Point spread function
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The point spread function (PSF) defines the propagation of electromagnetic radiation from a point source. It is defined in spherical coordinates for a Lambert-type radiator. Its unit is <math>[m^{-2}]<math>. It is a useful concept in Fourier optics.
In 3D microscopy (like in Confocal laser scanning microscopy), a PSF is the image of a single point object. The degree of spreading (blurring) of this point object is a measure for the quality of an optical system.
PSFs play an important role in the image formation theory of fluorescence microscopy. The reason for this is that in incoherent imaging systems such as fluorescent microscopes the image formation process is linear and described by linear system theory. This means that when two objects A and B are imaged simultaneously, the result is equal to the sum of the independently imaged objects. In other words: the imaging of A is unaffected by the imaging of B and vice versa. (The sum is of the light waves which may result in destructive and constructive interference at non-image planes.)
As a result of the linearity property the image of any object can be computed by chopping up the object in parts, image each of these, and subsequently sum the results. When one chops up the object in extremely small parts, i.e. point objects of varying height, the image is computed as a sum of PSFs, each shifted to the location and scaled according to the height of the corresponding point. In conclusion: the imaging in the fluorescent microscope is completely described by its PSF.
In microscope image processing knowing the PSF of the measuring device is very important for restoring the image with deconvolution.
The experimental determination of a PSF is usually tricky, due to the difficulty of finding sub-resolution (point-like) radiating sources. Quantum dots and fluorescent beads are usually considered for this purpose.