I. Introduction

During the last decade numerous publications[1]–[4] have appeared which use the wave-front measuring technique invented by Bruning et al.[5] This method allows measurements of wave-front deviations in the λ/100 region with repeatabilities in the 10⁻³ λ region.

Although this method is already used in industry, investigations of systematic errors are scarce.[6] Furthermore, some statements in the literature concerning systematic error sources are at least incomplete or inconsistent concerning the attainable accuracy. Only a few investigations of nonenvironmental systematic error sources have been carried out.[4] For this reason we feel justified in discussing some systematic error sources in more detail than hitherto. Our study contains theoretical as well as experimental results. For the experiments we used the interferometric setup given in Fig. 1. This is a conventional Twyman-Green interferometer equipped with a piezoelectric transducer to vary the reference phase controlled by a highly accurate capacitive distance meter and with a 32 × 32-element matrix as well as a CCD line array with 256 elements as a detector. The photoelectrical signal from the detector is converted into an 8-bit word which is fed into a K1520 microprocessor system. The system calculates the phase distribution according to the fundamental relations given in the literature.[5] The piezotransducer is controlled by the same microprocessor. The results of the phase measurement can be displayed graphically. In Fig. 2 the wave aberrations of a micro-objective of N.A. = 0.32 are shown where the contour line distance is λ/32. [Wave aberrations W(x,y) and the phase deviation Φ(x,y) which is a measured quantity are connected by the relation Φ(x,y) = 2kW(x,y) with k = (2π)/λ. The factor 2 results from the double passage in the Twyman-Green interferometer. Apart from the factor 2k the expressions for phase deviations and wave aberrations are used in the text synonymously.]

The CCD line array allows only for measurement of one central section through the 2-D wave aberration, but the lateral resolution of the CCD array is 8 times higher than with the matrix array which is essential for the error analysis. Therefore, the experimental investigations were carried out mainly with the CCD array. It will become clear from the context that the restriction to one dimension can be balanced by choosing the experimental conditions accordingly.

So far the software enables calculation of the phase deviations; elimination of linear and/or an additional quadratic term in the pupil coordinates; averaging operations; difference and summation operations of consecutive runs and other data manipulations discussed in the text; and display of the phase as contour line plot, pseudo-3-D plot, and as curve for the CCD data.

(A run comprises at least the data acquisition time for a set of values enabling the calculation of the phase to be measured.)

II. Errors Caused by Reference Phase Deviations from Ideal

The basic equation for wave-front measuring interferometry was given by Bruning et al.[5]:

Equation (1) assumes that the r samples are taken equidistantly and that r runs over whole periods of the intensity distribution. Only in this case Eq. (1) is valid. If the above assumptions are violated more complex equations will arise from the least-squares formalism underlying Eq. (1).

Here, we shall discuss weak violations of the exact conditions which may be caused by reference-phase inaccuracies or by thermal drifts and mechanical relaxation during the data acquisition time of a single run. Assuming a two-beam interference pattern we can write for the intensity I′ as the measured quantity:

While ψ_r is a physical quantity, φ_r is a mathematical one stored in the computer memory. For obvious reasons we introduce an error ɛ_r of the reference phase φ_r:

Using the orthogonality relations of the sine and cosine functions[7] and assuming that ɛ_r is small, we can put cosɛ_r = 1 and sinɛ_r = ɛ_r. Then the following expression can be derived:

The quantities C and S can be interpreted as Fourier coefficients of the error distribution ɛ = ɛ(r). In this interpretation only contributions correlating with sin2φ_r, cos2φ_r are essential. The interpretation as Fourier coefficients is also suggested by the approximate Fourier analysis contained in the algorithm of Eq. (1). In Eq. (7) the mean value of the phase error

Therefore, in the case of many phase steps/period the influence of this error on the phase to be measured will be weak although noticeable with high-precision measurements. However, microcomputer limitations and interferometer drifts make high-speed measurements using only few reference-phase steps desirable.[3],[4]

Equation (1) becomes extremely symmetric and simple if only four phase steps are taken, i.e., φ_r = (r − 1) (π/2), r = 1,…,4 and, therefore, the phase Φ can be calculated from

In Fig. 3(b) a section through the intensity pattern in the detection plane is shown. The cos-type fringes correspond to the phase distortion except that the spatial frequency of the phase variations has doubled.

One could argue that in normal measuring conditions a zero fringe pattern is adjusted, but it is obvious that phase variations due to the existing wave aberrations occur and therefore distortions are inevitable.

The averaging quality of the phase measurement procedure due to Eq. (1) can be studied by a comparison of runs with considerably different reference-phase step numbers. In Fig. 4 two deviation pictures are given made with the difference technique used in Fig. 3(a) but with only one arbitrarily introduced reference-phase error in the first run. The result of an error-free second run was substracted. In Fig. 4 top sixteen reference-phase steps were used and below four steps. The averaging effect for many steps (as stated above) becomes obvious in this example.

Therefore when only four steps are used it is especially necessary to assure the accuracy of the single phase steps by means of calibration runs. For this purpose the following procedure was programmed and carried out:

One makes four adjustments with the reference-phase values 0,φ,3φ,4φ. The corresponding intensities may be I₀,I₁,I₃,I₄, where the index gives the multiplicator of the reference-phase value φ. This value can be chosen in such a manner that 4φ is just below the maximum elongation of the PZT mirror. Provided that φ is approximately known, the four equations,

As will be shown next the averaging of different runs as usual to improve the accuracy can be combined with an error-compensation method in the following way. One makes two runs where the second run has a reference-phase offset of π/2. From Eqs. (4) and (5) it follows after some calculation that

If we define

This case is of special interest. To show this we assume ɛ_r to be a power function of the step index r:

In this connection it is interesting to note that the spherical Fizeau interferometer generates an increasing reference-phase error proportional to r caused by the axial translation of one of the spherical surfaces (for this purpose see [Ref. 2]). The coefficient is a function of the aperture angle of the translating sphere. With the just discussed procedure the influence of ɛ_r can be canceled. The application of this procedure to the spherical Fizeau should make corrections unnecessary.

So far we omitted a random error function. The superposition due to Eq. (12) leads to a decrease of the standard deviation by a factor of √2. Therefore, it seems reasonable to apply this averaging method instead of the simple averaging technique used so far because it maintains the ability to reduce the statistical fluctuations and eliminates calibration errors of the reference phase shifter, linear drifts, mechanical relaxation effects, etc., fairly well.

There is the possibility of averaging the results of two runs where the second run is carried out with the π/2 reference-phase offset. Also in this case a smoothing effect occurs under the above assumptions. From Eq. (7) one can derive the approximation:

Figure 5 shows the cancellation of the errors of two runs according to the two possibilities discussed. The reference-phase error introduced was chosen as in Fig. 3. Figure 5(a) gives the two runs superimposed on each other with three fringes/diameter. Figure 5 (b) gives the result of an a posteriori averaging operation on Φ. In Fig. 5(c) the averaging has been applied to numerator and denominator before the calculation of Φ for a different phase profile with a surface deviation of λ/256 per scale value. Note that the sensitivity has doubled in comparison with Figs. 5(a) and (b).

III. Errors Caused by Extraneous Interference Patterns

The influence of extraneous light amplitudes on the accuracy has been discussed to some extent by Bruning et al.[5] Here, we shall show that there is a Φ dependence of this phase error and propose an elimination technique working also for strong disturbances.

In the presence of extraneous coherent light the intensity distribution is no longer a two-beam distribution. Let three complex amplitudes,

interfere with each other; then a three-beam interference pattern results:

The remaining distortion δΦ is given here without proof:

The efficacy of the algorithm is demonstrated in Fig. 6. In this experiment a glass plate has been introduced into the interferometer to get a systematic error of known origin. Figure 6(a) shows a single run with extraneous light disturbances; Fig. 6(b) shows the average of two runs where the phase in the object arm has been increased by π. This improvement is unfortunately sensitive to the accuracy of the phase offset π in the object arm. This can be demonstrated by averaging two runs where the second run is made with a phase offset that is position dependent. This can be arranged by tilting the object surface before the data sampling of the second run. The average is then amplitude-modulated as can be seen from Fig. 7.

Therefore, we favor a total elimination of such distortions by the following procedure:

Take two runs with the phases Φ and Φ + χ, respectively, and calculate separately the numerators and denominators for each run. These quantities are substituted into the following equation:

To verify our statement we again introduced a glass plate into the reference arm, to get strong extraneous fringes, and we made several experiments.

Figure 8 shows a distorted interferogram. The corresponding phase distribution is given in Fig. 9. In Fig. 10 the runs are superimposed one on the other to show the phase shift introduced in the object arm. A total cancellation occurs if the algorithm of Eq. (19) is programmed and executed as shown in Fig. 11. Because the phase shift deviates somewhat from π the usual averaging of the two runs does not work well as can be seen in Fig. 12.

The influence of the amount of phase shift introduced can be studied in the following manner. We make two runs where the second run is made with a position-dependent phase shift in the object arm. Here the algorithm of Eq. (19) fails to work in the neighborhood of phase shifts of 2nπ (n is a whole number). This is shown in Fig. 13.

Figure 14 shows the (η − Φ) dependence of the phase error ΔΦ. Although η is constant along the section through the interferogram, periodic deviations due to Eq. (16) occur.

This can be seen more clearly in Fig. 15 where a series of pictures with different relative adjustments of the phase to be measured Φ and the phase η of the disturbance are given. The CCD array lies in the vertical direction in the interference photograph on the right of Fig. 15 so that Φ is constant or sgnη = sgnΦ and sgnη ≠ sgnΦ in the order from top to bottom. The deviation of the wave front has been eliminated by subtracting two runs where the second run had a Φ offset of nearly π.

IV. Coherent Noise

Because highly coherent laser light is used some high frequency spatial phase modulation is generated, the so-called coherent noise. Connected with this phase modulation a certain degree of uncertainty occurs because small alterations in the interferometer adjustment may introduce different phase modulations. High frequency phase modulation becomes disturbing when the two interfering waves take different ray paths through the whole setup.

Within the interferometer (beginning and ending at the beam splitter) the two waves have different ray paths resulting in high frequency Φ variations even in the case of parallel adjustment (see Fig. 1).

In addition there are differences due to relative tilts or slopes of the interfering wave fronts while passing the optics preceding and following the interferometer. Small glass inhomogeneities or dust particles on the surfaces of optical elements lead to diffracted waves which interfere with the background wave. Although the diffracted wave is weak, the wave front resulting from the interference of the scattered wave field and the background is no longer smooth. When the slope of two wave fronts differs, the diffracted waves are sheared relative to each other in the observation plane. The amount of shear is proportional to the slope difference of the two interfering wave fronts at some point in the interferometer exit.

To get an insight into the problem at stake we make a few estimations:

Each phase distortion results in intensity modulation (referred to in the literature as dust diffraction[6]). So one can derive the modulation amplitude of the phase from the intensity fluctuation of one of the beams. The complex light amplitude u_b of the background wave (Fig. 16) is superimposed by the amplitude of the scattered wave u_n having any phase φ_n within the interval 0…2 π relative to the background wave.

The resulting complex wave amplitude u_r has a phase within the interval Φ ± ΔΦ. The maximum phase modulation caused by diffraction at disturbances is ΔΦ = |u_n/u_b|. Because of the weakness of the scattered field we neglect the interference between the waves from different scatterers and restrict the discussion to the case of two-beam interference between the background and one scattered wave. So one gets

The detected spectrum of spatial frequencies of the disturbances is owing to the number of independent samples taken by the detector lying in the region of 1/P < ν < 1/p, where p is the period and P is the length of the CCD line array.

From Fig. 17 and from an estimate of the geometry one may conclude that there is a dominant spatial frequency (here in the range of 10/P – 15/P) of the phase modulation in the interfering wave fronts. For the phase of a distorted plane wave we assume here a periodic oscillation of Φ:

Hitherto, only the contributions from disturbances generated outside the interferometer were taken into account. From the scatterers within the interferometer, especially from the optics in the test arm, nearly the same contribution to the phase modulation originates. This can be studied if we assume a parallel adjustment (shear Δx = 0) of the interfering beams. In this case the expression for the intensity distribution can be factorized so that the influence of the scatterers outside the interferometer on the phase is canceled by the algorithm of Eq. (1). The remaining disturbances are given in Fig. 20 having approximately half of the amplitude in comparison with Fig. 19.

The considerations so far hold in the same way for conventional fringe evaluation interferometry when laser light is used. It also is handicapped by coherent noise from the imaging optics.[8]–[11]

Because coherent noise disturbs the measurement in any case it, therefore, seems advisable to suppress high frequency phase variations of the wave front which are caused by dust diffraction. This can be achieved partly by filtering and partly by averaging the light intensity in lateral dimensions.

Filter operations can be carried out in the focal plane at the interferometer output. Because in the last consequence only 32 × 32 pixels are used for a 2-D evaluation, the stop may have an angular dimension of 32 λ/field diameter. This restriction of the plane wave spectrum leads to a smoothing effect as shown in Fig. 21.

For the purpose of averaging in lateral dimensions an intermediate image of the surface to be tested is picked up by a moving scatterer. This is imaged in a further step on the detector (CCD or other). In this incoherent imaging process it is easy to curtail the MTF of the whole process by defocusing. In Fig. 22 the result with and without such defocusing is shown.

Several of the measures mentioned so far should be combined. This is especially important if one is urged to use high coherent light from a laser.

To summarize, these measures are (1) measuring in parallel adjustment; (2) using a stop in the exit pupil of the interferometer; (3) averaging by a moving scatterer which is out of focus in an intermediate image plane following the interferometer exit (in some cases the special detector regime provides a certain amount of lateral averaging[10]); and (4) using well-corrected beam shaping optics in the test arm of the interferometer.

Furthermore, the optics in the whole imaging system should be clean and free from scratches and inhomogeneities to the highest possible degree.

Conclusions

Three types of errors have been considered.

Errors caused by inaccurate reference phases (caused by mirror movements or by thermal drifts, etc.) can be diminished by (a) use of a parallel adjustment and only small wave aberrations in the interferometer; (b) use of the averaging technique contained in Eq. (12); (c) calibration of the reference mirror driver; and (d) use of multiple sampling (R > 4).

Furthermore, the averaging technique contained in Eq. (12) can be used to eliminate errors inevitably introduced by shifting the spherical test glass in a digital spherical Fizeau interferometer because the reference phase depends linearly on the movement[2] [as required by Eq.(14c)].

Errors introduced by extraneous fringes can be totally eliminated by making two runs, where the second has an object phase offset of π, and by a suitable combination [due to Eq. (19)] of the measured intensity values.

Errors generated by dust diffraction can be controlled by measurements in parallel adjustment and by filtering and lateral averaging of the intensity data.

With our present interferometric setup we could attain a repeatability of λ/200 in the ±3σ range, respectively, of λ/600 in the ±σ range as can be deduced from Fig. 23.

Appendix

Concerning Eqs. (11)–(12a) we give here a derivation of Eq. (12a) on the basis of Eq. (12) and the described experimental procedure.

Let the intensities during the first run be denoted by $I_r^{(1)}$ and during the second run by $I_r^{(2)}$. The actual reference phases given by the translation of the reference mirror are indicated by ${\psi }_r^{(1)}$ and ${\psi }_r^{(2)}$ respectively [see Eq. (2)], where according to our assumptions the following holds:

(A1)$$\begin{array}{ll}{\psi }_r^{(1)}\hfill & ={\phi }_r-{\varepsilon }_r^{(1)},\hfill \\ {\psi }_r^{(2)}\hfill & ={\phi }_r+\frac{\pi }{2}-{\varepsilon }_r^{(2)},\hfill \end{array}$$

where φ_r are the ideal reference phase values and ${\varepsilon }_r^{(1)}$, ${\varepsilon }_r^{(2)}$ are the errors of the reference phase values for the two consecutive runs.

Due to Eqs. (12) and (1) we have

(A2)$$tan\stackrel{\sim }{\mathrm{\Phi }}=\frac{\text{∑}_{r=1}^R{I}_r^{(1)}sin{\phi }_r+\text{∑}_{r=1}^R{I}_r^{(2)}sin\left({\phi }_r+\frac{\pi }{2}\right)}{\text{∑}_{r=1}^R{I}_r^{(1)}cos{\phi }_r+\text{∑}_{r=1}^R{I}_r^{(2)}cos\left({\phi }_r+\frac{\pi }{2}\right)},$$

with

(A3)$$\begin{array}{ll}{I}_r^{(1)}\hfill & =I_0\{1+Vcos[\mathrm{\Phi }-{\phi }_r+{\varepsilon }_r^{(1)}]\},\hfill \\ I_r^{(2)}\hfill & =I_0\left\{1+Vcos\left[\mathrm{\Phi }-\frac{\pi }{2}-{\phi }_r+{\varepsilon }_r^{(2)}\right]\right\}.\hfill \end{array}$$

Using the orthogonality relations Eqs. (A3) become:

(A4)$$tan\stackrel{\sim }{\mathrm{\Phi }}=\frac{\text{∑}_{r=1}^{R}cos[\mathrm{\Phi }-{\phi }_r+{\varepsilon }_r^{(1)}]sin{\phi }_r+\text{∑}_{r=1}^{R}sin[\mathrm{\Phi }-{\phi }_r+{\varepsilon }_r^{(2)}]cos{\phi }_r}{\text{∑}_{r=1}^{R}cos[\mathrm{\Phi }-{\phi }_r+{\varepsilon }_r^{(1)}]cos{\phi }_r-\text{∑}_{r=1}^{R}sin[\mathrm{\Phi }-{\phi }_r+{\varepsilon }_r^{(2)}]sin{\phi }_r}.$$

Equation (A4) is evaluated by the addition formulas for sinx and cosx with the approximations cosɛ_r = 1; sinɛ_r,. = ɛ_r:

(A5)$$tan\stackrel{\sim }{\mathrm{\Phi }}=\frac{2Rsin\mathrm{\Phi }+\text{∑}_{r=1}^R[{\varepsilon }_r^{(1)}+{\varepsilon }_r^{(2)}]cos\mathrm{\Phi }+(C_2-C_1)cos\mathrm{\Phi }+(S_2-S_1)sin\mathrm{\Phi }}{2Rcos\mathrm{\Phi }-\text{∑}_{r=1}^R[{\varepsilon }_r^{(1)}+{\varepsilon }_r^{(2)}]sin\mathrm{\Phi }-(S_2-S_1)cos\mathrm{\Phi }+(C_2-C_1)sin\mathrm{\Phi }},$$

with

$$C_i=\text{∑}_{r=1}^R{\varepsilon }_r^{(i)}cos2{\phi }_r\text{and}{S}_i=\text{∑}_{r=1}^R{\varepsilon }_r^{(i)}sin2{\phi }_r(i=1,2).$$

Equation (A5) is identical to Eq. (12a).

As pointed out in Eqs. (13)–(14c) reference-phase errors ${\varepsilon }_r^{(1)}={\zeta }_1(r-1)$ and ${\varepsilon }_r^{(2)}={\zeta }_1(r-1+a)$ (r is a whole number) lead only to an additional offset term arctanɛ′. For arbitrary R(R > 3) the cumulative error shall be denoted as ɛ_R. Then the slope ζ₁ of the error function is

(A6)$${\zeta }_1={\varepsilon }_R/R.$$

Since

$${\varepsilon }^{\prime }=\frac{1}{2R}\text{∑}_{r=1}^R{\zeta }_1(r-2+a)$$

where a = R/4 [because (2π)/R = (π/2)/a], if the reference phase is driven through one period of the interference pattern, the offset becomes

(A7)$$arctan\frac{\varepsilon R}{2}\left[\frac{5}{4}-\frac{1}{R}\right].$$

This offset due to linear calibration errors of the PZT driven reference mirror is in general of no consequence because the phase to be measured is known only up to a function f(x,y) = a + bx + cy + d(x² + y²), with a, b, c, and d as constants. This ambiguity is eliminated by a least-squares algorithm. Therefore, the offset of the type of Eq. (A7) is eliminated (remaining systematic error < λ/300) even if ɛ_R depends on (x² + y²) as is the case with the spherical Fizeau interferometer² (arctanɛ_R is approximated by ɛ_R), at least as long as the numerical aperture of the surface is below 0.6.

A time-saving algorithm with R = 4 and only 5 intensity values I₁…I₅ corresponding to reference phase values φ_r = 0,π/2,π,(3/2π),2π can be derived from Eq. (A4), i.e.,

(A8)$$tan\stackrel{\sim }{\varphi }=\frac{2I_2-2I_4}{I_1-2I_3+I_5}.$$

Calibration errors contribute an offset of arctan(ɛ₄/2). Because only simple arithmetic is used this formula is especially suitable for μP use.

Figures

 

Fig. 1 Scheme of the Twyman-Green interferometer with the phase measuring technique.

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Fig. 2 Contour lines and pseudo-3-D plot of the wave aberrations of a microobjective of N.A. = 0.32, where the contour line distance is λ/32.

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Fig. 3 Demonstration of the consequences of reference-phase errors. (a) Difference of two consecutive runs with arbitrary reference-phase deviations. First run: φ₁ = 0, φ₂ = π/2 + π/16; φ₃ = π + π/8, φ₄ = (3/2)π + (3/16)π. Second run: φ₁ = π/2, φ₂ = π + (π/16), φ₃ = (3/2)π + π/8, φ₄ = 2π + (3/16)π. The scale value on the left-hand side corresponds to a surface deviation of λ/256. (b) Intensity distribution with fringe adjustment. Note that the number of fringes is half of the number of maxima in (a).

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Fig. 4 Averaging effect in dependence of the phase step number. For the purpose of display an alifu representation has been chosen. The alifu program adds a linear function to the deviation data. From contour line to contour line a λ/64 increase has been introduced artificially (on the left). The perspective plot shows the same with a λ/128 increase from section to section (on the right). Top: Difference of two runs, where the first run had a phase error; i.e., one value of 16 had a deviation of π/8, Below: same as above but with only four phase steps per run and the same phase error for 1 value of 4.

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Fig. 5 Demonstration of reference-phase error cancellation. (a) Two runs superimposed, second run with π/2 reference-phase offset, 3 fringes/diameter, one division = λ/128 surface deviation. (b) A posteriori averaging of the two runs of (a). (c) Averaging of the numerator and the denominator of another phase profile with λ/256 sensitivity two runs (on the left) in comparison with error-free parallel adjustment evaluation (on the right).

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Fig. 6 Phase error due to extraneous fringes. (a) Phase error due to glass reflection introduced into the object arm of the interferometer, scale value: λ/64 surface deviation. Extraneous fringes can be perceived by slightly inclining the figure. (b) Average of two runs, where the second was made with a π offset of the object wave front, same sensitivity as in (a).

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Fig. 7 Average of two runs where the second was carried out with a tilt of λ from left to right. Scale value: λ/64 surface deviation. The beat frequency of the extraneous fringes is identical to the frequency of the fringes to be evaluated.

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Fig. 8 Intensity distribution overlaid with extraneous fringes. (a) Fringe pattern occurring if a glass plate has been introduced in one arm of the interferometer. (b) Section through the fringe pattern from top to bottom of (a). Extraneous fringes modulate the useful interference pattern.

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Fig. 9 Phase distribution associated with the intensity distribution of Fig. 8; scale value: λ/128 surface deviation.

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Fig. 10 Two runs superimposed on each other. The second run was made with a phase offset of c ·(2/3)π.

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Fig. 11 Total cancellation of the phase disturbances due to the algorithm of Eq. (19).

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Fig. 12 Averaging of the two runs of Fig. 11 a posteriori. Cancellation of extraneous fringes is only poor because of the phase mismatch of the object phase between the two runs.

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Fig. 13 Demonstration of the cancellation efficiency of the algorithm due to Eq. (19). The second run was carried out with an additional tilt (compare Fig. 7), which means that Φ has been changed by the additional phase χ exceeding the required limits. Note the phase jumps in the neighborhood of χ = 2nπ. The cancellation works sufficiently well for the greater part of the phase interval which can be outlined from the jump-corrected second curve.

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Fig. 14 Demonstration of the (η − Φ) dependence of ΔΦ. (a) Fringepattern adjusted in such a manner that along a perpendicular section η is constant (b) Phase disturbances due to a Φ variation (scale value: λ/128 surface deviation).

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Fig. 15 Series of pictures demonstrating the influence of Φ on the resulting erroneous deviation. (a) Φ = const: left, phase distribution; right, inference pattern. (b) sgnη = sgnΦ: left, phase distribution; right, interference pattern. (c) sgnη ≠ sgnΦ: left, phase distribution; right, interference pattern (scale value: λ/128 surface deviation).

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Fig. 16 Schematic illustration of the relation of the coherent noise amplitude to the background signal used in the phase measurement: u_b = background wave amplitude; u_n = noise amplitude; ΔΦ = maximum deviation from background phase Φ.

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Fig. 17 Scan through the intensity distribution in the detector plane. No filtering operations administered. Parallel adjustment of the interference pattern.

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Fig. 18 Same as Fig. 17 but with a moving scatterer at the pinhole position of the illuminating system. Note the smoothing effect of the moving scatterer.

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Fig. 19 Phase distribution for a wedge adjustment of c. 10 fringes. Scale value: λ/256 surface deviation.

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Fig. 20 Same phase distribution as in Fig. 19 but taken in parallel adjustment; same scale value as in Fig. 19.

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Fig. 21 Same as Fig. 20 but with spatial filter inserted.

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Fig. 22 Rotating scatterer inserted into the ray path at the position of an intermediate image of the surface to be tested: on the left, scatterer defocused; on the right, scatterer sharply imaged onto the detector array, spatial filter removed. Scale value: λ/256 surface deviation.

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Fig. 23 Phase difference of two consecutive runs made in parallel adjustment. For the display of the difference we used the alifu representation. alifu means the addition of a linear function to the phase distribution. Here (λ/64)/diameter were added: on the left, contour line plot, on the right, pseudo-3-D plot, because of the number of sections two consecutive lines are approximately λ/128 apart.

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