Jessica Rowbury discusses the technologies used to test LEDs and how test equipment manufacturers are keeping up with the simultaneous demand for accuracy and lower cost
In December last year, Isamu Akasaki, Hiroshi Amano and Shuji Nakamura were awarded the 2014 Nobel Prize in Physics for the invention of the blue light emitting diode. The blue LED, which in turn is the basis for white LEDs, led to a transformation of lighting technology, as white light sources that were brighter, more energy efficient and longer-lasting were able to replace older, inefficient lamps. The invention has contributed to reducing the world’s energy consumption among other big benefits, such as increasing the quality of life for people in developing countries with cheap, solar powered LEDs
Those using LEDs now expect a consistent, high-quality product, which puts demands on the test equipment. However, test equipment manufacturers must find the right balance between cost, speed and accuracy for their products.
LEDs are superior to incandescent light sources not just because they are more energy efficient, but because they can be engineered to have desired affects on people, plants, and animals by adjusting the total output power, wavelength or colour emission. This can be applied to almost anything, from enhancing alertness and activity in humans, stimulating growth in plants, or even to boost the life and appearance of an aquarium: ‘For example, LED aquarium lighting designers develop their LED light specifically to emit frequencies that coral reefs tend to like while giving the other desired effect of tank lighting,’ said Jason Pierce at spectroscopy company, StellarNet.
These new applications, however, have placed increasingly stringent demands on the optical characterisation of LEDs. This is particularly the case for white LEDs, where the production process means that they tend to vary considerably from one another. To produce white LEDs, a blue emitting chip is covered by yellow phosphor to create an overall impression of white light. ‘Deviations already start when producing the wafers to get this blue chip all LEDs are based on. The blue wavelength is always a little bit different − the conversion efficiencies are always a little bit different. And, if you put a yellow phosphor on top of that blue chip, the phosphor layer itself varies. It is not possible to produce the same white LED after LED; each LED is individual,’ explained Dr Matthias Hoeh, product manager at Instrument Systems, which provides light source characterisation equipment.
And as white LEDs are used more for aesthetic lighting, and for applications that involve the general consumer, getting consistent results is vital. ‘The perceived expectation in the market is that if a consumer buys a particular luminaire or light bulb, they would expect that if they bought another one then they would have the same colour,’ said Richard Austin, president of Gamma Scientific. ‘That’s a particularly difficult challenge to make sure the production of white LEDs is consistent.’
In order to get precise and reproducible results, LEDs are tested at several stages during production, from the wafer level all the way through to sorting on instrumentation.
Although LED sorting is the largest application, as David Creasey, vice president of sales and marketing of spectroscopy company Ocean Optics, pointed out, more and more manufacturers are testing at earlier stages during production, in order to maximise yield and keep costs down. ‘As the price of the LED drops, then you need to improve your yield to make money, so you need to make sure you get a more accurate measurement earlier in the process, more at the wafer level,’ he said.
And, by making measurements at the start of production, manufacturers are able to have more control over the characteristics of the LEDs, added Instrument Systems’ Hoeh. ‘[LED manufacturers] can combine the right chip with the right phosphor to get the emission they want. For this they need extensive measurements, for the knowledge to control the process.’
Making the measurement
There are different machines that can be used to test LEDs, but commonly spectrometers or spectroradiometers are used as they can provide the full spectrum in order to characterise the colour, relative power and absolute spectral intensity of LEDs. Optical spectrometers analyse the spectral characteristics of light radiation, and by adding optical probes and absolute calibration, this type of measurement system is turned into a spectroradiometer.
In a typical laboratory set-up, the LED is placed at the side port of an integrating sphere, which is used to collect the emitted light, and this is then coupled into a spectroradiometer to obtain the measurement data. ‘You can then interpret it in terms of, for example, the radiance flux measured in Watts or the photometric flux measured in lumen, for example,’ explained Hoeh. The integrating sphere can also be coupled with a spectrometer for measuring the colour, relative power and absolute spectral intensity of LEDs.
If LEDs are testing during sorting, then the spectrometer can be integrated onto a production line with a cosine corrector, which allows a known amount of light to be collected at an 180° angle. ‘Light passes right past [the spectrometer], and if you can match the speed of that movement to the integration time − it might take about 20 or 30 milliseconds to take a measurement, for example − that determines the speed at which the LEDs can go past it,’ noted Creasey.
White LEDs are typically more challenging to measure than green or red LEDs because of the blue spectral region, according to Creasey: ‘Most white LEDs typically have a gap in the blue. If you were to draw a spectra, it would go up very sharply, and instead of being a nice flat top across all of the wavelength range, it is as if someone has stuck their finger in the intensity profile in the blue,’ he said. ‘The challenge is that you’re measuring a little bit of light in the blue, and a lot of light in the red − you have to make sure that the spectrometer has the dynamic range so you get accurate measurements in the red where there is a lot of intensity, and also accurate measurements in the blue where there is less intensity.’
To increase the accuracy of measuring in the blue spectral region, some instruments use back-thinned, back-illuminated, CCD sensors, Hoeh pointed out: ‘Not so much blue light is absorbed by the CCD before reaching the photosensitive area [in a back-thinned sensor]. We optimise our optical components, optical system, for very low stray light in the spectroradiometer. This is very difficult to do.’ The detector must also have good thermal stability for it to give accurate and consistent results over long periods of time.
Another feature which improves sensitivity in the blue region is the design of the optical gratings. Concave gratings have a reduced number of optical surfaces than Czerny-Turner gratings, which have separate focusing and collimating mirrors. ‘Concave grating design causes less light scatter than typical optics called crossed Czerny-Turner,’ Pierce told Electro Optics.
Speed, accuracy and cost
As the LED market is growing and production is increasing, so is the need for higher-speed, lower cost testing equipment. ‘What’s driving the business is speed and volume. As people move from the incandescent bulb to the LED, there are more of them being made, and people want to make them faster,’ Creasey remarked. ‘Customers are always asking for smaller, faster, cheaper.’
Over the last few of years, however, the price of LEDs has lowered considerably, and it is quality that is becoming ever more important, driving a demand for highly accurate testing equipment. ‘Now there is an increasing demand for high quality products, so now that the prices have dropped to an acceptable level, there is always a competition between lower pricing and demand for higher quality,’ commented Hoeh.
‘There is a drive for more accuracy in testing compared to ten years ago where accuracy was less important. But as LEDs get used more and more, and it moves into places like indoor and housing lighting, it becomes more and more important,’ added Dr Kong Loh, CEO of Gamma Scientific. ‘Accuracy is becoming a higher priority.’
LED quality is particularly important in applications that might change how the final product is perceived by the consumer, for example backlighting for displays. ‘Backlighting for displays, for mobile displays or tablets, for example, there we would like to have homogenous light on the display, so that the upper right corner looks the same as the centre, for example,’ noted Hoeh. ‘This is where we need high-quality testing equipment, even in the production line, because we have large variations.’
But, although the demand for quality is increasing, there are still applications where the need for lower-cost equipment is greater than the need for quality, added Hoeh: ‘There are other applications where colour quality is not so important, where cost-down is required, so you just do a very simplistic pass-fail test − for example, a status LED just needs to show if a device is working.’
Therefore, test equipment manufacturers are looking for the right balance between cost, accuracy and speed − but ideally without compromising one for the other. ‘If you can provide accurate results at a lower cost and higher speed, then this is what differentiates our products. The challenge is speed, accuracy, and cost, and finding the optimum for that,’ said Austin of Gamma Scientific.
‘The challenge for Instrument Systems is to bridge this gap between the demand for lower cost and at the same time higher quality,’ added Hoeh.
As LEDs are used more to create an aesthetic impact, for example in architectural or museum lighting, it is important to measure how the human eye perceives colour, which can affect how a person views an artefact or building. Using spectroscopic instruments allows for more in depth analysis of the emission spectrum, which makes them the best instruments for measuring human perception, Creasey pointed out: ‘The spectra allows you to see the full spectrum of the LED, which allows you to measure chromaticity − a measure of what the eye perceives the colour to be. The most challenging thing is that you have to train the instrument to operate like the human eye,’ he said. ‘If you use a filter-based system, it only has three or four sensors, so it is very difficult to mimic the human eye.’
But there are still improvements to be made, not so much to the test equipment, but in the scientific understanding of how the human eye perceives light. Just like research into how wavelengths of light affect the growth of different plant species, there are ongoing studies looking into how the human eye perceives light. This in turn is important to improve measurement techniques. ‘There is research being done on the effect of blue radiation on people and the circadian rhythm sensor in the eye. In order to have good, consistent results in human factor studies you really need accurate spectroradiometric data, and that is one thing that is really lacking in the biosciences in general, where accurate spectroradiometry has typically not been a focus,’ Austin remarked.
Understanding how humans perceive light can be challenging for a range of reasons, such as the variations that exist from individual to individual, and between cultural backgrounds, explained Hoeh: ‘In Asia, a more bluish light is preferred in an indoor environment, and in Europe a more yellowish light is preferred. It is very difficult to find a commonly accepted qualitative evaluation.’
As well as the trend for higher quality products, there is also a trend towards incorporating smart features into LEDs. Smart lighting, which allows either indoor or outdoor lights to work automatically under certain conditions, is becoming more popular among consumers. This can be for energy-saving reasons, for example office lights only switching on when somebody enters the room, or for aesthetic reasons, such as restaurant lights dimming in the evening. ‘To combine high-quality LEDs with a smart control really gives a benefit to the customer,’ said Hoeh.
As these smart features become more popular, it is expected that in the near future, the demand for LED measurement systems will increase. ‘There will be many different facets of smart lighting that must be explored, tested, and ultimately measured,’ noted Pierce at Stellarnet. ‘There will be many new businesses designing smart lighting products and learning how to measure lights for the first time. Ultimately this means the lighting test and measurement industry will be very useful for them.’