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[[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 being the impulse response of a focused optical system. The PSF in many contexts can be thought of as the extended blob in an image that represents a single point object. In functional terms it is the [[spatial ___domain]] version of the [[Optical transfer function|optical transfer function of the 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) of the point object is a measure for the quality of an 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 [[linear system]] theory. This means that when two objects A and B are imaged simultaneously, 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 system, i.e. the PSF is the same everywhere in the imaging space, the image of a complex object is then the [[convolution]] of the true object and the PSF. ==Introduction==
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