Automated LED test system enables consistent colour

23rd July 2015
Source: MADE USA
Posted By : Barney Scott
Automated LED test system enables consistent colour

Now that the LED lighting transformation is now in full swing, there is another less conspicuous revolution going on in the manufacturing of LED lamps - the more comprehensive testing of colour values and brightness for improved lighting consistency. The potential for output variations that seem to be inherent in LEDs can be problematic for lamp manufacturers, light fixture producers and customers who desire consistent colour and brightness.

This may be especially true of customers who have dramatic architectural lighting installed, with LED fixtures that can travel continuously down hallways, around corners, and up stairways.

This “consistency revolution” is particularly concerned with the more complete testing of the performance of light engines, the power source of LED lamps. Composed of the combination of one or more LED modules, together with an LED driver (also known as electronic control gear, or ECG), the variations in light engine output account for most of the variations in LED colour temperature, brightness and other output characteristics.

If LED light engines are not subjected to more comprehensive and consistent testing, it is likely that those LED output characteristics will continue to vary as current LEDs are replaced by rapidly changing LED technologies.

The lighting industry has sold, or “binned”, LEDs by brightness (“intensity”) and colour (“CCT”) for decades, so in that sense LED binning is really only an extension of an existing paradigm.

With literally hundreds of separate colour and intensity “bins,” it will be difficult for end users to get accurate replacement lamps, since it may be problematic to trace the binning number, or the chip used in the original lamp’s light engine may have been superseded with a model that produces somewhat different light characteristics.

To alleviate this problem, many LED light engine manufacturers provide 100% “functional testing” of lamps along with first article and periodic inspections of a production run. However, functional testing merely validates that the lamps will illuminate, and usually don’t analyse the CCT (Correlated Light Temperature), CRI (Colour Rendering Index) or brightness of lamps. If there was an improperly marked or binned reel of components it would be possible to produce a significant number of units that had improper performance characteristics.

The more complete solution would be the comprehensive testing of 100% of LED light engines, and those who manufacture those items are developing more sophisticated testing systems that will automatically and thoroughly test them before they are sold to lamp manufacturers.

Companies such as MADE (Manufacture And Design Electronics – formerly CI Lighting), Auburn Hills, MI, a supplier of US manufactured LED light engines to the industry, are moving toward 100% comprehensive testing, control and data reporting and recording, plus production controls via a robotically-controlled test cell that will inspect and record the circuit boards for each light engine, including colour temperature, power (voltage and or current readings), brightness (luminous flux), and colour rendering index

By testing and controlling those factors, light engine manufacturers will enable the production of LED lighting with much greater consistency in power, colour and brightness attributes, saving time and binning efforts in the process. MADE, for example, now applies this type of testing to all types of customer LED light engine designs, including those for retrofit products such as the light strips that replace fluorescent lamps and ballasts.

“This inspection technology will provide our customers the assurance that the extensive binning processes we employ are producing the desired results in the final products they manufacture,” explains Donald Bernier, President, MADE. “We can be 100% sure that the binning is providing the desired 3-step McAdam ellipse colour results as well as provide voltage and current binning characteristics for the individual boards as well.  This will allow our customers who use a constant current driver to match light output from one module to the next, thereby providing consisting colour and intensity results.”

An added benefit of this approach is that production will be stopped if product is out of tolerance, thereby reducing the risk of scrapping product. Reducing such waste will enhance cost competitiveness and provide competitive advantages - both price and reliability - even with offshore suppliers.

Mike MacLeish, Director of Engineering at SPI Lighting (Mequon, WI), agrees that the consistency of LED colour attributes and brightness have a definite influence on lighting quality and applicability. SPI Lighting, which uses LED light engines from MADE, produces a broad range of fixtures including architectural lighting for workspaces and building exteriors, high-power asymmetric commercial lighting, and linear lighting for spaces such as offices and retail stores.

“In our industry, and particularly with architectural lighting, you have lighting fixtures where you view the source directly,” says MacLeish. “So it is critical to make certain that the colour temperatures and brightness of the lamps are consistent. I won’t release a design unless I’m sure those requirements will be met.”

Like other lighting manufacturers, when SPI creates lighting fixtures the lamps are graded by the colour temperature that the light produces, which varies from “warm,” such as a candlelight colour, up to “daylight,” which is equivalent to the bright light you see outside.

“This measure is called CCT, correlated colour temperature, which is gauged in degrees Kelvin,” MacLeish explains. “When it comes to LED lighting, we look at 2,700 Kelvin as “warm,” and 5,000 to 6,000 as a “true bright” or cool temperature. This is important to users because when they put lighting in their kitchens they don’t want it to look like the lighting in a doctor’s office; on the other hand, the doctor doesn’t want his office to have the look of candlelit fire.”  

Another factor, the CRI (colour rendering index) is also an important measure to lighting manufacturers that is inspected by the light engine testing system. CRI is the ability of the light source to be able to portray the true colour of the objects it is illuminating in their true colour form, which is how the colour would look in natural sunlight. The latter is the temperature that would enable people being able to see the true colour of paint chips or other colour samples.

“CRI is measured from 0 to 100, and a good colour rendering ranges from the upper 80s to mid-90s on the index,” MacLeish says. “Examples good uses for this higher range would be a grocery market where the owner wants the customers to see red apples as a healthy red, and the green apples to look the authentic green.”

Both colour condition temperature (CCT) and the colour rendering index (CRI) are very important to the lighting industry, he adds.

It is noteworthy that even today’s more robust binning systems can fall short of providing the consistency that is promised by the 100% light engine testing system. One reason is that the electroluminescence - the luminescence produced by voltage to an LED lamp - as well as colour of the light is determined by the lamp’s light engine microchip. Since the chips used in LEDs continue to change over time, their manufacturers often recommend a “blending” technique to achieve the desired lamp colour correctness and facilitate more accurate binning. However, this technique may not produce the consistent results that lighting manufacturers are looking for.

“When we go from production lot to production lot, we have concerns that blending - ‘recipe’ - that chip manufacturers recommend to achieve a common colour within the colour temperature tend to vary from one generation of the LED product to the next,” explains MacLeish. “With different chips we see shifts that can make them fall outside the visual parameters to a point where the differences becomes noticeable.”

The 100% testing system can make adjustments during production runs in order to compensate for chip variations and achieve more uniform colour and brightness characteristics.

In addition, in conjunction with the 100% inspection process, this test system stores all pertinent LED light engine lighting characteristics - from lot to lot - in a database that can be made accessible to lighting manufacturers.

This database could prove valuable to manufacturers for use in handling warranty claims and as an extended service to enable them to match the colour and brightness of an LED lamp from a given production lot years later when the lamp eventually requires replacement.

“Having the ability to match an LED light with one produced years ago would be an important capability, MacLeish notes. “We might know exactly what model the chip is, and that same chip might not be available, so the chip manufacture will provide us the closest model, and that is usually within an acceptable range. To actually tune a chip down to a very close colour mix of a lamp that requires replacement is really fantastic.”

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