A spatial filter is a device used to alter or correct aberrations in coherent light source outputs. Aberrations are undesirable characteristics in the projected beam of light caused by badly-made, dirty, or damaged optics, often characterized by weak, concentric rings of light around the central beam. The spatial filter typically consists of a collimator lens that focuses the beam of light and passes it through a tiny aperture. This allows only the desired part of the beam to pass while excluding the aberrations. Spatial filters are commonly used to remove imperfections from high-intensity, precision light sources such as lasers.
The integrity of high-intensity, finely-focused or coherent light sources are dependent on a number of elements, the most important being the quality of the optics used in the generation of the beam of light. Issues such as poor production standards, contamination, and damage of any sort can cause the light output to develop aberrations or imperfections. These can affect the beam to such an extent that it no longer performs its intended function, particularly in the case of high-precision devices such as lasers. Aberrations are often visible as fuzzy, concentric rings of weaker light around the central output beam.
A common method of restoring the desired fidelity of light outputs is the use of a spatial filter. These devices refocus the light and filter out the undesirable parts of the beam, leaving only the original output. This is achieved by focusing the light beam using a separate lens and directing it at thick foil plate with a very small aperture, or hole, in its surface. The size of the hole is carefully calculated to suit the focal length of the lenses used in the device, the beam's wavelength, and the extent of the imperfections present in the beam.
This procedure effectively blocks the aberrant portions present in the beam of light, allowing only the desired portion to pass through the aperture. The procedure allows for extremely high fidelity in the resultant light output. The size of the spatial filter aperture is critical, however, and small miscalculations can have an adverse effect on the output beam. If the hole is too small, a significant power loss may be experience, while, if too big, little or no aberration filtering will take place. In many commercially-available spatial filter devices, microscope objective lenses are also used because of their inherent high quality.