Collimated light

A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. Such a beam cannot be created, due to diffraction. A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. Such a beam cannot be created, due to diffraction. Light can be approximately collimated by a number of processes, for instance by means of a collimator. Perfectly collimated light is sometimes said to be focused at infinity. Thus, as the distance from a point source increases, the spherical wavefronts become flatter and closer to plane waves, which are perfectly collimated. Other forms of electromagnetic radiation can also be collimated. Collimation of X-rays is important in radiology. Reducing the size of the beam by collimation reduces the volume of the patient's tissue that is irradiated, and reduces intensity in the periphery of the beam. Peripheral x-rays can be absorbed by the patient's tissues and can generate scattered photons, which travel in many directions and cause film fog, reducing the quality of the x-ray image. The word 'collimate' comes from the Latin verb collimare, which originated in a misreading of collineare, 'to direct in a straight line'. Laser light from gas or crystal lasers is highly collimated because it is formed in an optical cavity between two parallel mirrors which constrain the light to a path perpendicular to the surfaces of the mirrors. In practice, gas lasers can use concave mirrors, flat mirrors, or a combination of both. The divergence of high-quality laser beams is commonly less than 1 milliradian, and can be much less for large-diameter beams. Laser diodes emit less-collimated light due to their short cavity, and therefore higher collimation requires a collimating lens. Synchrotron light is very well collimated. It is produced by bending relativistic electrons (i.e. those moving at relativistic speeds) around a circular track. When the electrons are at relativistic speeds, the resulting radiation is highly collimated, a result which does not occur at lower speeds. The light from stars (other than the Sun) arrives at Earth precisely collimated, because stars are so far away they present no detectable angular size. However, due to refraction and turbulence in the Earth's atmosphere, starlight arrives slightly uncollimated at the ground with an apparent angular diameter of about 0.4 arcseconds. Direct rays of light from the Sun arrive at the Earth uncollimated by one-half degree, this being the angular diameter of the Sun as seen from Earth. During a solar eclipse, the Sun's light becomes increasingly collimated as the visible surface shrinks to a thin crescent and ultimately a small point, producing the phenomena of distinct shadows and shadow bands. A perfect parabolic mirror will bring parallel rays to a focus at a single point. Conversely, a point source at the focus of a parabolic mirror will produce a beam of collimated light creating a Collimator. Since the source needs to be small, such an optical system cannot produce much optical power. Spherical mirrors are easier to make than parabolic mirrors and they are often used to produce approximately collimated light. Many types of lenses can also produce collimated light from point-like sources. This principle is used in full flight simulators (FFS), that have specially designed systems for displaying imagery of the Out-The-Window (OTW) scene to the pilots in the replica aircraft cabin.