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Determination of the principal distance and the location of the perspective centre in low magnification SEM photogrammetry
Author(s) -
Boyde A.
Publication year - 1975
Publication title -
journal of microscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.569
H-Index - 111
eISSN - 1365-2818
pISSN - 0022-2720
DOI - 10.1111/j.1365-2818.1975.tb04040.x
Subject(s) - magnification , perspective (graphical) , photogrammetry , principal (computer security) , computer vision , geography , artificial intelligence , computer science , operating system
SUMMARY The principal distance, D , from the centre of perspective in the SEM optical projection to the tilt axis of the specimen stage must be accurately determined before photogrammetric evaluation of stereoscopic pairs of micrographs can proceed. A precise procedure for measuring D is described in which the specimen stage X micrometer is used to measure the width of the field scanned for a particular width of the CRT, when the specimen stage is moved along the electron beam axis by amounts measured with the stage Z micrometer. The Z micrometer is calibrated with an external dial gauge. A plot of field width against Z extrapolated to zero gives the location of the perspective centre. In SEM photogrammetry, it is usual to leave the lens currents unchanged whilst recording the stereo‐pairs. The values of D measured with a constant final lens current show that the perspective centre is located close to the final aperture in its conventional position. Previous determinations of D for Stereoscans have used a changing lens current to keep the specimen in focus at varying Z , and found a virtual centre several millimetres above the final aperture. The value of D so obtained should only be used if the micrographs were recorded with dynamic or automatic focusing systems.