Figure 4.

The enhancement of the support of the OTF by means of standing wave excitation. (A) The circular passband for standard fluorescence microscopy with cutoff frequency r. (B) The cloverleaf-shaped OTF of HELM. The four circular regions 1, 2, 3, and 4 are relocated copies of the circular passband from A and correspond to the components B⁺, B⁻, C⁺, and C⁻ from Eq. 5, respectively. The displacement u is equal to the spatial frequency of the harmonic excitation. The ratio of u to r displayed in this figure corresponds to the real situation, because the cutoff frequency of the overall system is ≈20% less than that one expected from the nominal NA of our objective.