Lens chromatic aberration

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,... 

One of the methods under development at AT T Bell Laboratories for submicron lithography is deep ultraviolet projection photolithography. (O Fine line definition is obtained by use of 248 nm light and a lens of large numerical aperture. Because of the large chromatic aberration of the quartz lens a spectrally line-narrowed krypton fluoride excimer laser is used as a light source. 

Spherical aberration in the objective lens is particularly important, and there is no convenient way of correcting it. It arises because the outer zones of the lens focus more strongly than the zones closer to the axis. As with chromatic aberration, this produces a disk of confusion of radius Ars, which is given by... 

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