TECHNOLOGY
METROLOGY
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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
Measurements are
getting faster; inaccuracies have few places to hide
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. l
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