Hello, I am quite new to Gwyddion. I have some questions about the way that Gwyddion calculates the 1-D PSD (roughness along the fast scan axis). I want to understand exactly that how it works. I tried to dig in, and find the source code, but I could not find it. Here are my questions:
Suppose that my scan size is 1um*1um, and my window size is 1024 pixels by 1024 pixels. 1) My first question is about the x and y axis units of the 1-D PSD. One accepted unit is 1/m for the spatial frequency and m^3 for the y axis. However, I am more used to the rad/m for x axis and m^3/rad for the y axis. This is very important for me, because there is a factor of 2*pi difference between these two systems of units. I looked into the manual, and I believe that the x axis in the PSD graph (spatial frequency Kx) that Gwyiddion generates has the unit of rad/m and NOT 1/m. It would be great if you can confirm this for me. In this case, I think that it is worth that this new unit is written on the graph. OK, now, let's switch to the y axis (the PSD). According to the manual, the autocorrelation Gx has the unit of m^2, which is completely correct. Then, according to the mathematical expression for the 2-D PSD, the unit of this PSD should be m^4/rad. If we accept this, the unit of W1, which the PSD that I need, should be m^3 (I just used the integration definition in the manual to deduce this), as correctly shown on the PSD graph by Gwyddion. It would be great you can confirm this as well. I just want to make sure that it is not m^3/rad. 2) According to the manual, W1 can be calculated using two different approaches. One is integrating the 2-D PSD. The other is using the FFT on scan lines. Can you tell me exactly which one is implemented in Gwyddion? 3) When I inspected the generated PSD graph, I realized that the length of x vector (spatial frequencies Kx) is half of my window size, i.e., 512 and NOT 1024. Also, the minimum spatial frequency exactly corresponds to my largest spatial size (1um), however, the highest spatial frequency did not make sense. My smallest pixel size is 1um/1024. This should determine the highest spatial frequency in the graph. However, the highest spatial frequency of the PSD graph was exactly half of this, i.e., 512/1um. Can somebody explain the reason for me? 4) On the PSD options, there is an option called "Fixed res." Can you explain me that what this does? I realized that changing the number almost radically changes the calculated PSD. Thanks, ------------------------------------------------------------------------------ Everyone hates slow websites. So do we. Make your web apps faster with AppDynamics Download AppDynamics Lite for free today: http://p.sf.net/sfu/appdyn_d2d_feb _______________________________________________ Gwyddion-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/gwyddion-users
