Developments In Interferometry

Interferometry is used as a routine quality monitoring test for optical components and systems at wavelengths ranging from the visible to above 10 microns. The technique has been available as a commercial tool since the early 1970s and a number of well respected manufacturers such as Precision-Optical Engineering, FISBA OPTIK, Zygo and Wyko Optical Metrology*, have over the years, through healthy competition, ensured that the latest developments have been incorporated into the instrumentation as the technology has evolved. In the infrared the main wavebands used are from 3 to 5 microns and from 8 to 12 microns. The visible and infrared wavebands form a natural division in the field of interferometry. Interferometers are used in a host of applications. In the visible region, this includes the evaluation of glass or plastic optical components such as flats, lenses, and prisms, as well as precision metal components as diverse as bearings and computer disks and polished ceramics with 0.5% to 100% reflectivity. They also have extensive applications in the field of ophthalmology, for contact lenses and contact lens moulds.  Infrared interferometry is used for the evaluation of optical systems and materials used in this region of the spectrum and is used extensively in military applications. In addition, operating at wavelengths of 8-12 microns allows interference fringes to be produced from certain aspheric surfaces, and the shape of ground surfaces can be measured, which would be impossible using visible interferometers. This review aims to highlight some of the key developments in the field of interferometry and does not claim to be exhaustive.  

Interferometry principles and basic designs


The technique involves measuring the distortions in a wavefront from a coherent beam of light interacting with the test piece compared to a reference beam. The interference patterns which result from differences between the test beam and the reference beam appear as a set of black and white fringes which yield information which can be related to surface form errors or optical waveform distortion errors. In this way, geometrical aberrations in optical systems, badly manufactured optical components and inhomogeneities in materials can all be readily identified. The two most commonly used designs for commercial interferometers are the Fizeau and Twyman-Green geometries shown in Figure 1. Both types are used in commercial visible interferometers. The Twyman-Green configuration is generally used in infrared interferometers such as the INTERFIRE 10.6 from Precision-Optical Engineering).  

Developments in optics

In the early interferometers, the light source was a thermal source and the interference pattern was projected onto a screen and viewed by eye. Thermal sources did not have sufficiently narrow spectral lines to provide a high level of coherence, so in the Fizeau arrangement, the beam splitting surface had to be very close to the surface under test, and in the Twyman-Green system the reference mirror had to be adjusted on a rail to equalise path lengths. For infrared applications, thermal sources did not have sufficient intensity to stimulate the detector. All this changed with the arrival of usable lasers, which gave a long coherence length and an abundance of power to stimulate the sensor, coupled with the development of suitable sensors, particularly the pyroelectric vidicon camera and more recently infrared focal plane arrays. For visible applications, the HeNe laser is used extensively, while in the infrared, HeNe, CO or CO2 lasers are used. The amount of power available varies greatly and it is becoming difficult to obtain a low power CO2 laser so the most sensitive sensors tend to be ignored and pyroelectric sensors are used. Using a 3.39 micron HeNe laser, the power tends to be low so a cooled staring array sensor is preferred. Laser light sources greatly extended the complexity of optical systems that could be tested. Another major development has been the use of optical fibres. Delivery of the laser light to the beam splitter by optical fibre means that the laser can be located away from the interferometry optics, which has made a major contribution to the ability of manufacturers to miniaturise systems. Another significant development has been the use of CCD cameras, which allowed fringes to be viewed directly on a monitor, followed by frame grabber technology, which allowed the images of fringes to be acquired into a computer for measurement and analysis. The compact size of high resolution CCD cameras means that they can be fully integrated into the interferometer, and systems are available which offer an excellent 1000 X 1000 pixel resolution. The interferometer is essentially a comparator, and the ultimate accuracy of measurements is determined by the quality of the reference flats and spheres used as the external calibration system in the particular measurement configuration. Reference optics should be traceable - usually to an national or international standard. External standards are typically supplied with an accuracy of 30-60 nm (lambda/20 to lambda/10) but lambda/30 and lambda/40 calibration flats are also available. Repeatability of measured surface shape is typically in the range of 5-10 nm (about lambda/100).

Fringe and phase shift analysis - the software revolution

Once the interference fringes have been obtained they need to be measured and analysed. The simplest and original method, illustrated in Figure 2 involved taking a photograph of the fringes and measuring them with a ruler! The arrival of the PC and frame grabbers brought a revolution in fringe analysis. In its simplest form (Figure 3), typically 200 - 300 digitised points are taken along the fringes using maxima and minima detection routines. Increasing computing power allowed an ever increasing range of complex fringe analysis functions to be carried out. These include analysis of multiple fringes and full quantitative wavefront analysis including Modulation Transfer Function (MTF), Point Spread Function (PSF) and slope error. Wavefront shape can be fitted to Zernike polynomials enabling the calculation of Seidel Aberrations. The software can also flag pass/fail criteria based on irregularity, power, peak to valley wavefront value and rms wavefront aberration. Higher accuracy of measurements can be achieved using a technique known as phase shifting. This involves using a piezo transducer to move the reference optic by around lambda/2 and utilising dedicated phase shift analysis software to provide full analysis of circular, multiple, low contrast and nulled fringes. Results can be displayed directly in terms of ISO and DIN standards. An example screen display from phase analysis is shown in Figure 4.  Phase measuring interferometry is the more accurate technique since it offers higher density and uniform sampling of the interference pattern, and better phase resolution. However, despite the sophistication of the software currently available, it should be emphasised that automatic fringe or phase shift analysis is not a deskilled function and an understanding of what is happening in the optical system can still be very important in the interpretation of results. This is of particular importance in infrared applications.

Signal-to-noise in infrared interferometry

Infrared interferometry relies on sensing using an infrared camera. The cameras exhibit random noise on the video signal which is most obvious in the 3 to 5 micron waveband, since affordable lasers tend to be of low power (typically 5-10 mWatts). High signal gain is therefore necessary to provide sufficient contrast for recording fringe patterns or analysing fringe patterns using either static or phase shifting software. The profile of the laser beam is also important when laser power is marginal since this may cause contrast variations across the interferogram. This is particularly noticeable when using HeNe lasers at 3.39 microns where perhaps only 5 to 10 mWatts of power is available. This can have the effect of making fringes appear both fainter and thinner towards the edge of the pupil. When analysing with static fringe analysis packages care needs to be taken to check that the digitised fringes accurately represent the real shape of the fringe pattern. With the reducing cost of cooled staring arrays in the 3-5 micron band, with much lower Noise Equivalent Temperature Difference than pyroelectric vidicons, high signal gain can be used without the penalty of increased noise.

The problems raised by laser power profiles are greater when using phase shifting systems, since at least with static fringe analysis software some manual correction of digitised points is allowed. Phase shifting systems have a threshold contrast below which phase will not be measured. Reducing the threshold too far means that the system can pick up on random noise in the picture and allocate  phase values which are significantly larger than any other area of the pupil, producing a gross distortion of the results. There is therefore a fine balance between ensuring that phase information is available over the whole pupil without random noise causing a problem. These problems can be alleviated through the use of either a high quality higher power laser or a highly sensitive, low noise affordable detector. At 10.6 microns, however, these problems don't occur. In fact, excess energy from the laser source needs to be removed, using for example CaF2 filters or wire grid polarisers. Polarisers are essential in tunable laser system since the availably absorbing filters do not act uniformly across the waveband. There is more than sufficient power available to enable the pyroelectric vidicon camera to be set to low gain, thereby reducing noise to an insignificant level.

Diffraction effects in infrared interferometry

Some of the experimental conditions for infrared interferometry are rather different to those in the visible region, so special care must be taken in the setting up of the measurement, and the results analysed in as rigorous a manner as possible. Infrared optical systems are being produced with smaller optical apertures than previously for reasons of weight, cost and configurability. This trend has been aided by improvements in the detector technology. The result is that the IR lens for a camera system may have an aperture comparable in size to a visible camera system. However the wavelength in the IR system may be as much as twenty times greater than in the visible band. This makes the effects of diffraction much more dominant in IR interferometry, requiring that optical pupils are well focused onto the interferometers camera sensor surface. Because of the double pass nature of most commercially available interferometers, the aperture is seen twice by the interferometer - if the two images of the optical aperture are not conjugate with one another then some diffraction is inevitable. The effects of this must be minimised to enable accurate analysis. It is important that any analysis of the interferograms takes into account the limitations of the method. Figure 5 shows the effect of diffraction on the pupil imagery in the interferometer. In the unfocused case, diffraction rings can clearly be seen inside the pupil. This has the effect of breaking up the interference pattern such that the fringes are discontinuous or vary in width due to contrast changes. Any automatic fringe analysis program must be used with caution when analysing such fringes. In these cases the peak to valley wavefront aberration reported will be worse than the actual aberration for the lens under test. While Figure 5 shows the effect of diffraction within the optical pupil, the other common case is when the diffracted radiation appears outside the pupil. This has the effect of blurring the pupil edge so its size is uncertain and, because the diffracted radiation has a different curvature to the main beam, any interference fringes will curve within this zone. The effect is very similar to that seen with Spherical Aberration especially that of higher order and if not corrected can lead to a large departure in measured vs. actual performance. This situation is shown schematically in Figure 6. Here, if information is known about the size of the pupil within the interferogram, it is possible to mask out much of the effects of diffraction simply by ignoring those parts outside the pupil.

There will be cases where it is impossible to achieve good pupil imagery without going to great expense. In such circumstances, double pass interferometry can still provide a confidence test on system performance, but ultimate proof may require a single pass test, either by interferometry or by non coherent broad band MTF testing or similar.

Miniaturisation of visible systems

As we have seen, the Twyman-Green configuration can have the laser beam delivered via an optical fibre. Since the laser no longer needs to be an integral part of the instrument, the basic interferometer can be miniaturised. Indeed by incorporating a phase shifter and a CCD camera, the instrument becomes an interferometer probe. Instruments such as the Digital Compact Interferometer 2 (DCI 2) from FISBA OPTIK  physically measure 110 mm x 45 mm x 70 mm excluding the objective, yet features a high resolution 1000 x 1000 pixel CCD camera.  A large choice of interchangeable modular objectives brings versatility for measurement of plane and spherical surfaces in a variety of instrumental set ups by mounting the interferometer on an adjustable, vibration isolated stand incorporating a test sample stage featuring 4 degrees of freedom. This arrangement can easily be used in the factory, workshop and laboratory.  

FISBA's µPhase instrument is even smaller, measuring just 61 mm x 37 mm x 28 mm without the objective and is suitable for normal resolution requirements in production areas and optical workshops for repetitive measurements and serial-production testing. However, because of its extremely small size, it can be fitted directly onto a wide range of machines or incorporated into OEM instruments to make in situ measurements. Such equipment includes lathes, polishing machines, optical manufacturing machines and contact lens manufacturing systems. These ultra-small interferometers also have applications in the measurement of cylindrical components. Detailed discussions of any of these specific applications is beyond the scope of this review.

Miniaturisation of infrared Systems

The size of infrared interferometers has also reduced over the years. Twenty years ago a typical open plano IR interferometer with a 150~200 mm aperture would have covered a 12ft x 4ft table. Slow optics tended to be used for the sake of optical safety. The advent of new manufacturing processes, particularly diamond machining, has meant that accurate fast optics can be used which greatly reduce the size of the interferometer. The sensor packages have tended to remain about the same size, but smaller lasers have been available in the 8 to 14 micron band leading to overall packages with lengths of the order of 500 mm for a 35 mm output pupil. Increasing the aperture requires external beam expanders which can be geared to the actual application Ü a 30 to 150 mm beam expander would be similar in size to the interferometer.

  * Wyko Optical Metrology was acquired by Veeco Instruments Inc. in 1997 .

Figure 1. Fizeau and Twyman-Green interferometer configurations

Figure 2. Simple fringe analysis

Figure 3. Digitisation of fringe centres

Figure 4. Screen display from 'µShape' phase shift analysis software from FISBA OPTIK

Figure 5. Diffraction effects in the infrared region

Figure 6.Pupil imagery effects on fringes in the infrared region


Authors: David Page, Technical Manager, Precision-Optical Engineering, 42 Wilbury Way, Hitchin, Herts, UK, SG4 0TP. Tel: +44 (0)1462 440328. Fax : +44 (0)1462 440329. E-mail:  Internet :

and: Ian Routledge, Armstrong Optical Ltd, Poplar Farm, Caldecott, Chelveston, Northants, NN9 6AR 
Tel: 01933 622222 Fax: 01933 622226. E-mail: Internet