The smallest differences in optics and materials require the most sensitive of metrology tools, as Rob Coppinger discovers
When dealing with photons and angstroms your measurement system has got to be good – and whether it is optics, sensors, laser light or what the light is bouncing off, more accuracy and higher resolutions are what the market is demanding.
Beyond the laser there can be few components of any photonics technology more important than the optics, and Armstrong Optical provides systems for measuring optics quality; in particular, their centre thickness. ‘What we’ve designed is a low-cost, non-contact system to get down to five to 10 microns accuracy for measuring the centre thickness of lenses,’ Rob Roach, Armstrong’s sales manager for the optics market told Electro Optics. ‘It would be nice to go to higher precision but then you’re going to £80,000 worth of equipment in a temperature-controlled lab, not an optical workshop. We found people asking for a non-contact gauge for soft optical materials with tens of microns of accuracy.’
Armstrong Optical has received interest from organisations in Europe for using the system for quality assurance, checking batches of 10 to 20 lenses, and it can be deployed for batch quality-control of hundreds of lenses. Its use as an offline optimisation tool in production is another application for customers.
‘It is material-independent,’ explains Roach. ‘Silicon, silica; it is also form-independent; we can do unpolished rough surfaces, polished surfaces, coated surfaces.’ Roach has several systems in the user community already. ‘We have interest for five or six more systems,’ he adds. Roach finds that selling one system to a customer can encourage them to buy another for production – and sometimes Roach’s customer’s customers also realise that they could benefit from having the same measurement system as their supplier. ‘The customer’s end user can be interested in a system, as having the same tool for measurement ensures consistency from that supplier and others, without increasing the potential to damage components.’
Demonstrating the technology to potential clients, Roach explains that the system was shown to be able to detect a lens’ centre thickness after a high-performance coating had been added. As well as accuracy, another area of improvement is automation. But Roach sees drawbacks with this: ‘One improvement is automation, but if you automate it you end up with a product that is quite expensive. Why would users want it automated, when the automation costs become unjustifiably significant for customers used to making manual thickness measurements?’ he says.
For Roach’s customers, high-productivity throughput is not the goal. These companies are not churning out 10,000 lenses. They are more likely to be producing high-value lenses, and machining such lenses to micron tolerances can mean a number of in-process checks during production, for consistency. ‘It’s not for high-volume applications, its manual; you put the lens in, you record it, and rework if required. The non-contact technology means no damage to the component during these in-production measurements,’ he adds.
Trioptics’ research and development manager is Dr Iris Erichsen. She finds that her company’s markets and customers are asking for automation. ‘More people want completely automated systems and this is something we have seen for a while, which is the same for all of these markets.’ Trioptics’ markets are wide-ranging, from mobile phones to automotive to space. ‘We’ve provided measurement systems to almost anything you want to measure,’ says Erichsen. ‘We are not limited to any one measurement method. We have a wide range of techniques in terms of what we want to measure.’ As well as parameters such as effective focal length and the radius of the curvature of surfaces, Trioptics’ equipment measure the imaging quality of objective lenses, very small, <1mm diameter camera optics for mobile phones and very large lenses. As Erichsen explains, for the mobile phone market, ‘this measurement is done on the laboratory level but also in high-volume production. You can imagine, with the mobile phone industry, you get millions of lenses in one day in Asia – and we have a lot of instruments there because they have 100 per cent testing there.’
In Erichsen’s view, measuring imaging quality means measuring the system as it is. ‘We have systems to measure positioning errors of single elements in these optical systems. So we measure it from the centre and check if they are in the right position in terms of distance to the next lens, and these sorts of things, because if you are measuring imaging quality and you find it is bad, you want to know why it is bad. We have the instruments to tell what element is wrong in your complete system.’
The R&D manager points out that, with Trioptics technology, the complete fixed system does not have to be disassembled to make these checks. The checks can also be made when an optical system is being set up in a manufacturing environment.
‘We have a lot of instruments to measure angle. It is a very robust system; it is very precise, depending on the application; and you can get lab versions. We usually do customised systems. We also have basic instruments, but there is almost no instrument that is not adapted to customers’ needs,’ explains Erichsen. ‘We do a lot of development with a view to what the customer wants.’
What the customer wants is leading to more and more complex and smaller optics for that mobile phone industry. Erichsen points to the increasing resolution, with phones now having eight- to 12-megapixel resolution cameras.
Cameras are equally important for metrology in Ian Johnstone’s markets. Johnstone works for Armstrong Optical, and his customers use thermography cameras for examining composite structures – complex parts made of carbon fibres, set in a resin. Johnstone is applying the photonic technology of thermography cameras to quality assurance. ‘We use pulse phase lock-in, or active thermography, we input energy to the structure and then we look at the heat flow through the material.’ Defects show up as variations in heat flow.
The user has to synchronise the image capture with the excitation energy pulse. For composites and materials of similar slow heat behaviour uncooled thermography cameras with resolution of 1,024 x 768 pixels can be used for image collection. Such systems have frame rates of several hundred hertz and are used for low-heat flow rates. For a similar analysis with ceramics and metals, which have better heat flow, the energy input travels through the structure more quickly so the frame rate has to increase, perhaps up to a 3kHz line scan rate at reduced resolution. For high frame-rate operations, thermography cameras with cooled sensors of up to 1,280 x 1,024 resolution sensors are available. Whether the material is carbon fibre or ceramic, image analysis and controller software allows the synchronisation of the energy input sequence with the collection of the sequence of thermal images and the subsequent deconvolution of the data presentation of the results.
‘Big areas can be measured fairly quickly with active thermography, and it will show you where the defects are. Depending on the structure thickness and size of the defect, it may also allow the defect dimensions and position to be quantified,’ says Johnstone.
Chris Varney is CEO of Laser Components UK. His company sees the gradual incremental improvements that drive metrology to ‘ever increasing accuracy’. ‘We supply position sensing detectors, PSDs,’ explains Varney. One change he has seen is the growth of homogeneous silicon to form one-dimensional or two-dimensional detectors. ‘The 2D PSD is as a duo-lateral detector. Light incident on a slab of silicon produces a photoelectric current causing a current to flow to electrodes. No matter the shape of the light, the PSD can measure the photopic centroid.’ That response allows users to measure the position of x and y. As well as x and y, there are linear detectors that only measure in one axis, 1D. However, customers also require the ability to measure both axes, 2D. Laser Components’ detectors are very linear. Another option is to have non-linear detectors. ‘Some older detectors may be cheaper but not so linear,’ says Varney, adding that these do have drawbacks. ‘As they are non-linear, every 0.1mm of movement does not necessarily result in a measurement of 0.1mm, and plotting distance with measurement can show barrel distortion. Look-up tables are then required to calibrate the detector.’ The answer is to use detectors with good linear function, which are more expensive but it can save money because look-up tables and related electronics and software are not necessary.
Another area where Varney sees improvement is measurement of straightness and flatness. Measuring over a distance of up to 100m, a transparent PSD module acts as a beam splitter, splitting of a small portion of the laser light in flight, redirecting it to the sensor. The rest of the beam carries on. ‘One can place these at any distance at up to 100m, and can measure the straightness of an object to within about 25 microns with our system. Limitations are environmental vibration, temperature, air turbulence, and ambient light,’ says Varney. ‘For anyone looking to measure straightness or flatness that is an amazing accuracy over such a distance. It is hard to think of what else to use, for that sort of non-contact method. Popular measurement applications include large structures, where distortion and twisting is measured.’
One example he gives is of a train carriage. The carriage chassis needs to be rigid and not to deflect; with this technology that can be measured in real time. Another example is ships’ propeller shafts. ‘Areas like shafts, propeller shafts, are quite long and have large diameters. There is a worry about when they are not straight and when they slowly go off-centre,’ says Varney.
While metrology is primarily about detecting a signal, and understanding what that means in distance or time, metrological instruments also need to ignore noise. As Varney puts it: ‘Engineers have to remove the effects of ambient light. Using anti-reflection coatings and blocking coatings, unwanted stray light can be reduced, so improving the signal to noise of the system.’ The greater the blocking filters for the unwanted light, for example OD5 instead of OD4, either side of the wavelength of the light being used, the greater the system accuracy. ‘Some external applications require solar blocking filters to reduce the effect of sunlight when measuring position with PSDs,’ says Varney.
From infrared thermal cameras for production metrology to infrared blocking for laser measurements, metrology for and by photonics technology is not standing still. Measurements are getting faster and more accurate; inaccuracies have few places to hide.