New analytic results for the Zernike circle polynomials from a basic result in the NijboerZernike diffraction theory


 Ashlie Peters
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1 New analytic results for the Zernike circle polynomials from a basic result in the NijboerZernike diffraction theory A.J.E.M. Janssen Eindhoven University of Technology, Department EE and EURANDOM, LG.39, P.O. Box 53, 56 MB Eindhoven, The Netherlands Abstract. Several quantities related to the Zernike circle polynomials admit an expression, via the basic identity in the diffraction theory of Nijboer and Zernike, as an infinite integral involving the product of two or three Bessel functions. In this paper these integrals are identified and evaluated explicitly for the cases of (a) the expansion coefficients of scaledandshifted circle polynomials, (b) the expansion coefficients of the correlation of two circle polynomials, (c) the Fourier coefficients occurring in the cosine representation of the circle polynomials. Introduction Zernike circle polynomials are extensively used in the characterization of circular optical imaging systems with nonuniform pupil functions [] [8], and, more recently, for the computation of acoustical quantities arising from harmonically excited, baffledpiston radiators with nonuniform velocity profiles [9] []. The circle polynomials were introduced by Zernike [] in connection with the knifeedge test and his phasecontrast method and they play a fundamental role in Nijboer s thesis [3] (also see Ch. 9 in []), on the diffraction theory of aberrations where they were investigated in detail. Nowadays, the circle polynomials find widespread application in optical lithography, astronomy, ophthalmology and other fields dealing with diffraction phenomena involving nonuniformities in a circular, finite setting.
2 The circle polynomials are given for integer m, n with n m even and nonnegative by Z m n (ρ, ϑ) = R m n (ρ) e imϑ, ρ, ϑ < π, () where the radial polynomials R m n are given by R m n (ρ) = ρ m P (, m ) (ρ ), () n m with P (α,β) k the general Jacobi polynomial as in [8], Ch.. The circle polynomials are complete and orthogonal on the unit disk {(ρ cos θ, ρ sin θ) = (ν, µ) ρ = ν +µ, θ < π}, and can therefore be used to expand any pupil function P (ρ, θ) that is defined on and square integrable over the unit disk. Besides being complete and orthogonal, the circle polynomials possess a particular convenient form, in terms of Jinc functions (see Eq.(5) below), for their Fourier transforms. This result, which we shall call the basic identity of the NijboerZernike theory of optical aberrations (basic NZresult, for short) was given by Zernike in [] and applied by Nijboer in [3] to compute the optical pointspread function in and near the bestfocus plane for mildly aberrated, circular optical systems with lowtomedium numerical aperture (NA). The NijboerZernike approach to pointspread computation has been extended in recent years to cover the whole focal region for systems with arbitrarily high NA including the state of polarization and birefringence, see [4] [7] and a survey is given in [4]. The basic NZresult has yielded several useful analytic results in a variety of optical applications. In [5], this result is applied to derive expressions for the derivatives of the radial polynomials, with application to atmospheric turbulence. In [9] [], a formula for the Zernike expansion coefficients of a scaled pupil in terms of the coefficients of the unscaled pupil is derived. This formula is based on a concise expression for scaled radial polynomials in terms of unscaled radial polynomials, see Eq.(8) below, the proof of which is of similar nature as the one given in [5] and heavily depends on the basic NZresult. Results on pupil scaling find applications in optical lithography, where the NA of the optical system may be decreased deliberately for imaging enhancement of particular structures, and in ophthalmology, where pupil scaling is studied as a natural attribute of the human eye pupil, see [7] and []. The basic NZresult can also be used to express the Radon transform (the integral along a line of arbitrary distance p to the origin and arbitrary angle φ of the normal with the νaxis) of the circle polynomials, see Eq.(9) below. This result is used in [] to obtain a computation scheme of the DFTtype, with the inherent efficiency and accuracy that come with DFTalgorithms, for the circle polynomials of arbitrarily large degree n and order m, see Eq.() below. Equation () was derived by a different method in [3], while Eq.(9) was presented already in [4] by Cormack in a medical imaging context. In [5] [6], the problem of computing the Zernike expansion coefficients of the optical transfer function (OTF) from the coefficients of the pupil function is tackled using the
3 basic NZresult. This gives rise to integrals involving the product of three Bessel functions for which there are presented recursion relations in [6]. In the present paper, we give analytic solutions to three problems in optics using the basic NZresult. All these problems have as a common feature that they give rise, via the basic NZresult, to integrals involving the product of three Bessel functions that can be evaluated in analytic form. These problems / solutions concern determining the Zernike expansion of an arbitrary scaledandshifted circle polynomial, extending the pure scaling results in [9] [], finding the Zernike expansion of the OTF in terms of the expansion coefficients of the pupil function, thereby solving the problem in [5] [6] completely analytically, finding the Fourier expansion coefficients of the radial polynomials in which the radial variable ρ, ρ, is replaced by the numerically more convenient variable cos x, x π/. In the next section these three problems are described and motivated in more detail. A fourth problem that gives rise, via the basic NZresult, to integrals of the product of three Bessel functions, is the computation of the spatial impulse responses that occur in baffledpiston acoustic radiation using Zernike expansion of flexible membranes. This problem is discussed in [7] and [8], Sec. 6. Basic formulas and overview The circle polynomials are given for integer m, n with n m even and nonnegative by Eq.() and (). It is customary to refer to n as the degree and to m as the azimuthal order of Z m n. The circle polynomials form a complete orthogonal system of functions on the disk ρ, with Z m n (, ϑ) = e imϑ, which implies that and the orthogonality property reads explicitly π R m n () =, (3) Z m n (ρ, ϑ)((z m n (ρ, ϑ)) ρ dρ dϑ = π n + δ mm δ nn (4) with δ Kronecker s delta. In the sequel it will be convenient to set R m n = Z m n for integer values of m, n such that n m is odd or negative. A crucial property of the circle polynomials for diffraction theory is that their Fourier 3
4 transform has the particular simple form e πiνx+πiµy Zn m (ρ, ϑ) dν dµ = ν +µ = π e πiρr cos(ϑ ϕ) R n m (ρ) e imϑ ρ dρ dϑ = πi n J n+(πr) e imϕ, (5) πr where we have written ν + iµ = ρ e iϑ and x + iy = r e iϕ. Equivalently, in terms of Hankel transforms (of order m), we have (using J m (z) = i m m J m (z)) R n m (ρ) J m (πrρ) ρ dρ = ( ) n m J n+ (πr) πr. (6) This is the basic NZresult as given in [], Eq. (3) and [3], Eq. (.). By Fourier inversion in Eq.(5), using Hankel transforms of order m, it is seen that R n m (ρ) = ( ) n m J n+ (u) J m (ρu) du, ρ <. (7) This result, often attributed to Noll [5], is shown in [] to follow from the discontinuous WeberSchafheitlin integral, see [8], on p. 56. The integral on the righthand side of Eq. (7) converges uniformly in any closed set of ρ not containing, see Appendix A, and its value for ρ > is. Thus the equality in (7) holds pointwise and not just in an L sense. Also see [9], Appendix, Sec. A.. for a discussion of the result in Eq. (7). The results in Eqs. (6) (7) are basic to the proof of a number of results of the circle polynomials and their radial parts. In [5], the result in Eq. (7) was used to derive expressions for the derivative of Rn m in terms of Rpolynomials of azimuthal orders m ± by employing recurrence relations for Bessel functions and their derivatives. In [9] [], the two results in Eqs. (6) (7) were combined with recursion properties of the Bessel functions to produce the scaling formula R m n (ερ) = n (Rn n (ε) Rn+ n (ε)) R m n (ρ) = = ε n n + n + (Rn+ n + (ε) Rn+ n (ε)) Rm n (ρ). (8) Here m =,,..., n = m, m +,..., and the summation is over n = m, m +,..., n in which we recall the convention that R n + n = R n + n for the last term in either series. Although Eq. (8) is normally used for ε, ρ [, ], it should be emphasized that they are valid for all complex values of ε and ρ by analyticity. The result in Eq. (8) 4
5 is of interest to both the lithographic community and the ophthalmological community, see [7], [8], [9], [], []. In [], the formula R m n (p, ϕ) = n + ( p ) / U n (p) e imϕ, p, ϕ < π, (9) for the Radon transform of Z m n (ρ, ϑ) is used to show that R m n (ρ) = N N k= ( U n ρ cos πk N ) cos πmk N, ρ. () Here m =,,... and N is any integer > n + m, and U n is the Chebyshev polynomial of degree n and second kind. This formula is interesting since it gives the R m n (ρ) for m + n < N in the form of a discrete cosine transform, also see [3]. The result in Eq. (9) was discovered by Cormack in [4], but a proof can also be based on the result in Eqs. (6) (7) and the connection between Bessel functions and Chebyshev polynomials through the Fourier transform, see [8], on p In the present paper a number of new applications of the results in Eqs. (6) (7) are presented. A common feature of the problems we consider is that they all give rise to infinite integrals involving the product of three Bessel functions. In Section 3 we consider the problem of finding the Zernike expansion of scaledandshifted circle polynomials. That is, given a, b with a + b, we give explicit expressions, involving Jacobi polynomials, for the coefficients Knn mm (a, b) in the expansion Z m n (a + bρ e iϑ ) = n,m K mm nn (a, b) Zm n (ρ e iϑ ), ρ, ϑ < π, () see Fig. for the relation between the radial and angular variables of the full, centralized pupil and the reduced, shifted pupil. This result generalizes the scaling formulas in Eq. (8) which is the case with a = in Eq. (). Furthermore, it gives the analytic solution of the transformation problem for the aberration coefficients of an eye pupil when the pupil is scaled and displaced. This problem has a long history in the ophthalmological community, see [7] and [] for recent work and survey material, but no closedform solution seems to have been found thus far. Moreover, by expressing the scaledandshifted polynomials as linear combinations of the orthogonal terms Zn m one has a handle, via the transformation matrix elements (a, b), to tackle the important problem of assessing the condition of a finite set of K mm nn circle polynomials when they are restricted to subdisks of the unit disk. In Section 4 we consider the problem of computing the Zernike expansion coefficients of the correlation of two circle polynomials. Having these expansion coefficients available is of great interest when calculating transfer functions in optical imaging, see [9], [3] for early 5
6 P= ρe iθ =a+ ρbe i θ ` ` ρ P bρ ` θ a θ ` a+b Figure : Centralized, full pupil ρ e iϑ, ρ, ϑ < π, and shifted and reduced pupil a + b ρ e iϑ, ρ, ϑ < π, where a, b, a + b. work and motivation concerning this problem in computational optics. The complex pupil function of the lens is expanded as a series involving the circle polynomials. The resulting series with the appropriate Zernike coefficients represent the amplitude and phase of the complex pupil function, including wavelength shift and defocusing. Through the analytic expression for the expansion coefficients of the correlation of two circle polynomials, one has direct access to the optical transfer function over the full bandwidth of the imaging system. This mathematical device is very attractive when calculating, for instance, the pattern transfer in highresolution optical lithography. Hence, when m, n, m, n are integers such that n m and n m are even and nonnegative, and when we denote for two functions Z, Z L (R ) the correlation (Z corr Z )(ν, µ) = Z(ν + ν, µ + µ ) Z (ν, µ ) dν dµ, () we are interested in finding the numbers Γ mm m nn n such that (n + )(n + ) (Z m π n corr Zn m )(ρ, ϑ) = n + 4π n,m Note that Zn m corr Zn m Zn m ( Γ mm m nn n Zm n ( ρ, ϑ). (3) is supported by the disk around of radius and this requires ρ, ϑ) at the righthand side of Eq. (3). This problem has been considered in the optical context by Kintner and Sillitto in [5], [6] in the interest of computing the optical transfer function (OTF) from the Zernike expansion of the pupil function. In [5], [6] a number of results is obtained for the quantities Γ, but no closedform solution is given as 6
7 we obtain here. Moreover, the values of the lefthand side of Eq. (3) are expressed here as integrals involving the product of three Bessel functions. In optical simulations and/or experiments, a judicious choice of sampling points is important. It now turns out that replacing the radial variable ρ, ρ, by cos x, x π/, has the effect that the variation of the radial polynomials R n m is spread out uniformly over the xrange (see Fig. 5 in Section 7). This suggests an adequate pupil sampling strategy. In [3], Subsec..4, a matching procedure, using a separable set of sampling points (ρ k, ϑ l ) on the disk, for estimating the Zernike expansion coefficients of a pupil from its values at the sampling points is proposed. It turns out that choosing the radial sampling points as cos x, with uniformly spaced x between and π/, produces nearoptimal results, in the sense that the resulting method competes with Gaussian quadrature for all relevant azimuthal orders m simultaneously. The fact that the variation of R n m is spread uniformly over the xrange suggests, furthermore, to apply this substitution when integrals, involving the product of a radial polynomial and a function obtained from the pupil function after azimuthal integration, have to be computed. All this motivates consideration in Section 5 of the third problem: Finding the Fourier coefficients of the radial polynomials R n m in their cosinerepresentation. Here we aim at finding the Fourier coefficients a m nk in the representation n/ R n m (cos x) = j= a m n,n j cos(n j) x. (4) The a m nk will be found explicitly, and from this result it is seen that they are all nonnegative. It then follows from Eq. (3) that n/ R n m (ρ) a m n,n j = R n m () =, ρ. (5) j= That R n m (ρ), ρ, was proved by Szegö, see [3], 7.. on p. 64 and the references given there, and the nonnegativity of the a m nk was established earlier by Koornwinder [33], Corollary 6. on p. 3, as was communicated to the author by Erik Koelink [34]. The explicit form of the a m nk does not seem to have been noted before. With the explicit result for the a m nk available, the computation of the above mentioned integrals can be done directly and very explicitly by choosing the appropriate sampling points, of the form cos x with equidistant x, and using, for instance, DCTtechniques. The fact that the a m nk, being all nonnegative with sum over k equal to, are all small, renders this approach intrinsic stability. A word about the notation. We identify complex numbers z with their polar representation ρ e iϑ or their Cartesian representation ν + iµ, whatever is most convenient in a particular setting. Thus we write things like Z m n (a + ρ b e iϑ ), Z m n (ν, µ) (6) 7
8 to denote Zn m (ρ, ϑ) = R n m (ρ) e imϑ in which a + ρ b e iϑ = ν + iµ = z = ρ e iϑ (7) with a, b, a + b ; ρ, ϑ < π; ν, µ R, ν + µ ; ρ, ϑ < π. 3 Scaledandshifted Zernike circle polynomials We shall prove the following result. Theorem 3.. Let a, b with a + b, and let n, m be integers with n m even and nonnegative. Then Z m n (a + b ρ e iϑ ) = n,m K mm nn (a, b) Zm n (ρ e iϑ ), ρ, ϑ π, (8) where for n = m, m +,..., n = m, m +,... Here K mm nn mm (a, b) = Tnn (a, b) T mm n,n +(a, b). (9) T mm nn = = ( )p p J m m (au) J n (bu) J n+ (u) du = (q+p )! (p p )! (q q )! (p+q )! am m b n P (m m,n ) p p ( A ) P (m m,n ) p p (B ) when n n m m, (p+q )! (q q )! (p p )! (q+p )! am m b n P (m m,n ) q q ( A ) P (m m,n ) q q (B ) when n n m m, otherwise. () In Eq. () we have written p = n m q = n + m, p = n m, q = n + m. () Furthermore, P (γ,δ) k (x) is the general Jacobi polynomial as in [8], Ch. of degree k =,,... corresponding to the weight function ( x) γ (+x) δ on the interval [, ]. Finally, A = [ ( (a + b) )( (a b) )] / (a + b)(a b), () 8
9 [ / B = ( (a + b) )( (a b) )] (a + b)(a b). (3) Alternatively, we have A = sin α, B = sin β (4) with α, β such that α + β π/ and a = sin α cos β, b = cos α sin β. (5) That is, A and B can be obtained from the geometrical picture in Fig. where γ [ π, π] is such that sin γ = a + b. γ B A α b a β Figure : Geometric definition of angles α, β, γ and side lengths A, B from a, b, a + b in accordance with Eqs. (4) (5) and the rule of sines sin α = sin β = sin γ =. A B a+b Proof. By completeness and orthogonality of the circle polynomials, see Eq. (4), we have that K mm nn (a, b) = n + π π Z m n (a + b ρ e iϑ )(Z m n (ρ e iϑ )) ρ dρ dϑ. (6) We write ρ e iϑ = a + ρ b e iϑ in which ρ and ϑ are depending on ρ, ϑ with ρ, ϑ π. Then we get K mm nn (a, b) = n + π π R m n (ρ(ρ, ϑ )) e imϑ(ρ,ϑ ) R m n (ρ ) e im ϑ ρ dρ dϑ. (7) We now use Eq. (7) to rewrite R n m (ρ(ρ, ϑ )) in integral form and change the order of integration; this is allowed on account of Appendix A where we show that the integral on the righthand side of Eq. (7) converges boundedly for all ρ and uniformly, to 9
10 ( ) n m R n m (ρ), on any set [, ε] with ε >. Therefore, Knn mm (a, b) = n + ( ) n m π π J n+ (u) J m (uρ(ρ, ϑ )) e imϑ(ρ,ϑ ) e im ϑ dϑ R m m (ρ ) ρ dρ du. (8) χ U W γ V Figure 3: Geometric relation between angles χ, γ and side lengths U, V, W for Eq. (9). We use the addition theorem of Graf, see [8], on p. 363, C l (W ) e ilχ = C l+k (U) J k (V ) e ikγ (9) k= for integer l and C = J, where W, U, V, χ and γ are related as in the picture in Fig. 3. With the variables ρ, ϑ, ρ, ϑ as in the integral in Eq. (8) and displayed in Fig., we use Eq. (9) with W = u ρ(ρ, ϑ ), U = ua, V = u ρ b, χ = ϑ(ρ, ϑ ), γ = π ϑ, (3) where we note by the comments in [8] after and 9..8 on p. 363 that Eq. (9) can be used without any further restriction on U, V, W. Then we get J m (uρ(ρ, ϑ )) e imϑ(ρ,ϑ ) = J m+k (ua) J k (u ρ b) e ik(π ϑ ). (3) k=
11 Hence, in the case that m, π J m (uρ(ρ, ϑ )) e imϑ(ρ,ϑ ) e im ϑ dϑ = = π( ) m J m m (ua) J m (u ρ b) = π J m m (ua) J m (u ρ b), (3) and so Knn mm (a, b) = (n + )( ) n m J n+ (u) J m m (ua) J m (u ρ b) R m n (ρ ) ρ dρ du. (33) Using Eq. (3) for m < and noting that J m (z) = ( ) m J m (z) while for m <, it is seen that Eq. (33) holds for all integer m. ( ) n m ( ) m = ( ) n m (34) Next we use Eq. (6) with m, n, ub instead of m, n, πr to rewrite the integral in [ ] in Eq. (33), and we obtain Knn mm (a, b) = (n + )( ) n+n m m Using [8], first item in 9..7 on p. 36, J n+ (u) J m m (ua) J n +(ub) ub du. (35) J n +(z) z = (n + ) (J n (z) + J n +(z)), (36) it then follows that K mm nn (a, b) = ( ) n+n m m J n+ (u) J m m (ua) J n (ub) du + + J n+ (u) J m m (ua) J n +(ub) du, (37) and this establishes the equality in Eq. (9) with T s given in integral form by the first identity in Eq. (). The second identity in Eq. () follows from an application of a result of Bailey, the administrative details of which are deferred to Section 6.
12 We now list some special cases of Theorem 3.. a =. This gives the result of the scaling theory as developed in [9], [], also see Eq. (8). To see this, note that in Eqs. (4) (5) we have A = a = α =, B = b = sin β, (38) and in Eq. () only the cases with m = m give nonzero results. Furthermore, P (,n ) k () =, b n P (,n ) k (b ) = R n n +k(b), (39) and, see Eq. (), p + q = p + q, p p = q q (4) since m = m. Plugging all this in into Eq. () yields Eq. (8). b =. Only n = gives nonzero results in Eq. () and then also m = (since m n ). Now we have in Eqs. (4) (5) A = a = sin α, B = b = β =, (4) and, see Eq. (), q + p = q q, p p = p + q (4) since p = q = m = n =. Thus, when m, the first case in Eq. () yields a m P (m,) p where we have used that ( a ) P p (m,) ( ) = a m P p (,m) (a ) = Rm+p(a) m = Rn m (a), (43) P (γ,δ) k ( x) = ( ) k P (δ,γ) k (x), P p (,m) () =. (44) Therefore, we have in Eq. (8) with b = the trivial representation Z m n (a) = R m n (a) Z (ρ ), ρ, ϑ π, (45) with a similar result in the case that m. a + b =. We have in Eq. (4) (5) in this limit case α + β = π/, A = sin α, B = cos α, A = B = cos α. (46) As a consequence, all T s are nonnegative. A further interesting observation is that K mm nn (a, b) m n n m m. (47)
13 Hence, a Zernike circle polynomials Zn m can be identified from the set of integer pairs (m, n ) corresponding to nonzero coefficients when Zn m is scaled and shifted. In Subsection 7., a detailed computation based on Theorem 3. of the Zernike expansion of the scaledandshifted circle polynomial Z 4 is presented. The validity of the last identity for T in Theorem 3. will now be shown to extend to all complex values of a and b. First assume that a, b, a + b. We have from Eqs. () () that Zn m (ρ e iϑ ) = (ρ e ±iϑ ) m P (, m ) n m (ρ ), (48) where ± = sgn(m). Since we have that ρ e ±iϑ = a + b ρ e ±iϑ, (49) ρ = a + b (ρ ) + ab ρ cos ϑ, (5) Z m n (a + b ρ e iϑ ) = n+ m k= n m p k (ρ ; a, b) e ±ikϑ, (5) where p k (ρ ; a, b) depends polynomially on ρ, a and b. On the other hand, from Eqs. () (3), we have that for any polynomial p p( A ) p(b ) = p( x + y)p( x y) (5) is an even function of y = (( (a+b) )( (a b) )) / for any value of x = (a+b)(a b). Consequently, the righthand side of Eq. (5) contains only even powers of y. We conclude that any of the T s considered in Eq. () depends polynomially on a and b. Hence, the relation in Eq. (8) extends to all a, b C by analyticity in which Z m n (a + bρ e iϑ ) at the righthand side of Eq.(8) is to be replaced by the analytic extension W m n (a, b, ρ, ϑ ) of the righthand side of Eq.(5) to all complex values of a, b and ρ with ϑ < π. Note that for general complex a, b, ρ and ϑ, ϑ < π, does not need to hold even when a + bρ e iϑ. W m n (a, b, ρ, ϑ ) = Z m n (a + bρ e iϑ ) (53) ρ, ϑ < π (a =, b = exp(iα) in Eq.(53)), we have while For instance, when α is real and W m n (, e iα, ρ, ϑ ) = (e iα ρ e ±iϑ ) m R m n (e iα ρ ) (54) Z m n (e iα ρ e iϑ ) = (ρ e ±i(ϑ +α) ) m R m n (ρ ). (55) We do have equality in Eq.(53) when a and b are real and ρ, ϑ < π, and the definition of Zn m in Eqs.()() is used with general ρ. Accordingly, Theorem 3. is valid for all real a and b, with Zn m defined as in Eqs.()() with general ρ. 3
14 An important consequence of this extension is that now also the transformation matrices (Knn mm ( a/b, /b)) corresponding to the inverse transformation z a/b + z/b can be considered. Accordingly, when the degrees n, n are restricted to a finite set {,..., N}, the matrices corresponding to z a+bz and z a/b+z/b are each other s inverse. Having expanded the shiftandscaled circle polynomials in terms of the orthogonal functions Z m n, we have now the opportunity to deal with the problem of assessing the condition of the set of circle polynomials of maximal degree N as a linear system when they are restricted to an arbitrary disk in the plane. Indeed, the condition number is given as the squareroot of the ratio of the largest and smallest eigenvalue of the Grammian matrix, and this Grammian matrix and its inverse are expressible in terms of the appropriate transformation matrices (Knn mm (a, b)). Such an effort is already worthwhile for the case of restriction of circle polynomials to a disk ρ ε with ε < (pure scaling), and the author has found for this case useful and simple estimates for the magnitude of these condition numbers. This case is much simpler than the general case since the transformation matrices decouple per m, while the issue of analytic extension can be considered on the level of Eq.(8) that extends to all complex ε and ρ. We finally note that the approach, via the basic NZresult and Eq.(3), to prove Theorem 3. can be used to derive an addition theorem in which the general scaledandshifted circle polynomial Z m n (a + bρ e iθ ) is expressed as a finite Fourier series in θ where the Fourier coefficients are (again) in the form of the integral of the product of three Bessel function. This addition theorem generalizes the familiar one for the Legendre polynomials (m = ). 4 Zernike expansion of the optical transfer function In this section we assume that we have expanded the generalized complex pupil function P (ρ, ϑ) (vanishing outside ρ ) as P (ρ, ϑ) = n,m n + π γ m n Z m n (ρ e iϑ ), ρ, ϑ < π, (56) with coefficients γ m n that can be obtained by using orthogonality of the Z m n. Writing ν + iµ = ρ e iϑ with ν, µ R and identifying P (ν, µ) P (ρ, ϑ), compare end of Section, it is required to find the Zernike expansion of the OTF (optical transfer function) (P corr P )(ν, µ) = P (ν + ν, µ + µ ) P (ν, µ ) dν dµ (57) that vanishes outside the set ν + µ 4, see Fig. 4. Thus, considering the expansion in Eq. (56), it is required to compute (the Zernike expansion of) Zn m corr Zn m for integer n, m, n, m with n m and n m even and nonnegative. We maintain the p, qnotation of Eq. (). 4
15 Theorem 4.. We have, with ν + iµ = ρ e iϑ and ρ, (Zn m corr Zn m )(ν, µ) = = π( ) p p e i(m m )ϑ J n+ (u) J n +(u) J m m (ρu) u du, (58) and (n + )(n + ) (Z m π n corr Zn m )(ρ eiϑ ) = n + Γ mm m nn 4π n Zm n ( ρ eiϑ ), (59) n,m where Γ is nonvanishing for m = m m only, and in that case Γ mm m nn n = 8(n + )(n + )( ) n n n J n+ (u) J n +(u) J n +(u) du u. (6) ρ O θ Figure 4: Schematic representation of the autocorrelation function P corr P of a nonuniform pupil function P as an integral over the common region of two disks of unit radius with centers at and ρ e iϑ, respectively. 5
16 Proof. We have by Parseval s theorem and Eq. (5) that (F denotes the Fourier transform) (Zn m corr Zn m )(ν, µ) = = F [Zn m (ν +, µ + )](x, y) (F Zn m ) (x, y) dx dy = = e πiνx πiµy (F Z m n )(x, y) (F Z m n ) (x, y) dx dy = = i n n π J n+ (πr) J n +(πr) r e πiρr cos(ϑ ϕ) e i(m m )ϕ r dr dϕ, (6) where we have written x + iy = r e iϕ and where we use that νx + µy = ρ r cos(ϑ ϕ). Now π e πiρr cos(ϑ ϕ) e ikϕ dϕ = π( i) k e ikϑ J k (πρr), (6) and inserting this into Eq. (6) with k = m m, noting that i n n ( i) m m and substituting u = πr, we obtain Eq. (58). = ( ) p p Next to show Eq. (6), we note from orthogonality of the Zn m ( ρ eiϑ ), see Eq. (4), that Γ mm m nn n = (n + )(n + ) (Z m π n corr Zn m )(ν, µ)(zm n ( ν, µ)) dν dµ. (63) Again using Parseval s theorem, together with we obtain F(Z corr Z ) = FZ (FZ ), (F Z )( ν, µ))(x, y) = 4(FZ )(x, y), (64) Γ mm m nn n = = 4(n + )(n + ) π (F Z m n )(x, y)(f Z m n ) (x, y)(f Z m n ) (x, y) dx dy. (65) Then inserting Eq. (5) and using polar coordinates x + iy = r e iϕ, we obtain Γ mm m nn n = (n + )(n + ) n n n i π J n+ (πr) J n +(πr) J n +(4πr) dr r π e i(m m m )ϕ dϕ. (66) 6
17 The proof is completed by the substitution u = πr. Also note that n n and n have the same parity when m = m m. Notes.. We have by Eq. (36) that (Zn m corr Zn m )(ν, µ) = π and, for the case that m = m m, n + ( )p p e i(m m )ϑ [Q m m n+,n + Q m m n+,n + ] (67) Γ mm m nn n = ( ) n n n [Q n + nn + Q n + n+,n + Q n + n,n + + Qn + n+,n + ], (68) where Q k ij(a, b, c) = J i (au) J j (bu) J k (cu) du, (69) and where the Q s in Eq. (67) are evaluated at (a =, b =, c = ρ) and the Q s in Eq. (68) are evaluated at (a =, b =, c = ).. We shall use in Section 6 a result of Bailey [35] to show the following. We have Q n + nn = when n < n + n, and when n n + n, we have Q n + nn (,, ) = = ( (n + n + n ))! ( (n n n ))! ( (n + n n ))! ( (n + n n))! ( + )n+n P (n,n ) n n n () P (n,n) n n n (), (7) also see Note at the end of Section From a result of Bailey [35] it also follows that Q m m n+,n and Q m m n+,n + vanish when ρ, but this is already clear from the fact that the Z s are supported by the unit disk. The result in Eq. (67) takes a more complicated form when < ρ <, see Section 6 for more details. 4. In Subsection 7., a detailed computation, based on Theorem 4., for the OTF corresponding to P = Z is presented. 5 Cosine representation of the radial polynomials We shall prove the following result. Theorem 5.. Let m, n be integers with n m even and nonnegative. 7
18 Then R m n (cos x) = n/ where for integer k with n k even and nonnegative j= Here ε =, ε = ε =... = (Neumann s symbol), and where p = n l, q = n + l Proof. We have from Eq. (7) a n j cos(n j) x, (7) a k = ε k p! q! s! t! ( )l (P (γ,δ) p ()). (7), s = n r, t = n + r, γ = l r, δ = l + r, (73) l = max(m, k), r = min(m, k). (74) Rn m (cos x) = ( ) n m J n+ (u) J m (u cos x) du. (75) Next we note that Rn m (cos x) = (cos x) m P (,m) (cos x) (76) has nonvanishing Fourier components b k e ikx only for integer k of the same parity as m. For such k we shall show that π π n m J m (u cos x) e ikx dx = J m k ( u) J m+k ( u). (77) Indeed, abbreviating the coefficient of e imy in by C m, we have by the generating function e iz sin y = m= J m (z) e imy (78) 8
19 that π π = π C m J m (u cos x) e ikx dx = π C m = π C m = C m π π e iu sin(x+y) e iu sin(x y) e ikx dx = m,m = m, m =, m m = k π J m ( u) J m ( u) e iu cos x sin y e ikx dx = J m ( u) J m ( u) ei(m +m )y = e im (x+y) im (x y)+ikx dx = = J m k ( u) J m+k ( u). (79) Then we have at once from Eq.(75) that π π Rn m (cos x) e ikx dx = ( ) n m When k and m k, we have then π π R m n (cos x) e ikx dx = ( ) n m Q n+ m k J n+ (u) J m k ( u) J m+k ( u) du. (8), m+k (,, ), (8) see Eqs. (69) and (7). For this case, the result follows from Eq. (7), the fact that P p (δ,γ) () = ( ) p P p (γ,δ) () and the fact that a k = a k. When k and m k, the result follows in a similar manner, by using that J m k ( m k u) = ( ) J k m ( u) and Eq. (7) together with some easy administration with signs. Notes.. It is straightforward to generalize the result of Theorem 5. to the representation of Rn m (v cos x) with v. Now Q n+ m k (v, v, ) appears in Eq. (8) and a k in Eq. (7) becomes, m+k p! q! a k (v) = ε k s! t! ( v)l P p (γ,δ) (x)p p (γ,δ) ( x), (8) where x = ( v ) /. Also, see Section 6 and Eq. ().. In using Theorem 5. it is convenient to note that P p (γ,δ) () = p ( p + γ ) ( p + δ p j p j j= 9 ) ( ) p j, (83)
20 see [8],.3. on p Alternatively, P p (γ,δ) the hypergeometric function F. can be evaluated per Eq.(88) in terms of 3. In Subsection 7.3, a table is presented in which the radial polynomials R m n, integer n, m with n, m 8 and n m and even, are given in polynomial and in the cosine representation of Theorem Infinite integrals involving the product of three Bessel functions We consider in this section the integrals A λµν (a, b, c) = J λ (ax) J µ (bx) J ν (cx) dx, (84) where a, b, c > and λ, µ, ν are nonnegative integers. There is quite some literature on these integrals and more general instances of them, see [36], Sec on pp. 4 45, [35], [37], Sec. 9.3 on pp , [38], Sec on pp These general results can become quite unmanageable; it is the point of this section that, in the special cases that we consider, often concise and manageable results appear, often in terms of the radial polynomials themselves. For the case that a + b < c (and a >, b >, c > ), the integral in Eq. (84) is given by Bailey, [35], Eq. (8.) (with a minor correction in which the c µ+ν+ in the denominator at the righthand side should be replaced by c), as J λ (cu sin α cos β) J µ (cu cos α sin β) J ν (cu) du = = Γ( ( + λ + µ + ν)) sinλ α cos λ β cos µ α sin µ β c Γ(λ + ) Γ(µ + ) Γ( ( λ µ + ν)) F [ ( + λ + µ ν), ( + λ + µ + ν) ; λ + ; sin α] F [ ( + λ + µ ν), ( + λ + µ + ν) ; µ + ; sin β], (85) where α, β are such that α + β < π/. In Eq. () the choice c =, a = sin α cos β, b = cos α sin β ; λ = m m, µ = n, ν = n + (86)
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