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P H O T O G R A P H Y C O N T E S T Sponsored by A.J. Hirsch |
Grace Coyle
Third Place
2006 OAPT 4U Photo Contest
Star Filters and Diffraction
Star filters consist of a piece of glass (or other clear material) etched with a grid pattern. They are able to make light from a point source appear like a star. The number of points on the star depends on the grid pattern engraved on the star filter.
Light from a point source travels in all directions. When it passes through a star filter, the sharp, grooves of the grid pattern cause the light to be diffracted.
In the first photograph, a star filter was not used. The candlelight and reflections in the glass appear bright, as they would in real life. These, light sources in the second photograph appear much brighter, and they look like eight pointed stars. An eight point star filter was used to create this effect.
An eight point star filter is etched with two pairs of perpendicular lines. Light is diffracted through these etchings. When it passes through a horizontal etching, it is diffracted vertically, and vice versa. As a result, light is diffracted four different ways and four different 'light lines' are created. These four lines intersect at the centre of the light source, making an eight-pointed star.
The closer these etchings are together, the blurrier the star. Light can also be diffracted through the space between the etchings, and the closer these etchings are together, the more diffraction occurs - creating the blurry effect.