LED binning explained
Slight variations in the LED manufacturing process, particularly in regards to lumens, colour temperature and LED voltage, means individual LEDs are extremely difficult to make exactly alike. LEDs tend to be similar but rarely exact. In order for a light to be, say, red, manufacturers need a technique to ensure all of their LEDs are indeed red.
The technique manufacturer’s use is called LED binning.
But what is LED binning, and why is it important to use?
definition of LED binning
The technique manufacturers have developed to classify their LEDs is called LED binning. That is, LEDs are placed into similar categories, or bins, with the category/bin defined by the similarity in lumens, colour and voltage. Lumens and colour are the most important parameters in LED variability. Fortunately, binning according to lumens is relatively strait forward to define and measure. Binning LEDs according to colour is more complex and to understand the process it is necessary to introduce chromaticity diagrams.
the CIE 1931 chromaticity diagram
The CIE 1931 Chromaticity Diagram (Figure 1) describes colour as seen by the human eye in full daylight. The diagram describes colour along x and y axes with individual co-ordinates related to a unique colour. The CIE 1931 is a 2-dimensional diagram of colours with the same brightness (intensity). The distribution of colours, in reality, is 3-dimensional where the third axis relates to brightness. The 2-dimensional co-ordinates of CIE 1931 are related to specific colours. For example, the co-ordinate 0.3, 0.3 is white light, whereas 0.0082, 0.5384 is greenish cyan and 0.7066, 0.2934 is HeNe laser red. All colours in the visible spectrum are represented on the chromaticity diagram.
Figure 1. The CIE 1931 Chromaticity Diagram. Source: http://en.wikipedia.org/wiki/CIE_1931_color_space
Watch a YouTube Video on an Introduction to Colorimetry
Experiments in the 1930’s and 1940’s by the American physicist, David MacAdam, showed there are regions on the chromaticity diagram that contains all colours which are indistinguishable to the colour at the centre of observation. That is, an ellipse could be drawn around a single point on the chromaticity diagram where, within that ellipse, the human eye cannot distinguish different colours. Figure 2 shows the 25 ellipses drawn from MacAdam’s experiments on various colours in the chromaticity diagram. Within these ellipses, the test subject could not distinguish different colours.
Figure 2. MacAdam Ellipses drawn on the CIE 1931 Chromaticity Diagram. Source: http://en.wikipedia.org/wiki/MacAdam_ellipse
LED binning according to colour is based MacAdam’s finding’s that humans perceive colour within a region, or a concept known as “just noticeable differences”. In the CIE 1931 Chromaticity Diagram, a series of boxes are drawn to indicate regions of just noticeable differences. Within each of these individual boxes, the human eye cannot perceive any difference in colour. Therefore LED manufacturers indicate the co-ordinates of their LED on the chromaticity diagram and end-users can reliably use this information to achieve the correct colour light needed in their application.
LED colour mixing
Furthermore, information on the chromaticity co-ordinates of LEDs can be used to mix two or more LEDs together to create different colours. Just as mixing blue and yellow paint will give green paint, mixing two different LED colours will produce a third colour. Therefore if an application requires a light of a certain colour, and there is no LED available of that colour, then using an LED manufacturer’s chromaticity co-ordinates, and their LED binning process, can create the desired colour light.
white light standards and ANSI
Mixing LED bins to create a different colour is a common practice to create the white light required for indoor LED lamps such as in offices and homes. Several manufacturers, such as Cree and Phillips, adhere to standards for the production of LEDs for white light. The ANSI C78.377-2008 LED binning standard, for example, specifies the range of chromaticities recommended for general lighting. Although LED manufacturers can produce high quality LEDs that closely match, it is extremely difficult to consistently output exactly the same LED. By manufacturing within the range of chromaticities recommended in ANSI, the production process is more efficient and the end product cheaper. Within the range of recommended chromaticities, two or more LEDs can be combined to give the correct colour output – just like if you’re missing green paint you can combine blue and yellow paint to achieve the same result.
Standards and codes, such as ANSI, draw parallelograms on the CIE 1931 chromaticity diagram which specifies the range of recommended chromaticities. These parallelograms are based on MacAdam’s experiments and findings of just discernible differences. The parallelograms are not drawn all over the CIE 1931 chromaticity diagram, but focus on a very specific area of the diagram related to white light (Figure 3) and the black body line (BBL).
Figure 3. The white light region of the CIE 1931 Chromaticity Diagram and example parallelograms (blue boxes). Source: Cree.
back body line, Planckian Locus, & correlated colour temperature (CCT)
The BBL was formalised within the realm of quantum physics and relates to the spectral emissions of an ideal black body radiator. As the ideal black body radiator begins to heat it turns red, yellow and, at very high temperatures, white. We have all seen this when we watch a metal that is being heated glow from red, yellow and white as it gets hotter and hotter.
In terms of LEDs, the BBL is related to the correlated colour temperature (CCT) and is expressed in Kelvins (K). CCTs of 3000K are defined as warm white and lie towards the yellow region of the chromaticity diagram (Figure 4). Neutral white ranges between 3500 and 4000K and cool white ranges between 4500 and 5000K. As seen in the BBL in Figure 4, a value of 5000K lies towards the blue portion of the chromaticity diagram so cool white is often described as having a bluish hue.
Figure 4. The BBL (black body line) drawn on the CIE 1931 Chromaticity Diagram. Source: Cree.
Watch a YouTube Video on How LEDs can be used to find Planck’s Constant
parallelograms and the ANSI standard
The ANSI standard draws parallelograms around the BBL on the CIE 1931 chromaticity diagram as seen in the example of Figure 5. The Planckian locus in Figure 5 is the BBL and to achieve white light an LED must have its co-ordinates lying on or very close to this line. As an example, a user wants to achieve a warm white LED light and must select LEDs based on a manufacturers LED binning. Firstly, the user should choose an LED bin with the CCT value of 2700K. Then any LED bin bounded by the chromaticity x,y co-ordinates as delineated by the first box on the right hand side of the diagram in Figure 5.
Figure 5. Parallelograms along the BBL as defined by ANSI. Source: Cree.
LED manufacturers and standards
Some manufacturers of LEDs, such as Cree, go further and subdivide the ANSI parallelograms, as shown in Figure 5, into smaller parallelograms. An example is shown in Figure 6 which demonstrates the Cree XLamp binning structure.
Figure 6. An example LED Binning from Cree for their XLamp. Source: Cree.
Cree even goes a step further and subdivides the LED bins in warm white, neutral white, and cool white, into smaller LED bins. This is demonstrated in Figure 7 which shows the warm white LED bins from Figure 6, subdivided into smaller LED bins. The motivation for such fine scale subdivisions is to achieve a greater quality and more consistent LED product.
Figure 7. An example LED Binning from Cree for their XLamp in the warm white region. Source: Cree.
instrumentation for LED binning
Edaphic Scientific has a range of low cost, portable, and easy to use spectrometers specifically designed for LED binning. The software with our spectrometers automatically generates CIE 1931 Chromaticity Diagrams, x,y co-ordinates, CCT, LED bins, CRI, CQS and more parameters.
LEDs are binned, or categorised, according to their brightness (lumens), colour (chromaticity diagram), and voltage. LED binning is an efficient method to overcome variations in manufacturing of LEDs to lead to a better quality and consistent light for the end user.
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LED Color Mixing: Basics and Background. Cree Technical Note CLD-AP38 Rev 1A. 2014.