Optical transfer function ($H(\mathbf{\nu })=\hat{p}(\mathbf{\nu })$)

The optical transfer function is the Fourier transform of the normalized intensity point spread function for incoherent imaging with translation invariance, using spatial frequency in cycles per unit length. It has $H(0)=1$. For an ideal circular pupil it is the normalized pupil autocorrelation, with cutoff $2\mathrm{NA}/{\lambda }_0$ in object coordinates: no overlap remains when two pupil copies are shifted apart by more than one pupil diameter.^[ref]

For a periodic intensity distribution, $C=(I_{\max }-I_{\min })/(I_{\max }+I_{\min })$. It should not be confused with a resist contrast parameter.

The magnitude of the normalized intensity optical transfer function. It describes transfer of sinusoidal intensity contrast in a linear, shift-invariant incoherent imaging model.