Binning, in relation to LEDs is a practice designed to maximize effective utilization in the production of LEDs. In LED production, a single round wafer is coated with materials to create a semiconductor which forms the ‘heart’ of the blue LED. This is then sliced into small rectangles, wire bonds are inserted and the phosphor is added as a coating within the enclosure. The whole thing is then encapsulated to create a finished white light LED. That coating process creates significant inherent variations that impact color temperature, lumens and voltage of the LED. The process is not capable of producing highly consistent and strictly controlled production of LEDs.
To combat this issue, manufacturers sort production into voltage, color and lumen bins. Light output and color temperature are the most critical bin criteria that impact product performance. To sort for light output, LEDs are individually measured and sorted by lumen output into prescribed ranges. LED suppliers create their own standard set of lumen bins and provide clear information on expected lumen performance for each of their ranges. The larger a bin size – the more variation you can expect around color temperatures and outputs – small bins have tighter control.
Manufacturers select only those LEDs that meet their acceptable performances ranges – Sedna LED have four performance ranges, which is higher than the average, many manufacturers have only two. Customers buying from manufacturers with more ranges can expect less variation in their products.
SDCM is an acronym which stands for Standard Deviation Color Matching- sometimes known as a “MacAdam ellipse”. A 1-step MacAdam ellipse defines a zone in the CIE 1931 2 deg (xy) color space within which the human eye cannot discern color difference. Most LEDs are binned at the four to seven step level, so you can often see color differences. The variable nature of the color produced by white light LEDs means a convenient metric for expressing the extent of color difference within a batch is the number of SDCM ellipses in the color space that the LEDs fall into. If the chromaticity coordinates of a set of LEDs all fall within 1 SDCM most people would fail to see any difference in the color. If the differences extend to a zone twice as big (2 SDCM) you will start to see a color difference. A 2-step MacAdam ellipse is better than a 3-set, and so on. The diagram illustrates a CIE color space diagram, where SDCM ellipses are shown at 10 times magnification.
Traditional lighting systems such as fluorescent and high-intensity discharge (HID) are controllable, though control is predominantly limited to On/Off and, in some applications, dimming. In contrast, LED lighting can deliver not only significant energy savings and improvement in source longevity, but also the ability to implement another dimension of control: color.
In lighting design, color quality is predicted and evaluated using correlated color temperature (CCT), which expresses the shade of white light, and color rendering index (CRI), which expresses how closely a source renders colors compared to an ideal source.
The development of LED lighting enables the ability to adjust CCT using separately dimmable arrays of warm- and cool-CCT LEDs; color mixing RGB+A LEDs; or by adding separately dimmable color LEDs to white light LEDs.
As the color of light can dramatically transform the appearance of spaces, people and objects, this capability offers amazing design possibilities. The primary benefit is aesthetic, though color is now being linked to circadian health.
Another interesting potential is CRI modulation. By mixing red, blue, green and yellow or amber LEDs, we can not only change the shade of white light but also its color rendering.
The primary question, of course, is why we would want to do this. The simple answer is energy savings. By gradually reducing the red component of an RGB+Y (or A) mix, CRI declines as luminous efficacy increases, while both CCT and intensity can be maintained. From 10 to 25 percent energy savings can be realized, depending on the application.
This type of control strategy could be effective under certain conditions. Namely, spaces where 1) the lights must remain On during periods where there is no occupancy, and 2) intensity and CCT must be maintained.
An example of an application where good savings might be realized is an airport concourse. Late at night, the lights must be On and at full output for safety, though much of the concourse is empty. In this application, we could zone the luminaires in the central circulation areas to operate at full output and normal CRI and CCT. In peripheral areas, however, CRI could be reduced to save energy based on a schedule or occupancy.
Energy savings could be improved by increasing CCT as well, but such a change would mix sources with different CCTs in the field of view, which would be objectionable from an aesthetics perspective.
In 2005, a study was conducted at the Massachusetts Institute of Technology (MIT) evaluating CRI modulation as an energy-saving strategy in an open office space and two private offices at MIT’s Media Lab. The research goal was to determine how far CRI could be reduced in both the immediate and peripheral areas before occupants noticed the change and/or found it objectionable.
Eight experimental OSRAM SYLVANIA tunable ceiling-mounted LED panels were installed in the open office and two in each of the private offices. The 13 subjects were graduate students with no prior knowledge of the study. While CCT was maintained at a constant 5,000K and light levels maintained at about 30 footcandles, CRI was gradually changed over three seconds from CRI of 89 to CRI of 68. Fifteen seconds later, an instant message pop-up questionnaire asked the occupant what they were doing and whether they noticed the change.
Out of 320 queries that received responses, 203 indicated occupants did not notice a change. Occupants were most sensitive to changes in their immediate area and when changes occurred simultaneously in immediate and peripheral areas. The least noticed changes were those that occurred solely in peripheral areas. These results suggest CRI modulation is likely to be an acceptable method of saving energy in spaces that must be illuminated at full output even though they’re only partially occupied.
At the time the final study results were released in 2008, CRI modulation was considered a theoretical but not a practical strategy. LED technology has progressed a great deal since then, making it much more viable. However, the requirement of color mixing and careful zoning currently poses an installed cost that challenges this control strategy’s economic viability based on the potential energy savings. That being said, LED and control technology continues to advance in terms of performance and cost, and as it does, CRI modulation may increasingly become viable as an energy-saving lighting control strategy.
In the meantime, energy savings may be realized by applying color in another fashion in occupied spaces—as an indicator. Imagine a private office zoned so lighting outside the occupant’s immediate area becomes a saturated red when he or she is on the phone and therefore should not be disturbed. This would save energy while indicating to coworkers what the occupant is doing. There are many applications where this approach could be viable.
Color tuning has opened a vast potential in lighting design and application, and we are just beginning to pioneer. While these applications focus on aesthetics with some interest in circadian health, this extraordinary emerging dimension of lighting control may also be used to maximize energy savings.
Beyond the phase out of 40 and 60 W standard general service incandescent lamps as part of the Energy Independence and Security Act of 2007, at the start of 2014 the Voluntary California Quality Bulb standard has now been in place for a year so this seemed like an appropriate point to see what has occurred in the market in response to this legislation. Slightly more than a year ago, the California Energy Commission (CEC) finalized guidelines for investor owned utilities on what type of LED replacement bulbs would be eligible for rebates in the residential space. They established a < 1 year transition time where utilities could continue to offer incentives for products which did not meet the new CEC “California Quality Bulb” specification but still meet the current ENERGY STAR LED Bulb requirements. The table below is a quick summary of some key CEC requirements and how they compare with the updated ENERGY STAR bulb specification, which will become effective this September.
In a nutshell, the CEC raised the bar to qualify for an incentive with a significant increase in the Color Rendering Index (CRI) and color control as well as adding 2 years to the minimum warranty period. At the time this legislation was being publicly reviewed, the main concerns raised by some manufacturers were that these requirements would increase power consumption and cost as described in a prior LED Journal article. Using the latest LED Lighting Facts database, I did a search of how many new 60 W equivalent (>800 lumens) A19 medium screw base lamps with a CRI greater than 90 and a color temperature of 2,700 or 3,000 K had been introduced. This lamp was selected as it is one of the most common types used in the residential segment. Recently, four new bulbs were added, all from different suppliers, indicating manufacturers are responding to the market opportunity. This is not surprising since California represents > 10 percent of the US households and tends to be a bellwether for energy efficiency trends in the country. Since the data entered into the LED Lighting Facts is incomplete, it is not clear if all the bulbs will meet all the constraints of the new CEC requirements as some are not released on the market.
One of the bulbs introduced last fall targeting the CEC requirement was the 2700K 800 lumen Cree TW (True White). Since Cree already had an 80+ CRI ENERGY STAR 800 lumen lamp, this offers a vehicle to compare power consumption and price. In this case, the added power for the high CRI implementation increased consumption by 4 W (~ 40 percent power increase to 13.5 W) and the Cree TW product is priced $7 higher; $19.97 versus $12.97 for their 80+ CRI version.
The remaining aspect to consider is on the incentive side. California has a patchwork of public utilities as well as three large investor owned utilities. The latter must have their energy efficiency incentive programs reviewed by the utility commission and all have taken a different approach for the “Quality Bulb” initiative. Based on a random retail pricing sampling in the three utility regions, the incentives specifically on the Cree high CRI lamp versus the comparable 80+ CRI lamp were $10, $7 and $0. So in some areas the full price difference is borne by the consumer, while at the other extreme, the CEC compliant bulb is $3 less than the 80+ CRI version.
As more CEC compliant bulbs come on the market, the situation will no doubt become more dynamic, but it would be interesting to analyze comparable store sales to see how different price points influence market adoption. This would be very useful in seeing how consumers respond to the higher performance “Quality Bulb” and if it encourages retailers to introduce these bulbs in other regions of the country. Of course the most interesting test is replacing an existing bulb with a CEC bulb in a normal household environment and seeing if anyone in the family notices the difference.