The Tiffen 4 x 4" Streak (3mm) Glass Filter creates multiple points of light, or "stars," streaking outward from a central light source. This can make lighting within the scene take on a more glittering, glamorous appearance. This effect is produced by a series of thin lines etched into the flat optical surface of a clear filter. The size and brightness of the star lines produced are a function of the size, shape, and brightness of the light source.
You have additional control through the choice of a particular spacing between the lines on the filter. These spacings are measured in millimeters and range from 1mm to 4mm. A 1mm spacing has twice as many lines as a 2mm spacing. It will produce a brighter star for any given source. Lines in one direction produce a two-pointed star, just a streak through the center of the light.
Note: Always test before critical situations