isophote

Isophote (from Greek iso, equal, and phos, light) is a curve or line of constant brightness on a luminous object or image. The term is used in astronomy, optics, photography, and computer vision to describe contours of equal intensity or radiance.

In astronomical imaging, an isophote is a contour of equal surface brightness projected on the sky. They are used to analyze extended objects such as galaxies and nebulae, to measure radial light profiles, and to fit models by projecting the light distribution as a series of ellipses. Changes in isophote shape with radius can reveal structural features such as bars, disks, or mergers.

In digital images, isophotes are the loci of points with the same intensity value. They can be used as a normalization or segmentation tool, as features for shape descriptors, and in aesthetic analyses. In practice, isophote contours can be extracted with level-set methods or image thresholding and are analogous to topographic contour lines.

In differential geometry and computer graphics, an isophote on a surface is the set of points where the angle between surface normals and a fixed direction is constant. These curves reflect the surface's curvature and are used in shading, relief modeling, and surface analysis.