[[Image:spherical-aberration-disk.jpg|thumb|269x269px|A [[point source]] as imaged by a system with negative (top), zero (center), and positive (bottom) [[spherical aberration]]. Images to the left are [[defocus]]ed toward the inside, images on the right toward the outside.]]
The '''point spread function''' ('''PSF''') describes the response of an imaging system to a [[point source]] or point object. A more general term for the PSF is a system's [[impulse response]],; the PSF beingis the impulse response or impulse response function (IRF) of a focused optical imaging system. The PSF in many contexts can be thought of as the extended blob in an image that represents a single point object, that is considered as a spatial impulse. In functional terms, it is the spatial ___domain version (i.e., the inverse Fourier transform) of the [[Optical transfer function|optical transfer function (OTF) of thean imaging system]]. It is a useful concept in [[Fourier optics]], [[astronomy|astronomical imaging]], [[medical imaging]], [[electron microscope|electron microscopy]] and other imaging techniques such as [[dimension|3D]] [[microscopy]] (like in [[confocal laser scanning microscopy]]) and [[fluorescence microscopy]].
The degree of spreading (blurring) in the image of a point object for an imaging system is a measure forof the quality of anthe imaging system. In [[coherence (physics)|non-coherent]] imaging systems, such as [[fluorescent]] [[microscopes]], [[telescopes]] or optical microscopes, the image formation process is linear in the image intensity and described by a [[linear system]] theory. This means that when two objects A and B are imaged simultaneously by a non-coherent imaging system, the resulting image 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'', owing to the non-interacting property of photons. In space-invariant systems, i.e. those in which the PSF is the same everywhere in the imaging space, the image of a complex object is then the [[convolution]] of the that object and the PSF. The PSF can be derived from diffraction integrals.<ref>{{Cite book|url=https://books.google.com/books?id=lCm9Q18P8cMC&q=diffraction+integral+point+spread+function&pg=PA355|title=Progress in Optics|date=2008-01-25|publisher=Elsevier|isbn=978-0-08-055768-7|language=en|pages=355}}</ref>
