[Gwyddion-users] Infos about PSD calculation

[Alessandro Gambardella]

Dears,
I have a question related to calculation of the Power Spectral Density as reported at page 69 of the manual. I cannot easily understand what exactely is that "h" that originates the nm^3 unit instead of the nm^4 as reported elsewhere for the PSD. Is that h the maximum height in the processed image? Or is it maybe the image resolution in nm? I cannot find any h definition in the previous paragraphs of the manual.
I only need to transform the nm^3 as Gwyddyon calculates into nm^4 for my purposes.

Thank you in advance for your reply

Alessandro Gambardella

[David Nečas (Yeti)]

Post by Alessandro Gambardella
I have a question related to calculation of the Power Spectral Density
as reported at page 69 of the manual. I cannot easily understand what
exactely is that "h" that originates the nm^3 unit instead of the nm^4
as reported elsewhere for the PSD. Is that h the maximum height in the
processed image? Or is it maybe the image resolution in nm? I cannot
find any h definition in the previous paragraphs of the manual.

Please see the FAQ

http://gwyddion.net/faq.php#faq015

Concerning the formula, h is the sampling step.

Post by Alessandro Gambardella
I only need to transform the nm^3 as Gwyddyon calculates into nm^4 for my purposes.

I would advise against this. The values in nm⁴ you often encounter have
a normalisation that depend on sampling. So you cannot compare PSDFs
from different measurements (different area, different sampling step).
The way in which these PSDFs preserve Parseval's theorem is valid but
not useful.

Instead, try to figure out what exactly the software that produces PSDF
in nm⁴ does and then recalculate it to the actual density, the same as
used in Gwyddion.

Regards,

Yeti