# Near field of a diffraction grating?

 P: 7 I know one can find the "near field" length of an illuminated diffraction grating by calculating the Talbot Length, but I also know this is not the complete story. What happens when you have instead a light source (like a laser beam) that coherently illuminates a certain number of grating bars/slits , say 8 or 9 open slits. How would one calculate the length of the near field then? it seems that the near field length would increase with the number of slits you coherently illuminate. Can someone provide me with a better equation or method for finding the near field length? Thanks in advance
 Sci Advisor P: 1,477 The standard approach for a scalar field is to; 1. Calculate the angular spectrum at the aperture (This is the Fourier transform of the aperture function). 2. Propagate the field forward by adding an angle-dependent phase. 3. Measure the "length" you need. Claude.

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