This page explains the Sun and Sky system in V-Ray for Blender.
Overview
The V-Ray Sun and Sky texture are special features which are provided by the V-Ray renderer. Developed to work together, the V-Ray Sun and Sky system reproduces the real-life sun and sky environment of the Earth. Both are coded so that they change their appearance depending on the direction of the V-Ray Sun.
For easier workflow, V-Ray Sun and Sky create an all-set-and-ready-to-use Sun and Sky system in Blender.
More information on the V-Ray Sun can be found on the V-Ray Sun page.
More information on the V-Ray Sky texture can be found on the V-Ray Sky page.
How It Works
The V-Ray Sun and Sky system in Blender introduces an easy way to manage both the V-Ray Sun light and the Sky texture together.
When created, V-Ray automatically sets a Collection named VRaySunSky with a V-Ray Sun light and V-Ray Sun Light Target under it in the Outliner.
The V-Ray Sun Light Target is a dummy object created in the viewport, that you can use to control the direction of the Sun.
You can also see that when you open the V-Ray Node Editor in the World mode, there is a newly created Sky texture attached to the Background, GI, Reflection, and Refraction sockets of the V-Ray Environment node.
The Sun and Sky system also automatically links the Sky texture with the Sun light. If you go into the Sky texture's properties (either in the Node side menu of the V-Ray Node Editor or in the World tab) in the Options tab, you will see the Sun Light is already linked.
References
Here is a list of references about the V-Ray Sun and Sky implementation, as well as general information about the illumination of the Sun.
A.J. Preetham, P. Shirley, and B. Smits, A Practical Analytic Model for Daylight, SIGGRAPH 1999, Computer Graphics Proceedings;
An online version can be found at http://www.cs.utah.edu/~shirley/papers/sunsky/
This paper includes source code examples and is the base for the VRaySun and VRaySky plugins.
R. H. B. Exell, The intensity of solar radiation, 2000
This page is available at http://www.jgsee.kmutt.ac.th/exell/IntensitySolarRad.pdf (Please note that this link is no longer valid).
This document contains information about the average intensity of the solar radiation, as well as some specific measurements.
R. Cahalan, Sun & Earth Radiation
This page can be found at http://climate.gsfc.nasa.gov/static/cahalan/Radiation/ (Please note that this link is no longer valid).
These pages contain a list of accurate solar irradiances across a large portion of the electromagnetic spectrum.
D. Robinson-Boonstra, Venus Transit: Activity 3, Sun & Earth Day 2004
This document can be found online at
http://sunearth.gsfc.nasa.gov/sunearthday/2004/2004images/VT_Activity3.pdf (Please note that this link is no longer valid).
Among other things, this document gives the distance from the Sun to the Earth and the size of the Sun derived from astronomic observations.
Hosek L, et al, An Analytic Model for Full Spectral Sky-Dome Radiance
This document can be found online at
Describes the Hosek sky model used by the VRaySun and VRaySky