view factor is related to the etendue. Nusselt analog A geometrical picture that can aid intuition about the view factor was developed by Wilhelm Nusselt, and is called the Nusselt analog. The view factor between a differential element dAi and the element Aj can be obtained projecting the element Aj onto the surface of a unit hemisphere, and then projecting that in turn onto a unit circle around the point of interest in the plane of Ai. The view factor is then equal to the differential area dAi times the proportion of the unit circle covered by this projection. The projection onto the hemisphere, giving the solid angle subtended by Aj, takes care of the factors cos(θ2) and 1/r2; the projection onto the circle and the division by its area then takes care of the local factor cos(θ1) and the normalisation by π. The Nusselt analog has on occasion been used to actually measure form factors for complicated surfaces, by photographing them through a suitable fish-eye lens. (see also Hemispherical photography). But its main value now is essentially in building intuition. See also Radiosity, a matrix calculation method for solving radiation transfer between a number of bodies. Gebhart factor, an expression to solve radiation transfer problems between any number of surfaces. References External links A large number of 'standard' view factors can be calculated with the use of tables that are commonly provided in heat transfer textbooks. List of view factors for specific geometry cases View3D, a computer program (FOSS) for calculating view factors in 2D and 3D. Heat transfer