Light Painting Tips and Techniques: Double Masking Using a Group…by Harold Ross

One function of this technique is to preserve the first mask that we created, which can sometimes be a little surgical in nature, and therefore take some time to create. The other, and probably more important function of this technique, is to allow us to have a little bit more creative freedom and how we bring in the information from a light painted layer. 

Photograph by Pavel Kolman