Optical Fourier Transforms (of letters)

In Optics f2f we frequently use the example of letters to illustrate Fraunhofer diffraction (Chapter 6), convolution (Chapter 10 and Appendix B), spatial filtering (Chapter 10) and the properties of Fourier transforms in general (Chapters 6, 10 and Appendix B). Below, we share a python code, based on eqns (6.35) and (9.9) in Optics f2f, that calculates the intensity patterns associated with the optical Fourier transforms of letters.

The code works by printing a letter as an image, e.g. an E


then digitising this image as an array, calculating the Fourier transform, and finally plotting the modulus squared as a colormap.


The code (last updated 2019-02-14 [1]) evaluates eqns (6.35) and (9.9) in Optics f2f for cases where the input aperture function has the shape of a letter.

from numpy import frombuffer, uint8, mean, shape, zeros, log, flipud, meshgrid
from numpy.fft import fft2, fftshift, ifftshift
from matplotlib import figure, cm
from matplotlib.pyplot import close, show, figure, pcolormesh
fig = figure(1,facecolor='white')
ax = fig.add_axes([0.1, 0.1, 0.8, 0.8]) #add axis for letter
ax.axison = False
ax.text(-0.5,-1.0, 'Z', fontsize=460,family='sans-serif') #print letter
fig.canvas.draw() #extract data from figure
data = frombuffer(fig.canvas.tostring_rgb(), dtype=uint8)
data = data.reshape(fig.canvas.get_width_height()[::-1] + (3,))
bw-=1.0 #invert
[xpix, ypix]=shape(bw)
mul=4 # add zeros to increase resolution
F=flipud(fftshift(fft2(H))) #perform Fourier transform and centre
F2=Fr*Fr+Fi*Fi #calculate modulus squared
fig = figure(2,facecolor='white')
ax = fig.add_axes([0.1, 0.1, 0.8, 0.8])
ax.axison = False
p=ax.pcolormesh(log(Fs),cmap=cm.afmhot) # plot log(intensity) pattern
p.set_clim(12,20) #choose limits to highlight desired region

[1] Note, code works using python 2 and 3. Cut and paste text into jupyter notebook. Last tested using jupyter with python 3.7.1 numpy 1.15.4 and matplotlib 3.0.2 .

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