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I am building a test measurement (optics) to look a rectangular slit opening (1mm x 15 microns). The slit opening is illuminated by a white LED and using a microscope objective to magnify it to 10X on an image sensor.

What would be the possible method to estimate if the image is in it's best focus?

Current method, using a line profile and look for peaking position.

Any other ideas ?

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  • $\begingroup$ What is the pixel size on your image sensor? 10x magnification of a 1um slit is not all that much... $\endgroup$ – Floris Aug 7 '15 at 5:46
  • $\begingroup$ Optical format 1/2.5-inch (4:3) Active imager size 5.70mm(H) x 4.28mm(V) 7.13mm diagonal Active pixels 2592H x 1944V Pixel size 2.2 x 2.2 mu $\endgroup$ – Rene Duchamp Aug 7 '15 at 14:41
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You could try using contrast. The contrast in the image is highest when the image is in best focus. You can analyze the contrast information and move the object, lens, or sensor into a position that gives the maximum contrast value.

The downside to this is that you will need to typically adjust your opto-mechanics through the full range of motion to find the peak contrast. In other words, you may need a brute force search to find the maximum contrast.

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Since you're imaging something that is basically a 2D rectangle function you would need an infinite amount of spatial frequencies reconstruct it in the Fourier domain.

You could take the FFT of the image and vary the image or object distance until you got the highest possible spatial frequencies.

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  • $\begingroup$ This is a method for post processing. Not practical in real time though. $\endgroup$ – Rene Duchamp Aug 19 '15 at 13:23
  • $\begingroup$ I find the 2D FFT of a 2592 by 1944 image to be extremely fast in MATLAB. Less than a second. $\endgroup$ – BeamLauncher Aug 19 '15 at 15:46

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