Extended Data Fig. 4: Image post-processing revealing the optical-field distribution of a single photonic-crystal unit cell.
From: Coherent interaction between free electrons and a photonic cavity
a, Raw image captured by the camera in greyscale. b, Black and white image binarized by adaptive thresholding, showing the features of the etched holes in the photonic-crystal membrane. c, The photonic-crystal holes are recognized by machine vision using the circular Hough transform. d, Calculation of the geometric distortion. The control points in the red grids are generated from the recognized hole centres. The green lines indicate the control point pairs used for image registration. e, Aligned image recovered from the geometric distortion. f, Cropped image. g, Contrast adjustment on selected image areas: recovery of the field amplitude by normalizing the electron current in the Si3N4 membrane relative to the holes.
