Automatic focusing of an optical system by extremum control

Deeley, E.M.; Allos, Janan E.

doi:10.1049/piee.1967.0025

Cited by 3 publications

(2 citation statements)

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“…3 The resolution, defined as the minimum distance between two points in the object which are capable of being resolved, is a minimum at best focus, and the contrast in the image is then a maximum. This situation is not true of the electron microscope, in which the large depth of focus only gives rise to small changes in the resolution, near to focus.…”

Section: Introductionmentioning

confidence: 99%

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Focusing aid for an electron microscope

Curling

Deeley

Temple

1969

Proc. Inst. Electr. Eng. UK

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An aid to focusing an electron microscope is described, employing the signal obtained by scanning the image. It is shown that use can be made of certain frequency components from the resulting signal, in order to provide an indication of the focus condition. A voltage is generated which is proportional to the mean power of these frequency components, and which, for most objects, has an extremum value at focus. The greatest sensitivity is obtained when phase-contrast effects are present, when the above voltage has a minimum value at focus. An experimental system has been built using a selenium electron-bombardment-induced-conductivity (e.b.i.c.) target scanned by a conventional raster, and, although targets of less than the highest quality were used, it has been shown that typical biological specimens, exhibiting phase contrast, can be focused with a reproducibility greater than is possible by eye. Simple theory predicts that, for such objects, all frequency components are equally useful as a measure of focus, but practical results show that only the upper half of the spectrum should be used, the lower frequencies tending to give rise to error in the direction of over-focus. The arrangement has also been used, in conjunction with a peak-holding controller, in an automatic focusing system.List of principal symbols V = output of mean squarer V o = value of V at minimum contrast / = focal length of microscope / 0 = focal length at minimum contrast N = mean-square noise added to V s(t) = signal resulting from scanning k = contrast /" = intensity in element of image I m = intensity in adjacent element I au = average, or background, intensity = phase angle between background and scattered waves 0 = angle between ray and optic axis at image M = magnification A = wavelength of electrons G ss (cu) = ideal power spectrum of s(t) 2tj = time taken to scan one resolution element t s = time interval between successive scans N o = electrons collected per second per unit area G nn (to) -power spectrum of image noise n(t) = noise added after filtering <«(0 2 ) = mean-square value of n(t)

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Section: Introductionmentioning

confidence: 99%

“…This situation can be contrasted with that in the conventional light microscope, where only the highest-frequency components are affected near to focus. 3 In practice, the image intensity varies continuously, rather than in steps as is suggested by separate resolution elements, so that the power spectrum is more realistically represented in the following form:…”

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confidence: 99%