up::[[Radiation]] View Factor is also called **form factor**, **shape factor** or **configuration factor** View factor is a metric of how well a surface can "see" another sufrace. If you draw a hemisphere from one flat surface (i.e. fisheye view), then how much of the fisheye view is blocked by a surface of interest It is a number between 0 and 1. It expresses the portion of radiative heat flux that leaves one surface that impacts another surface. It is purely based on geometry. View Factors are parts of [[Grey Body Factor]] ## 1. Definition view factors It depends on three factors - Area of the two surfaces  and  - distance between surface  - angle between surface normals ( and ) ### 1.1. Reciprocity View factor depends on what is source and target if the source and targets have different areas All radiation emitted from a face to all other faces in the domain should add up to 1 if it is radiating inside a closed domain ## 2. Calculating view factors Analytical calculation of view factors is complex even for the simplest of geometries. Hence numerical methods are used. Following methods are some of the techniques used - Double area summation - Nusselt Sphere technique - Crossed-String Technique - Monte-Carlo ray tracing - Contour integration - Hemi-cube - Quadrature - Newton Cotes Integrals ### 2.1. Double Area Summation Method Instead of doing an integration, we just split the area into small finite elements of area  and use summation ### 2.2. Nusselt Sphere Method To calculate the view factor of an area  (think a small satellite) to a large spherical surface 2 (think a planet) is calculated as follows - Draw a hemisphere of unit radius around  - Draw tangents from the  surface to the large surface 2 - Consider where the tangents intersect the unit hemisphere into a circle and project that down towards . The projected circle has an area  - Form factor is  divided by area of unit hemisphere's circular base (unit circle) ### 2.3. Monte Carlo Ray Tracing Method Monte Carlo method uses random numbers to perform integration. Let us suppose you have 4 surfaces A, B, C and D. If you shoot 1000 rays from surface A from bunch of random points on it in bunch of random directions (different azimuth  and elevation ), let us suppose 200 rays hit surface B, 150 hit surface C and 80 hit surface D. The rest escape to ambient. View factor  is 0.2,  is 0.15 and so on. In all cases sum of view factors should be 1. ## 3. References - [A Catalog of Radiation Heat Transfer Configuration Factors (thermalradiation.net)](http://www.thermalradiation.net/indexCat.html)