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The quality of spectroscopy

David Robson discovers how spectrometers are being applied in quality control for industry

The spectrometer is one of the scientist’s most treasured laboratory tools. It equips researchers with the ability to measure the exact wavelengths of light emitted and absorbed by a substance – telltale fingerprints that can indicate the chemical composition and atomic structure of an unknown material. The process is so fundamental that spectrometers are practically ubiquitous in scientific laboratories. Chris Lynch from PerkinElmer explains: ‘UV and visible spectrometry is one of the oldest and broadest techniques in the lab. It’s synonymous to the lab balance – every lab will have a UV/Vis spectrometer.’

It may seem obvious that the advantages of this precision could be effectively translated to  industry. Manufacturers frequently need to measure the exact appearance of a product (for example the colour of fabric for clothes or the output of a light bulb), which a spectrometer could do to a precision that is not possible with a simple colour sensor.

Spectrometers can also detect the chemical composition of a food product without performing chemical tests, and can be used as a feedback mechanism when controlling chemical processes in the electronics and semiconductor industry.

However, if the advantages seem manifold, the disadvantages of using a scientific tool on the hectic, dirty factory floor once outweighed them. The complexity of the instruments limited their usability; in the past you would have needed an undergraduate degree, if not a PhD in chemistry, to be able to operate and read useful information from the spectral readings of a typical spectrometer.

Also, the instruments were typically bulky and fragile, unable to withstand the robust environment of the factory floor compared to the clinical conditions of a laboratory, and too hefty to move between the different stages of the production line.

This situation is changing, with multiple optics suppliers now offering small, portable systems suitable for use in the field, as noted in the April/May 2007 issue of Electro Optics. A year later, and it seems this trend is escalating, with new applications on the factory floor to aid the manufacturing of high-value LEDs, displays, solar panels and architectural glass. What was once a highly complex scientific instrument is now a compact, smart device that can provide the rejection or approval of a product at the click of a button.

‘At the moment we’re just in the starting phase,’ says Benno Oderkerk, technical director from Avantes. ‘We’ve been working now for five to 10 years on inline spectrometers, but the real applications of high value are only just coming out.’

He says that, until recently, companies would never have even considered using a spectrometer in industrial applications. ‘It takes a while to penetrate the market, and the awareness that you need a spectrometer for these applications is just arising. We even have customers that don’t know what a spectrometer is, but are now using one.’

For many applications, ranging from the quality assurance of LED light output to the inspection of leather car seats, spectrometers are replacing traditional colour sensors, based on three chips that would measure the relative intensities of red, green and blue in the overall colour. Compared to a spectrometer, which measures the exact intensities of each wavelength in the spectrum, this gives a crude comparison, missing out important details that may be noticeable to a human. ‘A colour sensor normally looks at the base colour of a surface and not the way the light is reflected,’ explains Rob Randelman, president and CEO of Ocean Optics.

The USB2000, from Ocean Optics, is used to help the sorting of LEDs on a production line.

For example, a carpet appears differently to an observer if the pile is flattened in different directions; although they are both the same basic colour, the two regions reflect light in a different way, making them look subtly different. This phenomenon, in which one colour can appear differently depending on the way it reflects light, is very common, and a spectrometer would pick up on these subtle differences in the spectra of light to ensure that two products really do look exactly the same.

Whereas colour sensors only measure the visible spectrum, spectrometers can measure wavelengths in the infrared and ultraviolet, to find faults that would not be detected using other methods. ‘Spectrometers can also find flaws in a product that otherwise wouldn’t be evident to a typical colour sensor. For example, most LEDs have a packaging made of plastic and coated with different materials. These protective coatings may only be visible in UV light, which wouldn’t be detected with a typical colour sensor. You are looking at the surface chemistry, and not just the colour,’ says Randelman.

Chris Lynch, the polarimetry, UV, and fluorescence business manager from PerkinElmer, explains how the inspection of architectural glass, used as the façade of large tower blocks, also demonstrates the requirement for a precise description of the reflection and transmittance of the different wavelengths throughout the spectrum. The glass is often tinted to prevent glaring sunlight from hitting the offices, and it may be designed to insulate the building, to keep the offices warm in the winter and cool in the summer. The manufacturers would use a spectrometer to measure the intensity of light emitted at every wavelength from the ultraviolet through the visible to the infrared spectrum (which would include radiated heat energy) to ensure that it transmits exactly the correct wavelengths that it is meant to.

This detailed description of the product, across many wavelengths in the form of an accurate spectral analysis, is one of the key advantages of the spectrometer. The comprehensive  information contained within the spectral analysis can be used to decide an agreed standard between two parties – for example, the fabric producer and the clothes manufacturers – of parameters that could not otherwise be measured. This description can then be used as a specification to ensure the customer is getting the exact product they had asked for. ‘In many cases there is no ISO standard,’ says Ocean Optics’ Randelman, ‘but there is an agreed method between them. Spectroscopy provides a way of giving very specific parameters for this agreement…. it’s the only technique that can be used to provide information about the colour, reflectivity and material together.’

Spectrometers also allow a greater flexibility in the production process. In the past, manufacturers would have deployed dedicated equipment for each product, but this would not be practical in today’s industrial environment, where the product may change from week to week. Companies now need to be sure that the quality control process can be adapted for each new product going through the production line. The detailed spectral information captured by a spectrometer provides more than enough data to identify the characteristics of each new batch of products.

‘Companies must be more nimble about manufacturing,’ says Randelman. ‘For example, a bottling plant may handle water, soda or vitamin juice. You don’t want a mixture of the product, but a spectrometer would allow you to detect outliers by showing peaks in the spectral analysis where you weren’t expecting it.’

While spectrometers are frequently used to evaluate a product once it has been produced, they are also important in controlling the production environment. For example, spectrometers are used to control the plasma etching process that writes circuit patterns onto a semiconductor wafer. In this process, the highly reactive plasma is fired at the wafer to etch the fine structures into the material.

A spectrometer is key to this, determining that the plasma contains the correct mixture of chemicals to perform the task, as Marco Snikkers, commercial director of Ocean Optics explains: ‘Plasmas are a mixture of highly reactive gases, and the rate at which the etching occurs is linked to the combination of gases. If this is not correct, you may etch too deeply into the semiconductor. The spectrometer measures the emission lines of the gases and controls their composition in the vacuum chamber, providing a direct feedback to the gas valves to adjust their relative values.’

The Jaz spectrometers from Ocean Optics are portable and suitable for spectroscopic measurement in the field, such as assessing the quality of oranges.

With these emerging applications come new requirements. There is now a greater emphasis on the repeatability of readings so that spectrometers can handle the rugged industrial environment to produce consistent and reliable readings. They must also be interchangeable, so a replacement can be fitted without changing the whole production line. And while the devices are noticeably smaller than previous efforts, they are also smarter. While many users will still need to view the full spectral analysis, for some applications it would be preferential for this data to be held in the background: in automated systems a computer processor compares the analysis against predetermined parameters to decide for itself whether a product should be rejected or approved. The result is that the spectrometers of today can now be operated by virtually anyone.

‘When they were evolving, they became just a smaller copy of a big spectrometer – they just gave the spectrum on the screen,’ says Benno Oderkerk of Avantes. ‘What we have done is put a lot of intelligence in the electronics. Our spectrometers include a microprocessor that compares the spectral input with a database of parameters to determine whether it lies outside of a certain threshold, to give a “yes” or “no” output. It’s important to have a black box approach, which gives fast and reliable processing on board. We want it to be so the user simply inputs the optical information and is provided with a “yes” or “no” answer. Everything else should be contained within the box. ‘For the near future we will see a combination of different technologies. For example, they will use Bluetooth communication, and they will integrate spatial and spectral information using a GPS system.’

In addition to this new computational intelligence, it seems likely that spectrometers of the future will also be autonomous and self maintaining. Current spectrometers need to be calibrated by a human to give accurate readings, but Ocean Optics’ Randelman believes that spectrometers will one day be capable of performing the calibration process themselves. 

Emerging markets: solar cell production

‘The real applications of high value are only just coming out now,’ says Benno Oderkerk from Avantes. While it is difficult to pinpoint exactly what these emerging markets will be, it seems likely that the production of renewable energy sources could benefit from the advanced  information provided by an industrial spectrometer.

‘One of the developing areas is renewable energy sources and solar cell development,’ says Christopher Lynch from PerkinElmer. ‘Solar cells used to be made solely with silicon, but, with rising oil prices, there is a strong push to optimise solar cells with new optical thin film techniques instead.’

These new solar cells are made from glass coated in thin films of semiconductor materials, built up in layers to absorb all the possible light and convert it into electricity. While this is much cheaper than previous solar cell technologies, manufacturers are still keen to optimise the production process. ‘Solar energy is still four or five times more expensive than conventional energy sources, so there is a drive to reduce production costs,’ says Lynch.

Spectrometers could help with this, by measuring the optical characteristics of the thin films to ensure that they have indeed achieved the optimum absorption of light to provide the maximum amount of electricity. Solar cells are typically employed outdoors and subject to harsh weather, so spectrometers are also used to measure how this absorption of light changes over time in these harsh environments.