Tough Guy - The Photoshop: Professional Retouching and Compositing Tips, Tricks, and Techniques - page 371

Tip
If you don't want the light source to be quite so bright, change the blend mode to
Linear Dodge (Add) and adjust the Fill slider. It's a subtle difference, but using
the layer's opacity tends to make the light source appear gray.
2. Next we'll add some dust specks to the light sources using the thingys technique
from Chapter 3 . Choose File > Place Embedded (File > Place in earlier versions of
Photoshop), locate the thingys file, and click OK. Option/Alt-Shift-drag a corner
handle so that the file fills the entire layer. Press Return/Enter. Change the blend
mode to Screen so that only the light parts of the layer are visible.
3. Go to Filter > Blur > Motion Blur. Change the Angle to -45 and the Distance to 70
and click OK ( Figure 7.16 ) .