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The contrast transfer function (CTF) mathematically describes how aberrations in a transmission electron microscope (TEM) modify the image of a sample.Spence, John C. H. (1988 2nd ed) ''Experimental high-resolution electron microscopy'' (Oxford U. Press, NY) .Ludwig Reimer (1997 4th ed) ''Transmission electron microscopy: Physics of image formation and microanalysis'' (Springer, Berlin
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Earl J. Kirkland (1998) ''Advanced computing in electron microscopy'' (Plenum Press, NY). This contrast transfer function (CTF) sets the resolution of
high-resolution transmission electron microscopy High-resolution transmission electron microscopy is an imaging mode of specialized transmission electron microscopes that allows for direct imaging of the atomic structure of samples. It is a powerful tool to study properties of materials on the a ...
(HRTEM), also known as phase contrast TEM. By considering the recorded image as a CTF-degraded true object, describing the CTF allows the true object to be
reverse-engineered Reverse engineering (also known as backwards engineering or back engineering) is a process or method through which one attempts to understand through deductive reasoning how a previously made device, process, system, or piece of software accompli ...
. This is typically denoted CTF-correction, and is vital to obtain high resolution structures in three-dimensional electron microscopy, especially electron cryo-microscopy. Its equivalent in light-based optics is the
optical transfer function The optical transfer function (OTF) of an optical system such as a camera, microscope, human eye, or projector specifies how different spatial frequencies are captured or transmitted. It is used by optical engineers to describe how the optics pr ...
.


Phase contrast in HRTEM

The contrast in HRTEM comes from interference in the image plane between the phases of scattered electron waves with the phase of the transmitted electron wave. When an electron wave passes through a sample in the TEM, complex interactions occur. Above the sample, the electron wave can be approximated as a plane wave. As the electron wave, or wavefunction, passes through the sample, both the phase and the
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of ampl ...
of the electron beam is altered. The resultant scattered and transmitted electron beam is then focused by an objective lens, and imaged by a detector in the image plane. Detectors are only able to directly measure the amplitude, not the phase. However, with the correct microscope parameters, the phase interference can be indirectly measured via the intensity in the image plane. Electrons interact very strongly with
crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...
solids. As a result, the phase changes due to very small features, down to the atomic scale, can be recorded via HRTEM.


Contrast transfer theory

Contrast transfer theory provides a quantitative method to translate the exit wavefunction to a final image. Part of the analysis is based on Fourier transforms of the electron beam wavefunction. When an electron wavefunction passes through a lens, the wavefunction goes through a Fourier transform. This is a concept from Fourier optics. Contrast transfer theory consists of four main operations: # Take the Fourier transform of the exit wave to obtain the wave amplitude in back focal plane of objective lens # Modify the wavefunction in reciprocal space by a phase factor, also known as the ''Phase Contrast Transfer Function'', to account for aberrations # Inverse Fourier transform the modified wavefunction to obtain the wavefunction in the image plane # Find the square modulus of the wavefunction in the image plane to find the image intensity (this is the signal that is recorded on a detector, and creates an image)


Mathematical form

If we incorporate some assumptions about our sample, then an analytical expression can be found for both phase contrast and the phase contrast transfer function. As discussed earlier, when the electron wave passes through a sample, the electron beam interacts with the sample via scattering, and experiences a phase shift. This is represented by the electron wavefunction exiting from the bottom of the sample. This expression assumes that the scattering causes a phase shift (and no amplitude shift). This is called the ''Phase Object Approximation.''


The exit wavefunction

Following Wade's notation, the exit wavefunction expression is represented by: :\tau(r,z) = \tau_o \exp i\pi\lambda\int dz'U(r,z')/math> :\tau_o = \tau(r,0) :U(r,z) = 2mV(r,z)/h^2 Where the exit wavefunction τ is a function of both r in the plane of the sample, and z perpendicular to the plane of the sample. \tau_o represents the wavefunction incident on the top of the sample. \lambda is the wavelength of the electron beam, which is set by the accelerating voltage. U is the effective potential of the sample, which depends on the atomic potentials within the crystal, represented by V. Within the exit wavefunction, the phase shift is represented by: :\phi(r) = \pi\lambda \int dz' U(r,z') This expression can be further simplified taken into account some more assumptions about the sample. If the sample is considered very thin, and a weak scatterer, so that the phase shift is << 1, then the wave function can be approximated by a linear Taylor polynomial expansion. This approximation is called the ''Weak Phase Object Approximation.'' The exit wavefunction can then be expressed as: :\tau(r,z) = \tau_o + i\phi(r)/math>


The phase contrast transfer function

Passing through the objective lens incurs a Fourier transform and phase shift. As such, the wavefunction on the back focal plane of the objective lens can be represented by:
:I(\theta) = \delta(\theta) + \Phi K(\theta) \theta = the scattering angle between the transmitted electron wave and the scattered electron wave \delta = a
delta function In mathematics, the Dirac delta distribution ( distribution), also known as the unit impulse, is a generalized function or distribution over the real numbers, whose value is zero everywhere except at zero, and whose integral over the entire ...
representing the non-scattered, transmitted, electron wave \Phi = the Fourier transform of the wavefunction's phase K(\theta) = the phase shift incurred by the microscope's aberrations, also known as the ''Contrast Transfer Function:'' :K(\theta) = \sin 2\pi/\lambda)W(\theta)/math>
W(\theta) = -z\theta^2/2 + C_s\theta^4/4 \lambda = the relativistic wavelength of the electron wave, C_s = The
spherical aberration In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. Lenses and curved mirrors are prime examples, because this shape is easier to manufacture. Light rays that strike ...
of the objective lens
The contrast transfer function can also be given in terms of spatial frequencies, or reciprocal space. With the relationship \theta =\lambda k, the phase contrast transfer function becomes: :K(k) = \sin 2\pi) W(k)/math>
W(k) = -z\lambda k^2/2 + C_s\lambda^3 k^4/4 z = the defocus of the objective lens (using the convention that underfocus is positive and overfocus is negative), \lambda = the relativistic wavelength of the electron wave, C_s = The
spherical aberration In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. Lenses and curved mirrors are prime examples, because this shape is easier to manufacture. Light rays that strike ...
of the objective lens, k = the spatial frequency (units of m−1)


Spherical aberration

Spherical aberration In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. Lenses and curved mirrors are prime examples, because this shape is easier to manufacture. Light rays that strike ...
is a blurring effect arising when a lens is not able to converge incoming rays at higher angles of incidence to the focus point, but rather focuses them to a point closer to the lens. This will have the effect of spreading an imaged point (which is ideally imaged as a single point in the
gaussian Carl Friedrich Gauss (1777–1855) is the eponym of all of the topics listed below. There are over 100 topics all named after this German mathematician and scientist, all in the fields of mathematics, physics, and astronomy. The English eponym ...
image plane) out over a finite size disc in the image plane. Giving the measure of aberration in a plane normal to the optical axis is called a transversal aberration. The size (radius) of the aberration disc in this plane can be shown to be proportional to the cube of the incident angle (θ) under the small-angle approximation, and that the explicit form in this case is : r_s = C_s\cdot\theta^3\cdot M where C_s is the spherical aberration and M is the magnification, both effectively being constants of the lens settings. One can then go on to note that the difference in refracted angle between an ideal ray and one which suffers from spherical aberration, is : \alpha_s = \arctan\left(\frac\right) -\arctan\left(\frac\right) where b is the distance from the lens to the gaussian image plane and R is the radial distance from the optical axis to the point on the lens which the ray passed through. Simplifying this further (without applying any approximations) shows that : \alpha_s = \arctan\left(\frac\right) Two approximations can now be applied to proceed further in a straightforward manner. They rely on the assumption that both r_s and R are much smaller than b, which is equivalent to stating that we are considering relatively small angles of incidence and consequently also very small spherical aberrations. Under such an assumption, the two leading terms in the denominator are insignificant, and can be approximated as not contributing. By way of these assumptions we have also implicitly stated that the fraction itself can be considered small, and this results in the elimination of the \arctan() function by way of the small-angle approximation; : \alpha_s \approx \arctan\left(\frac\right)\approx\frac=\frac=\frac If the image is considered to be approximately in focus, and the angle of incidence \theta is again considered small, then : \frac\approx\tan\left(\theta\right)\approx\theta ~~ \text ~~ M\approx \frac meaning that an approximate expression for the difference in refracted angle between an ideal ray and one which suffers from spherical aberration, is given by : \alpha_s \approx \frac


Defocus

As opposed to the spherical aberration, we will proceed by estimating the deviation of a defocused ray from the ideal by stating the longitudinal aberration; a measure of how much a ray deviates from the focal point along the optical axis. Denoting this distance \Delta b, it is possible to show that the difference \alpha_f in refracted angle between rays originating from a focused and defocused object, can be related to the refracted angle as
: \sqrt\cdot\sin(\alpha_f)=\Delta b \cdot\sin(\theta' -\alpha_f) where R and b are defined in the same way as they were for spherical aberration. Assuming that \alpha_f<<\theta' (or equivalently that , b\cdot\sin(\alpha_f), <<, R, ), we can show that
: \sin(\alpha_f)\approx\frac = \frac Since we required \alpha_f to be small, and since \theta being small implies R<, we are given an approximation of \alpha_f as
: \alpha_f\approx\frac From the
thin-lens formula In optics, a thin lens is a lens with a thickness (distance along the optical axis between the two surfaces of the lens) that is negligible compared to the radii of curvature of the lens surfaces. Lenses whose thickness is not negligible are so ...
it can be shown that \Delta b / b^2 \approx \Delta f / f^2, yielding a final estimation of the difference in refracted angle between in-focus and off-focus rays as
: \alpha_f\approx\frac :


Examples

The contrast transfer function determines how much phase signal gets transmitted to the real space wavefunction in the image plane. As the modulus squared of the real space wavefunction gives the image signal, the contrast transfer function limits how much information can ultimately be translated into an image. The form of the contrast transfer function determines the quality of real space image formation in the TEM. This is an example contrast transfer function. There are a number of things to note: * The function exists in the spatial frequency domain, or k-space * Whenever the function is equal to zero, that means there is no transmittance, or no phase signal is incorporated into the real space image * The first time the function crosses the x-axis is called the ''point resolution'' * To maximize phase signal, it is generally better to use imaging conditions that push the point resolution to higher spatial frequencies * When the function is negative, that represents positive phase contrast, leading to a bright background, with dark atomic features * Every time the CTF crosses the x-axis, there is an inversion in contrast * Accordingly, past the point resolution of the microscope the phase information is not directly interpretable, and must be modeled via computer simulation


Scherzer defocus

The defocus value (z) can be used to counteract the spherical aberration to allow for greater phase contrast. This analysis was developed by Scherzer, and is called the Scherzer defocus. z_s = (C_s\lambda)^ The variables are the same as from the mathematical treatment section, with z_s setting the specific Scherzer defocus, C_s as the spherical aberration, and λ as the relativistic wavelength for the electron wave. The figure in the following section shows the CTF function for a CM300 Microscope at the Scherzer Defocus. Compared to the CTF Function showed above, there is a larger window, also known as a ''passband,'' of spatial frequencies with high transmittance. This allows more phase signal to pass through to the image plane.


Envelope function

The envelope function represents the effect of additional aberrations that damp the contrast transfer function, and in turn the phase. The envelope terms comprising the envelope function tend to suppress high spatial frequencies. The exact form of the envelope functions can differ from source to source. Generally, they are applied by multiplying the Contrast Transfer Function by an envelope term Et representing temporal aberrations, and an envelope term Es representing spatial aberrations. This yields a modified, or effective Contrast Transfer Function: K_(k) = E_tE_s(\sin 2\pi/\lambda)W(k)/math> Examples of temporal aberrations include chromatic aberrations, energy spread, focal spread, instabilities in the high voltage source, and instabilities in the objective lens current. An example of a spatial aberration includes the finite incident beam convergence.
As shown in the figure, the most restrictive envelope term will dominate in damping the contrast transfer function. In this particular example, the temporal envelope term is the most restrictive. Because the envelope terms damp more strongly at higher spatial frequencies, there comes a point where no more phase signal can pass through. This is called the ''Information Limit'' of the microscope, and is one measure of the resolution.
Modeling the envelope function can give insight into both TEM instrument design, and imaging parameters. By modeling the different aberrations via envelope terms, it is possible to see which aberrations are most limiting the phase signal. Various software have been developed to model both the Contrast Transfer Function and Envelope Function for particular microscopes, and particular imaging parameters.


Linear imaging theory vs. non-linear imaging theory

The previous description of the contrast transfer function depends on ''linear imaging theory''. Linear imaging theory assumes that the transmitted beam is dominant, there is only weak phase shift by the sample. In many cases, this precondition is not fulilled. In order to account for these effects, ''non-linear imaging theory'' is required. With strongly scattering samples, diffracted electrons will not only interfere with the transmitted beam, but will also interfere with each other. This will produce second order diffraction intensities. Non-linear imaging theory is required to model these additional interference effects. Contrary to a widespread assumption, the linear/nonlinear imaging theory has nothing to do with kinematical diffraction or dynamical diffraction, respectively. Linear imaging theory is still used, however, because it has some computational advantages. In Linear imaging theory, the Fourier coefficients for the image plane wavefunction are separable. This greatly reduces computational complexity, allowing for faster computer simulations of HRTEM images.Notes
prepared by Professor Laurie Marks at Northwestern University.


See also

*
Airy disk In optics, the Airy disk (or Airy disc) and Airy pattern are descriptions of the best- focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light. The Airy disk is of importance in physics, ...
, different but similar phenomena in light *
Optical transfer function The optical transfer function (OTF) of an optical system such as a camera, microscope, human eye, or projector specifies how different spatial frequencies are captured or transmitted. It is used by optical engineers to describe how the optics pr ...
* Point spread function * Transmission electron microscopy


References


External links


Contrast transfer function (CTF) correction

Talk on the CTF by Henning Stahlberg




Microscopes Protein structure