Contrast-to-gradient method for the evaluation of image resolution in scanning electron microscopy.

Abstract

In a previous study, we proposed the contrast-to-gradient (CG) method for evaluating image resolution. Here, the CG resolution is defined as a weighted harmonic mean of the local resolution, which is proportional to the quotient of the threshold contrast divided by the local gradient. The local gradient is calculated from the quadratic function that best fits the local pixel intensities over the region of interest (ROI) of 3 x 3 or 5 x 5 pixels in size. To refine the CG method, some modifications are carried out in the present study. Directional resolutions are employed to evaluate images, including astigmatism or strongly directional patterns as well as isotropic patterns. Here, CG resolution is redefined so as to keep the same value even for the image reversed in black-and-white contrast, because of no difference in the image information during contrast reversing. Besides, CG resolution is characterized to be independent of the brightness/contrast change unless these changes do not bring about both cut-off and saturation in the pixel intensities. Dependencies of the denoising effect and the resolution accuracy on ROI size are demonstrated as a function of image-noise.