime of LT70 or LT90. An LT70 of 40,000h for instance means that the panel still has 70% of its original light output after being in operation for 40,000h. The main boundary conditions to calculate the lifetime are luminous flux and input current. It is therefore important to point out the direct relationship between the brightness of an OLED (or any light source) and the rated product lifetime.

As a practical example, the LG Display OLED panel LL055RS1-62A1-OY1 is rated with a luminous flux of 75lm (Lumen) at a driving current of 150mA and LT70 of 40,000h. It is however possible to drive the panel with more than 150mA, 160% or 240mA are still safe according to the manufacturer. As there is a roughly linear relationship between current and flux, doubling the current means roughly doubling the luminous flux – the panel appears much brighter. The trade-off here is that any increase over the rated 150mA will entail a reduction in life time i.e. less than 40,000h.

What does that mean for a real life example?

Using the LG Display OLED-based desktop lamp on the maximum option (which equals 200% of its standard rated current), the panel life time will reduce from 40,000h to about 13,500h. That may sound like a dramatic reduction but putting that in relation to the average consumer usage of a desktop lamp of about 1,000h/ year or 3h/ day, this would still amount to 13.5 years. And remember at this point, the lamp would not be defunct, it would just mean that the brightness is reduced to 70% of its original value.


Physical characteristics (lm, cd, lx, cd/m², lm/W)

Luminous Flux (lm)

Luminous flux, measured in Lumen (lm), is a widely used metric to measure the output of a light source. It is important to note that the luminous flux gives the amount of light emitted in all directions; whether useful or not. To find out more about luminous flux, we recommend this Wikipedia article.

Luminous Intensity (cd)

Compared with luminous flux, luminous intensity measures the light output under a solid plane. For example in a classic desktop lamp, an increase in the beam diameter (for instance by exchanging the lampshade for one with a wider beam angle) would increase the luminous intensity. The luminous flux as a measure for the total light generated by the bulb, on the other hand, would remain the same.

To find out more about luminous intensity, we recommend Wikipedia.

Illuminance (lx)

The illuminance indicates which part of the luminous flux produced falls on a certain area. It is therefore indicated with lm / m2 or Lux (lx). The level of illuminance depends greatly on the structure of the lamp. So the distance between lamp and illuminating surface is, therefore critical; the greater the distance, the lower the illuminance. The illuminance is often listed in the requirements for interior lighting. More information can be found here.

Luminance (cd/m2)

Luminance is an indicator of how bright a surface will appear to an observer. It is commonly used in the display industry to characterize the brightness of a display, such as a computer or TV screen. Since an OLED lighting panel is not too technologically different from displays and is also a surface light source, luminance ratings are quite common in OLED literature. For further reading, please refer to Wikipedia.

Luminous Efficacy (lm/W)

Luminous efficacy defines the ratio between luminous flux and input power. A rated efficacy of 60lm/W means that the light source is emitting a flux of 60lm for every W of electricity used to power it.

To find out more about luminous efficacy, we recommend Wikipedia.


Case Study: Brightness

A commercial 40W bulb usually shows a luminous flux of 400lm on the label. This figure is only partially helpful as often just a portion of the radiation angle of the light bulb is used directly, as shown in the picture below. Thus, of the 400lm luminous flux stated on the light source, a good part of the brightness is lost inside the housing. One can therefore assume that in all lighting sources that cannot be used openly because of the resultant glare (e.g. LED), the indicated brightness of the light source is always much higher than its actual application.

For an OLED panel, the situation is slightly different. Looking at the example of below Sky Table Lamp from LG Display, the OLED panel is used openly without any shades or reflectors. This is possible due to the naturally diffusive effect of OLED light that does not produce any harmful glare. Consequently, the luminous flux of the lamp is equal with that of the light source. As a result, the actually perceived useful illuminance for the user can be similar when comparing the Sky lamp with a traditional desktop lamp, despite the fact that the luminous flux of the used OLED panel is with 250lm significantly lower than the flux of a 40W light bulb.

Reading Lamp
OLED-Lamp Sky

Case Study: Efficiency

The luminous efficacy is a commonly used measure of demonstrating the efficiency of a light source such as LED and OLED. It is important for the user to note the efficiency of an application rather than the light source in which it will be used. The following example demonstrates the difference. The figures each show a reading lamp but with different light sources: on one hand the classic light bulb, on the other, an OLED panel. Focusing on the efficacy, the bulb is specified to be around 10 lm/W. The OLED in contrast ranks in the range of 60 lm/W. By substituting the classic bulb for a retrofit LED bulb, a slightly higher luminous efficacy of 80 lm/W can be achieved. However, because the bulb has to be housed to reduce the glare, the useful luminous flux of 400lm is significantly reduced. The luminous efficacy of the lamp is therefore less than that of the bulb, since the electric power is independent of the operation, whether the bulb is covered by a shade or not. In the OLED lamp however, the situation is different. Because OLEDs can be used without a shade due to their diffuse light distribution, the luminous efficacy of the lamp is identical to that of an unshaded OLED panel.

Übliche Leselampe
OLED-Lampe Sky