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In optics, the Airy disk (or Airy disc) and Airy pattern are descriptions of the best focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light. The Airy disk is of importance in physics, optics, and astronomy. The diffraction pattern resulting from a uniformly-illuminated circular aperture has a bright region in the center, known as the Airy disk which together with the series of concentric bright rings around is called the Airy pattern. Both are named after George Biddell Airy. The disk and rings phenomenon had been known prior to Airy; John Herschel described the appearance of a bright star seen through a telescope under high magnification for an 1828 article on light for the ''Encyclopedia Metropolitana'':
However, Airy wrote the first full theoretical treatment explaining the phenomenon (his 1835 "On the Diffraction of an Object-glass with Circular Aperture").〔Airy, G. B., "On the Diffraction of an Object-glass with Circular Aperture," (''Transactions of the Cambridge Philosophical Society'', Vol. 5 ), 1835, p. 283-291.〕 Mathematically, the diffraction pattern is characterized by the wavelength of light illuminating the circular aperture, and the aperture's size. The ''appearance'' of the diffraction pattern is additionally characterized by the sensitivity of the eye or other detector used to observe the pattern. The most important application of this concept is in cameras and telescopes. Owing to diffraction, the smallest point to which a lens or mirror can focus a beam of light is the size of the Airy disk. Even if one were able to make a perfect lens, there is still a limit to the resolution of an image created by this lens. An optical system in which the resolution is no longer limited by imperfections in the lenses but only by diffraction is said to be diffraction limited. ==Size== Far away from the aperture, the angle at which the first minimum occurs, measured from the direction of incoming light, is given by the approximate formula: : or, for small angles, simply : where ''θ'' is in radians, ''λ'' is the wavelength of the light and ''d'' is the diameter of the aperture. Airy wrote this as : where ''s'' was the angle of first minimum in seconds of arc, ''a'' was the radius of the aperture in inches, and the wavelength of light was assumed to be 0.000022 inches (the mean of visible wavelengths).〔Airy, G. B., "On the Diffraction of an Object-glass with Circular Aperture," (''Transactions of the Cambridge Philosophical Society'', Vol. 5 ), 1835, p. 287.〕 The Rayleigh criterion for barely resolving two objects that are point sources of light, such as stars seen through a telescope, is that the center of the Airy disk for the first object occurs at the first minimum of the Airy disk of the second. This means that the angular resolution of a diffraction limited system is given by the same formulae. However, while the angle at which the first minimum occurs (which is sometimes described as the radius of the Airy disk) depends only on wavelength and aperture size, the appearance of the diffraction pattern will vary with the intensity (brightness) of the light source. Because any detector (eye, film, digital) used to observe the diffraction pattern can have an intensity threshold for detection, the full diffraction pattern may not be apparent. In astronomy, the outer rings are frequently not apparent even in a highly magnified image of a star. It may be that none of the rings are apparent, in which case the star image appears as a disk (central maximum only) rather than as a full diffraction pattern. Furthermore, fainter stars will appear as smaller disks than brighter stars, because less of their central maximum reaches the threshold of detection.〔Sidgwick, J. B., (''Amateur Astronomer's Handbook'' ), Dover Publications, 1980, pp. 39–40.〕 While in theory all stars or other "point sources" of a given wavelength and seen through a given aperture have the same Airy disk radius characterized by the above equation (and the same size diffraction pattern), differing only in intensity (the "height" of the surface plot at upper right), the appearance is that fainter sources appear as smaller disks, and brighter sources appear as larger disks.〔Graney, Christopher M., "Objects in Telescope Are Farther Than They Appear – How diffraction tricked Galileo into mismeasuring distances to the stars", (''The Physics Teacher'' ), vol. 47, 2009, pp. 362–365.〕 This was described by Airy in his original work:
Despite this feature of Airy's work, the radius of the Airy disk is often given as being simply the angle of first minimum, even in standard textbooks.〔Giancoli, D. C., (''Physics for Scientists and Engineers (3rd edition)'' ), Prentice-Hall, 2000, p. 896.〕 In reality, the angle of first minimum is a limiting value for the size of the Airy disk, and not a definite radius. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Airy disk」の詳細全文を読む スポンサード リンク
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