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Etendue or étendue () is a property of light in an optical system, which characterizes how "spread out" the light is in area and angle. From the source point of view, it is the product of the area of the source and the solid angle that the system's entrance pupil subtends as seen from the source. Equivalently, from the system point of view, the etendue equals the area of the entrance pupil times the solid angle the source subtends as seen from the pupil. These definitions must be applied for infinitesimally small "elements" of area and solid angle, which must then be summed over both the source and the diaphragm as shown below. Etendue may be considered to be a volume in phase space. Etendue is important because it never decreases in any optical system. A perfect optical system produces an image with the same etendue as the source. The etendue is related to the Lagrange invariant and the optical invariant, which share the property of being constant in an ideal optical system. The radiance of an optical system is equal to the derivative of the radiant flux with respect to the etendue. The term ''étendue'' comes from the French ''étendue géométrique'', meaning "geometrical extent". Other names for this property are acceptance, throughput, light-grasp, collecting power, optical extent, and the AΩ product. ''Throughput'' and ''AΩ product'' are especially used in radiometry and radiative transfer where it is related to the view factor (or shape factor). It is a central concept in nonimaging optics.〔Julio Chaves, ''Introduction to Nonimaging Optics'', CRC Press, 2008 (978-1420054293 )〕〔Roland Winston et al.,, ''Nonimaging Optics'', Academic Press, 2004 (978-0127597515 )〕〔Matthew S. Brennesholtz, Edward H. Stupp, ''Projection Displays'', John Wiley & Sons Ltd, 2008 (978-0470518038 )〕 ==Definition== An infinitesimal surface element, dS, with normal n''S'' is immersed in a medium of refractive index ''n''. The surface is crossed by (or emits) light confined to a solid angle, d''Ω'', at an angle ''θ'' with the normal n''S''. The area of d''S'' projected in the direction of the light propagation is . The etendue of this light crossing dS is defined as : Because angles, solid angles, and refractive indices are dimensionless quantities, etendue has units of area (given by dS). 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「Etendue」の詳細全文を読む スポンサード リンク
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