in my last article I mentioned the “additive alpha” calling it a hidden feature. This has intrigued some, of course, so that I am here to give some explanation. In 3dmax there is a feature of the material called “additive opacity”. 
the additive alpha is a technique widely used in videogame world to achieve special effects luminescent. Of course it is often useful even in traditional rendering. the additive alpha is a physically incorrect behavior of the material, in which light passing through it increases instead of being filtered. In blender to obtain such effects is normally used in the compositor: mixing various layers in add mode (this becomes difficult when the layers become hundreds or intersect). For example, a simple plan with a material additive alpha and a image of a star, duplicated and intersected dozens of times, produces an effect like this:
There is no point increasing the parameter “emit”, where the object is bright is not transparent and where it is trasparent is not bright and no change occurs on the background!
the only way is to use the material nodes. The solution was to multiply only the refracted part of the material while maintaining alpha. But in blender the slot of multiply node allows you to have White as the maximum (value = 1) so the result would be of equal or less bright than the original.
The problem has been solved thanks to the intuition of Giuseppe “Renderdemon” Canino, who suggested to multiply the color of the original material by using as a multiplier the node “Value” with a value higher than 1.
Once you understand how to do it, it was not difficult to create a series of materials with different characteristics to achieve different effects like those seen in the previous article.
Fernando Luceri