Image Transforms
UNITS IN THE 2D IMAGE AND FOURIER DOMAINS
Images are typically functions of space (hence the units of measurement in the image domain are m, cm, mm, µm, etc.).
In the 2D Fourier domain, the unit of frequency is cycles/ mm, cycles/ m, etc.
The unit Hertz is not used in image processing.
If the distance to the viewer is taken into account, frequency is expressed in terms of cycles/ degree of visual angle subtended.
Frequency is sometimes expressed in line pairs/ mm.
Spatial resolution may be expressed in terms of: sampling interval (mm), width of the point spread function (mm), size of the laser scanning spot used for sampling (µm), or the smallest object or separation viable (mm).