Lighting can be enhanced in ways that go beyond what exists in nature. Star filters create points of light, like "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. These lines act as cylindrical lenses, diffracting light points into long thin lines of light running perpendicular to the etched lines. Lines on the filter positioned horizontally produce vertically oriented star lines.
The size and brightness of the star lines produced are first 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. Generally these spacings are measured in millimeters. A 1mm spacing has twice as many lines per unit area as a 2mm spacing. It will produce a brighter star for any given source. Spacings offered generally range from 1mm to 4mm, as well as both narrower and wider for specialty effects.
The number of directions that lines run in determines the number of points produced. Lines in one direction produce a two-pointed star, just a streak through the center of the light. There are 4, 6, 8, 12 and more points available. With an 8 or 12 point filter, the many star lines will tend to overpower the rest of the image, so use them carefully. Although the more common types have a symmetrical arrangement of points, they can also be obtained with asymmetric patterns, which tend to appear more "natural," less synthetic.