Lens aberrations chromatic aberrations

There are three common types of lens aberrations. An aberration is the failure of a lens to produce an exact point-to-point correspondence between an object point and an image point, or in other words, an image distortion. The types of aberrations are spherical, chromatic, and curvature of field as illustrated in Figure 22A-22C. Spherical aberration is the failure of the lens to focus light onto the same focal plane (Fig. 22A). Chromatic aberration is when the different colors focus on different focal planes (Fig. 22B), which makes the image appears blurry. Curvature of field is when the plane of sharpest focus is curved due to this image curvature the whole image cannot be in focus (Fig. 22C). 

In equations (3) and (4), a is the semi angle of the illuminating convergence cone, is the coefficient of chromatic aberration, AE is the thermal energy spread of the electrons of charge e and AI/I and AV/V are the relative fluctuations of lens current and high voltage, respectively. 

The effect this has on a HRTEM image can be understood as follows. When the electron beam is not perfectly monochromatic, the chromatic aberration, Q, of the objective lens causes the electrons of different wavelength (energy) to be focused at different levels. This results in a focus spread,, and the effect can be understood by integrating the CTF for all focus values represented by the focus spread. For low spatial frequencies, this will not result in noticeable differences, but, for high frequencies, where the CTF varies rapidly with defocus, the variations will start to cancel each other out, thereby limiting the information that is transferred in the image. 

A laser lens for electrons has spherical aberration, which has a negligible effect on estimate (6), and also chromatic aberration, which is a more serious matter. For the particular numerical example we are discussing here, the electrons would have to be monochromatic within something on the order of 0.01%. 

Thermal lens microscopy (TLM) is a type of photothermal spectroscopy. TLM depends on the coaxial focusing of the excitation and probe laser beams (see Figure 7.20). Which is achieved using the chromatic aberration of a microscopic objective lens [731]. The excitation beam can be provided by a YAG laser (532 nm) [846,1021] or an Ar ion laser (514.5 nm [846] or 488 nm [732]).The probe beam can be provided by a He-Ne laser (632.8 nm) [846,1021], After optical excitation of the analyte molecules, radiationless relaxation of the analytes occurs,... 

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