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- I don't understand why a spatial frequency like 1/pixel_width gives line pairs per millimeter rather than lines per millimeter.
Hello,
I'm having some difficulties in getting my head around the relationship between pixel size and lines/mm or line pairs/mm.
My thinking is the following: I'm assuming we have a sensor with square pixels of side length p (given in mm) and we have a line pattern, so a pattern with white and black lines of equal width (eg a Ronchi ruling). In the ideal case, where our pixels are exactly aligned with the lines, we can capture one line per pixel. So the thinnest line that we can capture has width p. That corresponds to a frequency of 1/p lines per millimeter or 1/2p line pairs per millimeter. If the lines are offset by eg half a pixel then there's aliasing and we can't accurately resolve the pattern. So the best resolution we can reliably get is at the Nyquist frequency of 1/2p lines per millimeter or 1/4p line pairs per millimeter. I'm assuming a perfect lens, a perfect sensor and no diffraction.
To me that sounds like it would make sense. But it doesn't seem to square with some of the things that I read online.
Thanks for your help!
I'm having some difficulties in getting my head around the relationship between pixel size and lines/mm or line pairs/mm.
My thinking is the following: I'm assuming we have a sensor with square pixels of side length p (given in mm) and we have a line pattern, so a pattern with white and black lines of equal width (eg a Ronchi ruling). In the ideal case, where our pixels are exactly aligned with the lines, we can capture one line per pixel. So the thinnest line that we can capture has width p. That corresponds to a frequency of 1/p lines per millimeter or 1/2p line pairs per millimeter. If the lines are offset by eg half a pixel then there's aliasing and we can't accurately resolve the pattern. So the best resolution we can reliably get is at the Nyquist frequency of 1/2p lines per millimeter or 1/4p line pairs per millimeter. I'm assuming a perfect lens, a perfect sensor and no diffraction.
To me that sounds like it would make sense. But it doesn't seem to square with some of the things that I read online.
- For example, on page 3 of https://isl.stanford.edu/~abbas/ee392b/lect09.pdf it says: "Assuming a square pixel with width (pitch) p, the spatial Nyquist rate in each dimension is 𝑓_𝑁𝑦𝑞𝑢𝑖𝑠𝑡 = 1/2𝑝 and is typically reported in line pairs per millimeter (lp/mm)". To me that sounds like 1/2𝑝 gives line pairs per millimeter when in my logic it gives lines per millimeter.
- Another example is the MTF of an ideal sensor. According to eg https://www.opto-e.com/media/docs/Resources/metrology-white-paper.pdf the MTF of an ideal sensor is given by |sinc(w)|, which is the Fourier transform of a rectangular 'pixel' signal. On page 5 it says that the cutoff frequency is 1/p. They give an example where a 5um pixel results in a cutoff frequency of 200 line pairs per millimeter. In my logic 1 / 0.005 results 200 lines per millimeter (so 100 line pairs per mm).
Thanks for your help!