Keywords:Image formation theory, Optical microscopy, Optical coherence tomography
We formulate a four-dimensional (4-D) image formation theory of all laser microscopy and optical coherence tomography (OCT) by defining 4-D aperture as the rate of Fourier components in sample F(fx, fy, fz) acquired by the signal light with angular frequency ω. The 4-D aperture is a key function that can evaluate and compare the optical resolutions of all laser microscopy and OCT. In microscopy where optical transfer function (OTF) can be defined, the OTF is calculated from the 4-D aperture. This OTF is a 4-D function OTF(fx, fy, fz,ωo) containing diverse information, one of which is a common three-dimensional OTF expressed by OTF(fx, fy, fz, 0). Even in microscopy such as OCT where OTF cannot be defined, the 4-D aperture still exists. Any laser microscopy including fluorescence confocal microscopy, OCT, and coherent Raman scattering microscopy have their unique 4-D apertures. Thus the 4-D aperture is the most fundamental physical quantity that describes the optical resolution of image formation systems.