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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 15 — Jul. 18, 2011
  • pp: 14300–14314
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Estimation of accuracy of optical measuring systems with respect to object distance

Antonin Miks and Jiri Novak  »View Author Affiliations


Optics Express, Vol. 19, Issue 15, pp. 14300-14314 (2011)
http://dx.doi.org/10.1364/OE.19.014300


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Abstract

It is well-known that the change in the distance from the optical system to the object affects the image quality. Optical measurement systems, which are aberration-free for a specified position of the measured object, are then limited by induced aberrations for other object positions due to the dependence of aberrations on the varying object position. The consequence of this effect is a change in measurement accuracy. Our work provides a theoretical analysis of the influence of aberrations, which are induced by the change in the object position, on the accuracy of optical measuring systems. Equations were derived for determination of the relative measurement error for monochromatic and polychromatic light using the dependence of the third-order aberrations on the object position. Both geometrical and diffraction theory is used for the analysis. The described effect is not removable in principle and it is necessary to take account to it in high accuracy measurements. Errors can be reduced by a proper design of optical measuring systems. The proposed analysis can be used for measurement corrections.

© 2011 OSA

1. Introduction

The accuracy of optical instruments that are used for measurements of distances and dimensions in science and engineering depends on a distance from the measurement device to the measured object. Photogrammetric systems, optical measuring instruments based on CCD sensors, fringe projection systems for 3D shape measurement, theodolites, level instruments, microscopes and projection measurement systems are mainly used for such kind of measurements [1

1. N. Suga, Metrology Handbook: The Science of Measurement (Mitutoyo Ltd., 2007).

,2

2. T. Yoshizawa, Handbook of Optical Metrology: Principles and Applications (CRC Press, 2009).

]. These instruments are designed by the manufacturer so that they provide optimum imaging properties only for a specific distance which is characteristic for different measurement instruments. It is well known that aberrations of optical systems depend on the distance from the optical system to the object [3

3. A. Miks, Applied Optics (Czech Technical University Press, 2009). [PubMed]

9

9. A. Walther, “Irreducible aberrations of a lens used for a range of magnifications,” J. Opt. Soc. Am. A 6(3), 415–422 (1989). [CrossRef]

]. This problem is analyzed in detail by Buchdal [4

4. H. A. Buchdahl, An Introduction to Hamiltonian Optics (Cambridge University Press, 1970).

]. Moreover, it is also described by Herzberger [6

6. M. Herzberger, Modern Geometrical Optics (Interscience, 1958).

,7

7. M. Herzberger, “Theory of image rrrors of the fifth order in rotationally symmetrical systems. I,” J. Opt. Soc. Am. 29(9), 395–406 (1939). [CrossRef]

], Wynne [8

8. C. G. Wynne, “Primary aberrations and conjugate change,” Proc. Phys. Soc. 65B, 429–437 (1952).

], and Walther [9

9. A. Walther, “Irreducible aberrations of a lens used for a range of magnifications,” J. Opt. Soc. Am. A 6(3), 415–422 (1989). [CrossRef]

]. The measurement error originates from the dependence of aberrations of the optical system of the measurement device on the distance from the device to the object. Aberrations of the optical system affect the measurement accuracy of the system, which can differ substantially from the optimum accuracy given by the manufacturer. The described effect is not removable in principle and it is necessary to take account to it in high accuracy measurements. It is presented that this effect can be reduced by a proper design of optical measuring systems.

The aim of our work is not a derivation of exact analytical formulas for errors due to varying object distance. Such formulas cannot be obtained in a simple analytical form. For example, with the use of the fifth order aberration theory [4

4. H. A. Buchdahl, An Introduction to Hamiltonian Optics (Cambridge University Press, 1970).

,7

7. M. Herzberger, “Theory of image rrrors of the fifth order in rotationally symmetrical systems. I,” J. Opt. Soc. Am. 29(9), 395–406 (1939). [CrossRef]

] one can derive very complicated equations, which can be hardly used in practice. It is important to estimate measurement errors with respect to varying object distance in practice, i.e. found out if measurement errors are tenths of millimeters, millimeters or centimeters. Optical measuring systems, such as autocollimators, theodolites, leveling instruments, photogrammetry systems, topography measurement systems, use low numerical apertures and have usually a small field of view, which makes possible to apply the third-order aberration theory for approximate analysis of such systems. Our work deals with a theoretical description of the mentioned effect in terms of geometrical and diffraction theory of optical imaging on the basis of the third-order aberration theory (Seidel aberration theory) [3

3. A. Miks, Applied Optics (Czech Technical University Press, 2009). [PubMed]

8

8. C. G. Wynne, “Primary aberrations and conjugate change,” Proc. Phys. Soc. 65B, 429–437 (1952).

]. The change in the object position with respect to the measurement instrument leads to the change in aberrations of the optical system and this change affects negatively the measurement accuracy. The aim of this work is to derive simple formulas for approximate estimation of errors of optical measurement systems using the Seidel aberration theory both for monochromatic and polychromatic light. Derived formulas can be used to estimate measurement errors for objects that are located in different distances from the optical measuring system, which is a very important task from a practical point of view. As far as we know such relations had never been published. It is possible to carry out approximate corrections of the measured data based on the derived equations in order to obtain higher measurement accuracy.

2. Change in aberrations with object position

Consider the problem of influence of the change in the object position on imaging properties of a general rotationally symmetrical optical system. Figure 1
Fig. 1 Imaging of two different planes by optical system.
shows imaging of two different planes by the optical system. Suppose that the optical system is aberration-free for imaging of the plane ξA, then an arbitrary point A of this plane is imaged as the point A´ in the plane ξ′A. Choose now the different plane ξB. An arbitrary point B of this plane will not be imaged by the optical system as the point B´, but as the circle of least confusion dB according to aberrations of the optical system, which originate from the fact that the planes ξA and ξB are not identical. It causes the deviation of imaging properties. The relationship between the size of the object and its image will not be further linear. If we use such optical system for measurement the mentioned effect causes the measurement error, which cannot be removed. If the optical system is aberration free for a specific position of the object, then it has aberrations for other object positions and the image has lower quality.

S=(g4SIgPg3SIIgP2g2SIIISIVgP3gSV),A=(a11a12a13a14a15a16a17014a1212a1312a1434a15a16a270016a13012a15a16a370001000000014a15a160),S0=(SISIISIIISIVSVSVI1).

The coefficients aij are given by the following formulas
a11=(gPg)4,a12=4(gPg)3gP,a13=6(gPg)2gP2,a14=2n2f2(gPg)2,
a15=4(gPg)gP3,a16=gP4,a17=nf(gPg)[3(gP21)3gP(gPg)+(gPg)2],
a27=nf(gPg)[2(gP21)gP(gPg)],a37=nf(gPg)(gP21),
where f′ is the focal length of the optical system. Equations (2) represent generally valid formulas for calculation of aberration coefficients of the optical system corresponding to varying distance of the object. Moreover, Eq. (2) can be modified into the form where the dependence on the magnification (position of the object) is explicitly expressed in contrast to previously published papers. We obtain the following linear system:
S=BG,
(3)
where

B=(b11b12b13b14b150b22b23b24b2500b33b34b350000b45000b54b55),G=(g4g3g2g1),
b11=SI,b12=4gP(SI+SII)nf,b13=6gP2(SI2SII+SIII)+2n2f2SIV,
b14=4gP3(SI+3SII3SIII+SV)4n2f2gPSIV+3nf,
b15=gP4(SI4SII+6SIII4SV+SVI)+2n2f2gP2SIV+gPnf(gP23),
b22=gPSII,b23=3gP2(SII+SIII)+nf(nfSIVgP),
b24=3gP3(SII2SIII+SV)2nf(nfgPSIV1),
b25=gP4(SII+3SIII3SV+SVI)+n2f2gP2SIV+gPnf(gP22),
b33=gP2SIII,b34=2gP3(SVSIII)nf(gP21),b35=gP4(SIII+SVI2SV)+nfgP(gP21),
b45=SIV,b54=gP3SV,b55=gP4(SV+SVI).

S2=S1B(G1G2).

Furthermore, we will focus on a special case which is interesting both from the theoretical and practical point of view. We will analyze the changes of aberrations with respect to the change in object’s position (change in magnification of the optical system) for the optical system which is aberration-free for objects at infinity. Assume now that we have the optical system, which is aberration-free for imaging the object at infinity and for the selected spectral range, i.e.

SI=SII=SIII=SIV=SV=SVI=0.

If we use Eq. (1) and set δx=δy=0, then the transverse ray aberrations δx,δy, which are induced by the change in position of the object, can be expressed as
δx=(1/2g)[(AY2AX+AX3)SI02tanwAXAYSII0+tan2wAXSIII0],δy=(1/2g)[(AX2AY+AY3)SI0tanw(AX2+3AY2)SII0+3tan2wAYSIII0],
(4)
where AX = xP 1/p 1 and AY = yP 1/p 1 are numerical apertures (in air) in the object space, w is the angle of field of view (tan w = y/p 1), and n = n' = 1 (the most common situation in practice). The coefficients SIo,SIIo,SIIIoare given by

SIo=f(gPg)   [(gP+g)2gPg3],SIIo=f(gPg)[gP(gP+g)2],SIIIo=f(gPg)(gP21).
(5)

Equations (4) and (5) are very interesting because these formulas show that aberration properties (aberration coefficients) of the considered optical system (aberration-free for the object at infinity) depend only on the focal length f', the angular magnification g, and the angular magnification gP between pupils of the optical system. In order to determine the shift of “geometric-optical energy centre of the circle of confusion” we calculate mean values of transverse ray aberrations. The mean values (centroid of the spots) δx and δy of the transverse ray aberrations δxand δy (Eq. (3) are given by [8

8. C. G. Wynne, “Primary aberrations and conjugate change,” Proc. Phys. Soc. 65B, 429–437 (1952).

]
δx=1πR202π0Rδxr  dr  dϕ=1πAM202π0AMδxA  dA  dϕ=0,
(6)
δy=1πR202π0Rδyr  dr  dϕ=1πAM202π0AMδyA  dA  dϕ=12gAM2tanwSII0,
(7)
where we used the following relations
Ax=Asinϕ,Ay=Acosϕ,
(r, φ) are polar coordinates in entrance pupil plane, R is entrance pupil radius, and

A=Ax2+Ay2=xP12+yP12/p1=r/p1.

As one can see from Eq. (7) the position of the centroid is affected only by coma coefficient in our case. Other aberration coefficients have no influence on the position of the centroid because integrals (6) and (7) of terms corresponding to aberration coefficients SIo and SIIIoare zero. However, aberration coefficients affect the radius of gyration over the exit pupil. This problem is treated in more detail in [11

11. J. B. Develis, “Comparison of methods for image evaluation,” J. Opt. Soc. Am. 55(2), 165–173 (1965). [CrossRef]

]. Concerning distortion one can clearly see from Eq. (3) that aberration coefficient SV is zero for all positions of the object. The expression
AM=12F0(gg)P
(8)
gives the maximum numerical aperture of the optical system in the object space, and F 0 is the f-number of the optical system for the object at infinity. We can write
tanw=yp1=ysPs1=yf(gPg),
where y is the size of the object, s 1 is the object distance, and sP is the position of the entrance pupil. We obtain for the relative error of measurement
δyy=ϕ(m,mP)F02,
(9)
where we denoted
ϕ(m,mP)=mmP+m2(12mP2)8(mPm)2,
(10)
m is the transverse magnification of the optical system (m = 1/g), mP is the transverse magnification in pupils of the optical system (mP = 1/gP), and is the image size. In measurement practice, optical systems with a telecentric path of the principal ray (object-side, image-side and double-sides telecentric lenses) are frequently used. Telecentric optical systems are treated in detail in [12]. In case of image-side telecentric optical system (mP), e.g. objective lenses of CCD cameras, we obtain for such systems from Eq. (10)

ϕ(m,)=m2/4.

Formulas (9) and (10) are very useful for estimation of measurement errors caused by varying object distance. These errors cannot be explained using the paraxial approximation. We had used the third-order aberration theory and obtained resulting formulas that have shown the magnitude of errors, both generally and for a special case of optical system without aberrations. In the last case (optical system without aberrations for objects at infinity) derived equations present approximate estimation of measurement accuracy obtained with optical instruments. Presented formulas extend knowledge in the field of optical metrology and other areas, such as geodesy, photogrammetry, fringe projection methods, etc. It is clear from the presented results that these errors have physical character and cannot be removed, but their influence can be reduced by a proper design of optical measuring systems. This can be done as follows. Firstly, we choose the range of angular magnification ggmin,gmax for which we want to optimize the optical system. We can write Eqs. (1) and (2) for several values of angular magnification ggmin,gmax. Then, using the least squares method we can calculate the values of the aberrations coefficients SI,SII,SIII,SIV,SV,SVI that minimize aberrations in the given range of the change of magnification g. Using aberrations coefficients SI,SII,SIII,SIV,SV,SVI we can calculate parameters of the optical system. These parameters then serve as a starting state (predesign) for further optimization of the optical design of the optical system using optical design software, such as OSLO, ZEMAX, CODE V, OPTALIX. The procedure is the same as in the Ref [10

10. A. Mikš, J. Novák, and P. Novák, “Method of zoom lens design,” Appl. Opt. 47(32), 6088–6098 (2008). [CrossRef] [PubMed]

]. The problem is also solved in a different way by Walther [9

9. A. Walther, “Irreducible aberrations of a lens used for a range of magnifications,” J. Opt. Soc. Am. A 6(3), 415–422 (1989). [CrossRef]

] who used mock ray tracing and numerical optimization in his work. Equations (9) and (10) are also interesting from a theoretical point of view because they say that if the optical system is aberration-free (SI=0, SII=0, SIII=0, SIV=0, SV=0, SVI=0) for the object at infinity, then the relative measurement error does not depend (within the validity of the third-order aberration theory) on the type and composition of the considered optical system, but it depends only on its transverse magnification m, transverse magnification mP between pupils, and f-number F 0. Using Eq. (9) we can calculate the ratio of the mean value (centroid) of transverse ray aberration to the image size in dependence on the transverse magnification m of the optical system.

3. Influence of wavelength on measurement accuracy

Let us now deal with the influence of the wavelength of light λ on the accuracy of measurement. It is well known that aberrations of the optical system vary with the wavelength of light, i.e. the optical system has chromatic aberration [3

3. A. Miks, Applied Optics (Czech Technical University Press, 2009). [PubMed]

5

5. W. T. Welford, Aberrations of the Symmetrical Optical Systems (Academic Press, 1974).

,7

7. M. Herzberger, “Theory of image rrrors of the fifth order in rotationally symmetrical systems. I,” J. Opt. Soc. Am. 29(9), 395–406 (1939). [CrossRef]

]. Assume that we have the optical system, which is aberration-free for imaging the object at infinity and for the selected spectral range λλ1,λ2 (e.g. visible spectral range), i.e.

SI(λ)=SII(λ)=SIII(λ)=SIV(λ)=SV(λ)=SVI(λ)=0.

In order to analyze the influence of chromatic aberration of the optical system on measurement accuracy for different object positions (object located at finite distance from the optical system), we have to differentiate Eq. (10). After some calculations we obtain
dϕλ=α(mP,m)(dyλy)β(mP,m)(dyPλyP)
(11)
where functions α(mP,m) and β(mP,m)are given by the following expressions
α(mP,m)=mPm[mPm(4mP23)]8(mPm)3,β(mP,m)=mPm[mPm(4mmP3)]8(mPm)3,
(12)
dyλ is the lateral chromatic aberration of the optical system for the transverse magnification m, and dyPλ is the lateral chromatic aberration in pupils of the optical system for the transverse magnification mP. Further, we focus on calculation of the lateral chromatic aberration dyλ of the optical system. Assume that the image and object space is air (n=n=1). It is known from the theory of chromatic aberrations that longitudinal chromatic aberration δsλ and lateral chromatic aberration dyλ can be expressed by the following general formulas [4

4. H. A. Buchdahl, An Introduction to Hamiltonian Optics (Cambridge University Press, 1970).

,5

5. W. T. Welford, Aberrations of the Symmetrical Optical Systems (Academic Press, 1974).

]
δsλ=h12σ2CI,
(13)
dyλy=1f(mmP)(m2s12CImPmsPs1CII)=dsλf(mPm)hP1h1HCII,
(14)
where CI is the coefficient of longitudinal chromatic aberration, CII is the coefficient of lateral chromatic aberration, and H=n(hP1σ1h1σP1) is the Lagrange-Helmholtz invariant. It holds that
s12CI=f2(gPg)2CIsPf(gPg)(CII+CIIP)+sP2CIP,
s1CII=f(gPg)CII+sPCIP,
where CIand CII are chromatic aberration coefficients for the object at infinity (s1), CIP and CIIP are pupil chromatic aberration coefficients.

s12CI=sPf(gPg)CIIP+sP2CIP,s1CII=sPCIP.
(15)

By substitution of Eq. (15) into Eqs. (13) and (14) we have

δsλ=sPg2[f(gPg)CIIPsPCIP],
(16)
dyλy=δsλ+sP2mPmCIPf(mPm).
(17)

We can derive for the chromatic aberration dyPλ in pupils
dyPλyP=1f(mPm)(mP2sP2CIPmPmsPs1CIIP),
(18)
where

CIP=CIP,CIIP=CIIP+sPs1(CIPCIIP).
(19)

By substitution of Eq. (19) into Eq. (18) we obtain

dyPλyP=mPsP2fCIPsPCIIP.
(20)

By substitution of Eq. (17) and Eq. (20) into Eq. (11) we obtain provided that CI=CII=0 the following relation

dϕλ=α(mP,m)(δsλ+sP2mPmCIPf(mPm))β(mP,m)(mPsP2fCIPsPCIIP).
(21)

Equation (21) enables to analyze the influence of chromatic aberrations of the optical system (aberration corrected for the object at infinity: δsλ=0, δyλ=0) on measurement accuracy for different object positions (object located in a finite distance from the optical system). If chromatic aberrations in pupils of the optical system are corrected, i.e. CIP=CIIP=0, then we obtain from Eq. (21) that dϕλ=0. In such case the optical system is achromatic and the measurement accuracy is not dependent on wavelength.

The relationship between the wave aberration and the ray aberration is given by [3

3. A. Miks, Applied Optics (Czech Technical University Press, 2009). [PubMed]

5

5. W. T. Welford, Aberrations of the Symmetrical Optical Systems (Academic Press, 1974).

]
δx=2FWX=mWAX,δy=2FWY=mWAY,
(22)
where X and Y are normalized coordinates of the intersection of the ray with the reference sphere in the image space, F is the f-number of the optical system, AX and AY are numerical apertures in the direction x and y in the object space. The wave aberration can be then calculated by integration, i.e.
W=1m(δxdAX+δydAY),
(23)
where the transverse ray aberrations δx,δyare given by Eq. (4).

4. Dependence of image quality on object position change

Considering wave properties of light and the finite size of optical systems, the image of the point in the object plane is the diffraction pattern in the image plane. The response of the optical system to the point signal is called the point spread function (PSF) [3

3. A. Miks, Applied Optics (Czech Technical University Press, 2009). [PubMed]

,10

10. A. Mikš, J. Novák, and P. Novák, “Method of zoom lens design,” Appl. Opt. 47(32), 6088–6098 (2008). [CrossRef] [PubMed]

,11

11. J. B. Develis, “Comparison of methods for image evaluation,” J. Opt. Soc. Am. 55(2), 165–173 (1965). [CrossRef]

,13

13. E. L. O'Neill, Introduction to Statistical Optics (Addison-Wesley Inc., 1963).

19

19. V. N. Mahajan, “Zernike polynomials and optical aberrations,” Appl. Opt. 34(34), 8060–8062 (1995). [CrossRef] [PubMed]

]. The shape of the PSF, i.e. the energy distribution in the diffraction pattern, depends on the position of the point in the object plane and on the distance from the optical system to the object plane. If wave aberration changes due to the variation of the object position, the shape and position of the peak of point spread function will also change and measurement errors occur.

Assuming the case that aberrations are small, then we can take as a criterion of the quality of optical systems for imaging the point object the normalized intensity in the peak of the diffraction pattern (Strehl definition, Strehl ratio) [10

10. A. Mikš, J. Novák, and P. Novák, “Method of zoom lens design,” Appl. Opt. 47(32), 6088–6098 (2008). [CrossRef] [PubMed]

,11

11. J. B. Develis, “Comparison of methods for image evaluation,” J. Opt. Soc. Am. 55(2), 165–173 (1965). [CrossRef]

,20

20. A. J. E. M. Janssen, S. van Haver, J. J. M. Braat, and P. Dirksen, “Strehl ratio and optimum focus for high-numerical-aperture beams,” J. Eur. Opt. Soc. Rapid Publ. 2, 07008 (2007). [CrossRef]

] that is defined as the ratio of the maximum of the point spread function of the real optical system to the maximum of the point spread function of the diffraction limited system (optical system without aberrations). It is well known [10

10. A. Mikš, J. Novák, and P. Novák, “Method of zoom lens design,” Appl. Opt. 47(32), 6088–6098 (2008). [CrossRef] [PubMed]

,11

11. J. B. Develis, “Comparison of methods for image evaluation,” J. Opt. Soc. Am. 55(2), 165–173 (1965). [CrossRef]

,13

13. E. L. O'Neill, Introduction to Statistical Optics (Addison-Wesley Inc., 1963).

15

15. W. B. King, “Dependence of the Strehl ratio on the magnitude of the variance of the wave aberration,” J. Opt. Soc. Am. 58(5), 655–661 (1968). [CrossRef]

] that the Strehl ratio of the optical system with small aberrations can be expressed in the form
I=1k02(W2¯W¯2)=1k02E0,
(24)
where E 0 is the variance of wave aberration and
W¯=1SSWdS,  and  W2¯=1SSW2dS,
(25)
where W is wave aberration, k 0 = 2π/λ0, and λ0 is the wavelength of light in the vacuum. With respect to the Strehl definition, we consider the optical system to be equivalent to the diffraction limited system if the Strehl definition is higher than 0.8 (E0λ02/196). Wave aberrations can be expressed e.g. using Zernike polynomials [11

11. J. B. Develis, “Comparison of methods for image evaluation,” J. Opt. Soc. Am. 55(2), 165–173 (1965). [CrossRef]

,16

16. V. N. Mahajan, “Strehl ratio for primary aberrations: some analytical results for circular and annular pupils,” J. Opt. Soc. Am. 72(9), 1258–1266 (1982). [CrossRef]

19

19. V. N. Mahajan, “Zernike polynomials and optical aberrations,” Appl. Opt. 34(34), 8060–8062 (1995). [CrossRef] [PubMed]

] or Seidel polynomials [5

5. W. T. Welford, Aberrations of the Symmetrical Optical Systems (Academic Press, 1974).

,9

9. A. Walther, “Irreducible aberrations of a lens used for a range of magnifications,” J. Opt. Soc. Am. A 6(3), 415–422 (1989). [CrossRef]

]. In our work we will use Seidel aberration polynomials for further analysis. The wave aberration W of the third order can be expressed as
W=W11rcos φ +W20r2+W40r4+W31r3cos φ +W22r2cos 2φ,
(26)
where r and φ are polar coordinates at the exit pupil sphere of the rotationally symmetric optical system, W 11 is the coefficient of tilt, W 20 is the coefficient of defocus, W 40 is the coefficient of the third-order spherical aberration, W 31 is the coefficient of the third-order coma, and W 22 is the coefficient of the third-order astigmatism. Aberration coefficients can be calculated from the following formulas [3

3. A. Miks, Applied Optics (Czech Technical University Press, 2009). [PubMed]

,11

11. J. B. Develis, “Comparison of methods for image evaluation,” J. Opt. Soc. Am. 55(2), 165–173 (1965). [CrossRef]

]
W20=(δst+δss)/2s08nF2, W11=δyzy02F, W40=δsK16nF2,W31=δyKt6F, W22=δstδss16nF2,
(27)
where s 0 is longitudinal defocus, y 0 is the transverse defocus, δyz´ is distortion, δsK´ is spherical aberration for the aperture ray that passes through the edge of the entrance pupil, δst´ is meridional astigmatism, δss´ is sagittal astigmatism, δyKt is meridional coma, F is the f-number of the optical system, and n´ is the index of refraction in the image space. By substitution of Eq. (26) into Eq. (25) we obtain

E0=W2¯W¯2=112W202+16W20W40+445W402+14W112+13W11W31+18W312+16W222.
(28)

Aberration coefficients W 11 and W 20 are unrestrained parameters that express the coordinates of the centre of the reference sphere. The position of the optimum image point [11

11. J. B. Develis, “Comparison of methods for image evaluation,” J. Opt. Soc. Am. 55(2), 165–173 (1965). [CrossRef]

], i.e. the point with the minimum E 0 and maximum Strehl definition, can be calculated from the necessary conditions for the extremum of function E 0

E0W11=0,E0W20=0.

The solution of these equations can be expressed as [11

11. J. B. Develis, “Comparison of methods for image evaluation,” J. Opt. Soc. Am. 55(2), 165–173 (1965). [CrossRef]

]

W11=23W31,W20=W40.
(29)

Formula (29) is valid only for the optimum image point. By substitution of Eq. (27) for coefficients W11,W31,W20,W40 into Eq. (29) one can calculate the position (coordinates s 0, y 0), where the Strehl definition is maximum with respect to the paraxial image point. It holds

s0=δst+δss2+δsK2,y0=δyZ+29δyKt.

The detailed calculation of aberration coefficients is described in Ref [3

3. A. Miks, Applied Optics (Czech Technical University Press, 2009). [PubMed]

5

5. W. T. Welford, Aberrations of the Symmetrical Optical Systems (Academic Press, 1974).

]. We can see that one needs to know aberration coefficients W 11 and W 31 for analysis of the error of measurement with the optical system. Using formula (29) we have
W11=y02F=23W31=29FδyKt,
(30)
where

δyKt=32gtanwAM2SII0.
(31)

Using formulas (4), (5), (8), (30) and (31), we obtain for the relative measurement error
y0y=ψ(m,mP)F02,
(32)
where we denoted

ψ(m,mP)=mmP+m2(12mP2)12(mPm)2.
(33)

Formulas (32) and (33) are showing the influence of the shift of the point spread function central peak of the optical system under consideration with respect to the position of the point spread function central peak of the ideal optical system. Equation (30) is the resulting formula for the relative measurement error that is calculated on the basis of the diffraction theory of optical imaging.

We can compare values of function φ(m,mP) that was calculated on the basis of the geometrical theory of optical imaging and values of function ψ(m,mp) that was obtained using the diffraction theory of optical imaging. We can see that the less accurate geometrical theory gives larger values of relative measurement errors than more accurate diffraction theory of optical imaging. We obtain by comparison of both functions

ϕ(m,mP)ψ(m,mP)=32.
(34)

One can see that the geometrical theory of optical imaging gives 50% higher estimate of relative measurement errors than the diffraction theory of optical imaging regardless of the value of the transverse magnification m of the optical system.

5. Examples and analysis

s=f12λ(fF)2.
(35)

In order to give some impression about the magnitude of aberration induced measurement errors we will calculate the relative measurement errors for the optical system, which is aberration-free (SI=0,SII=0,SIII=0,SIV=0,SV=0,SVI=0) for an object at infinity. Let the optical system has the f-number F 0 = 5 and the transverse magnification in pupils of the optical system is mP = 1. The focal length of the optical system is f=300  mmand the image size is y=5  mm.

Example 1

Firstly, we determine the relative measurement error that is obtained using the geometrical theory of optical imaging for values of the transverse magnification m = - 0.5 and m = - 1. The relative measurement error εg(m,mP) expressed in percents can be calculated according to Eq. (9)

εg(m,mP)=100δy/y=100ϕ(m,mP)/F02.

We obtain for the first case δy=0.0085  mm, εg(0.5,1)=0.17%, and for the second case δy=0.0125  mm, εg(1,1)=0.25%. Figure 2
Fig. 2 Spot diagrams for different values of transverse magnification m.
presents spot diagrams for different values of transverse magnification m. Transverse ray aberrations dx and dy in Fig. 2 are expressed in millimeters.

Example 2

Further, we can calculate the relative measurement error that is obtained using the diffraction theory of optical imaging for the same optical system and same values of the transverse magnification m = - 0.5 and m = - 1. The relative measurement error εd(m,mP) expressed in percents can be calculated according to Eq. (32)

εd(m,mP)=100y0/y=100ψ(m,mP)/F02.

Example 3

We will show how accurately describes the third-order theory a real example of a collimator objective lens. Consider two element achromatic objective lens with the focal length f' = 100 mm and the f-number F = 10. The design parameters of the objective lens are given in Table 1

Table 1. Achromatic Objective Lens (f' = 100 mm, F = 10)

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, where r is the radius of curvature and d is the axial thickness. Table 2

Table 2. Aberrations – Exact and Approximate Calculation Example 4

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presents values of longitudinal spherical aberration (ΔsIII,Δsexact), tangential coma (KIII, Kexact) and astigmatism (astIII,astexact), calculated using third-order aberration coefficients SI, SII, SIII (index ″III″) and the exact calculation by ray tracing (index ″exact″). As one can see from Table 2 the third-order theory gives sufficiently accurate results that are practically the same as the exact values obtained by ray tracing.

Furthermore, we will analyze the accuracy of error calculation using Eq. (9) by a comparison with exact calculations using ZEMAX software. Considering the optical system with parameters described in Table 3

Table 3. Parameters of the Optical System

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. This optical system has very small residual aberrations and one can consider the system practically as diffraction limited.

Assume now that the optical system forms images of objects in different distances from the optical system with the transverse magnification m. The paraxial image height was chosen y=10  mm. Table 4

Table 4. Comparison of Results Between Derived Equations and Exact Calculations with ZEMAX Software

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presents calculations using Eq. (9) and ZEMAX software. As one can see from Table 4, Eq. (9) gives results which differ from the exact calculation using ZEMAX only 7.6% on average. This can be considered as a good agreement. The third-order aberration theory provides sufficiently accurate results for estimation of measurement errors due to the varying distance of the measured object from the optical system, even in the case when we do not know detailed parameters of the optical system.

6. Summary

Acknowledgments

This work has been supported by the Czech Science Foundation grant number P102/10/2377.

References and links

1.

N. Suga, Metrology Handbook: The Science of Measurement (Mitutoyo Ltd., 2007).

2.

T. Yoshizawa, Handbook of Optical Metrology: Principles and Applications (CRC Press, 2009).

3.

A. Miks, Applied Optics (Czech Technical University Press, 2009). [PubMed]

4.

H. A. Buchdahl, An Introduction to Hamiltonian Optics (Cambridge University Press, 1970).

5.

W. T. Welford, Aberrations of the Symmetrical Optical Systems (Academic Press, 1974).

6.

M. Herzberger, Modern Geometrical Optics (Interscience, 1958).

7.

M. Herzberger, “Theory of image rrrors of the fifth order in rotationally symmetrical systems. I,” J. Opt. Soc. Am. 29(9), 395–406 (1939). [CrossRef]

8.

C. G. Wynne, “Primary aberrations and conjugate change,” Proc. Phys. Soc. 65B, 429–437 (1952).

9.

A. Walther, “Irreducible aberrations of a lens used for a range of magnifications,” J. Opt. Soc. Am. A 6(3), 415–422 (1989). [CrossRef]

10.

A. Mikš, J. Novák, and P. Novák, “Method of zoom lens design,” Appl. Opt. 47(32), 6088–6098 (2008). [CrossRef] [PubMed]

11.

J. B. Develis, “Comparison of methods for image evaluation,” J. Opt. Soc. Am. 55(2), 165–173 (1965). [CrossRef]

12.

http://www.schneiderkreuznach.com/pdf/div/optical_measurement_techniques_with_telecentric_lenses.pdf

13.

E. L. O'Neill, Introduction to Statistical Optics (Addison-Wesley Inc., 1963).

14.

J. J. M. Braat, S. van Haver, A. J. E. M. Janssen, and P. Dirksen, “Assessment of optical systems by means of point-spread function ” in Progress in Optics, E. Wolf, ed. (Elsevier, 2008), Vol. 51, pp. 349–468.

15.

W. B. King, “Dependence of the Strehl ratio on the magnitude of the variance of the wave aberration,” J. Opt. Soc. Am. 58(5), 655–661 (1968). [CrossRef]

16.

V. N. Mahajan, “Strehl ratio for primary aberrations: some analytical results for circular and annular pupils,” J. Opt. Soc. Am. 72(9), 1258–1266 (1982). [CrossRef]

17.

V. N. Mahajan, “Strehl ratio for primary aberrations in terms of their aberration variance,” J. Opt. Soc. Am. 73(6), 860–861 (1983). [CrossRef]

18.

V. N. Mahajan, “Zernike circle polynomials and optical aberrations of systems with circular pupils,” Appl. Opt. 33(34), 8121–8124 (1994). [CrossRef] [PubMed]

19.

V. N. Mahajan, “Zernike polynomials and optical aberrations,” Appl. Opt. 34(34), 8060–8062 (1995). [CrossRef] [PubMed]

20.

A. J. E. M. Janssen, S. van Haver, J. J. M. Braat, and P. Dirksen, “Strehl ratio and optimum focus for high-numerical-aperture beams,” J. Eur. Opt. Soc. Rapid Publ. 2, 07008 (2007). [CrossRef]

OCIS Codes
(080.0080) Geometric optics : Geometric optics
(110.3000) Imaging systems : Image quality assessment
(120.3930) Instrumentation, measurement, and metrology : Metrological instrumentation
(120.3940) Instrumentation, measurement, and metrology : Metrology
(120.4640) Instrumentation, measurement, and metrology : Optical instruments
(150.3045) Machine vision : Industrial optical metrology

ToC Category:
Instrumentation, Measurement, and Metrology

History
Original Manuscript: March 10, 2011
Revised Manuscript: April 21, 2011
Manuscript Accepted: May 17, 2011
Published: July 12, 2011

Citation
Antonin Miks and Jiri Novak, "Estimation of accuracy of optical measuring systems with respect to object distance," Opt. Express 19, 14300-14314 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-15-14300


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References

  1. N. Suga, Metrology Handbook: The Science of Measurement (Mitutoyo Ltd., 2007).
  2. T. Yoshizawa, Handbook of Optical Metrology: Principles and Applications (CRC Press, 2009).
  3. A. Miks, Applied Optics (Czech Technical University Press, 2009). [PubMed]
  4. H. A. Buchdahl, An Introduction to Hamiltonian Optics (Cambridge University Press, 1970).
  5. W. T. Welford, Aberrations of the Symmetrical Optical Systems (Academic Press, 1974).
  6. M. Herzberger, Modern Geometrical Optics (Interscience, 1958).
  7. M. Herzberger, “Theory of image rrrors of the fifth order in rotationally symmetrical systems. I,” J. Opt. Soc. Am. 29(9), 395–406 (1939). [CrossRef]
  8. C. G. Wynne, “Primary aberrations and conjugate change,” Proc. Phys. Soc. 65B, 429–437 (1952).
  9. A. Walther, “Irreducible aberrations of a lens used for a range of magnifications,” J. Opt. Soc. Am. A 6(3), 415–422 (1989). [CrossRef]
  10. A. Mikš, J. Novák, and P. Novák, “Method of zoom lens design,” Appl. Opt. 47(32), 6088–6098 (2008). [CrossRef] [PubMed]
  11. J. B. Develis, “Comparison of methods for image evaluation,” J. Opt. Soc. Am. 55(2), 165–173 (1965). [CrossRef]
  12. http://www.schneiderkreuznach.com/pdf/div/optical_measurement_techniques_with_telecentric_lenses.pdf
  13. E. L. O'Neill, Introduction to Statistical Optics (Addison-Wesley Inc., 1963).
  14. J. J. M. Braat, S. van Haver, A. J. E. M. Janssen, and P. Dirksen, “Assessment of optical systems by means of point-spread function ” in Progress in Optics, E. Wolf, ed. (Elsevier, 2008), Vol. 51, pp. 349–468.
  15. W. B. King, “Dependence of the Strehl ratio on the magnitude of the variance of the wave aberration,” J. Opt. Soc. Am. 58(5), 655–661 (1968). [CrossRef]
  16. V. N. Mahajan, “Strehl ratio for primary aberrations: some analytical results for circular and annular pupils,” J. Opt. Soc. Am. 72(9), 1258–1266 (1982). [CrossRef]
  17. V. N. Mahajan, “Strehl ratio for primary aberrations in terms of their aberration variance,” J. Opt. Soc. Am. 73(6), 860–861 (1983). [CrossRef]
  18. V. N. Mahajan, “Zernike circle polynomials and optical aberrations of systems with circular pupils,” Appl. Opt. 33(34), 8121–8124 (1994). [CrossRef] [PubMed]
  19. V. N. Mahajan, “Zernike polynomials and optical aberrations,” Appl. Opt. 34(34), 8060–8062 (1995). [CrossRef] [PubMed]
  20. A. J. E. M. Janssen, S. van Haver, J. J. M. Braat, and P. Dirksen, “Strehl ratio and optimum focus for high-numerical-aperture beams,” J. Eur. Opt. Soc. Rapid Publ. 2, 07008 (2007). [CrossRef]

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