NLS-Glossary

Diffraction is a spread or bending of a freely propagating wave when it encounters any type of obstacle, for example, sound waves become bent (deflected around) corners. When a light wave passes through an aperture (an opening in an opaque screen) it spreads out from the aperture and the exhibits regions of maximum and minimum intensity as a result of interference of spherical waves emitted from every point in the aperture in accordance with the Huygens-Fresnel principle.

The intensity distribution in the diffracted beam as registered by an observer is called the diffraction pattern. The light pattern of the diffracted beam does not correspond to the geometric shadow of the aperture. Diffraction phenomena are generally classified into two categories.

In Fraunhofer diffraction, the incident light beam is a plane wave (a collimated light beam) and the observation or detection of the light intensity pattern (by placing a photographic screen etc.) is done far away from the aperture so that the waves being received also look like plane waves. Inserting a lens between the aperture and the photographic screen enables the screen to be closer to the aperture. In Fresnel diffraction, the incident light beam and the received light waves are not plane waves but have significant wavefront curvatures. Typically, the light source and the photographic screen are both close to the aperture so that wavefronts are curved. Fraunhofer diffraction is by far the most important

Diffraction is a spread or bending of a freely propagating wave when it encounters any type of obstacle.

For example, sound waves become bent (deflected around) corners. When a light wave passes through an aperture (an opening in an opaque screen) it spreads out from the aperture and the exhibits regions of maximum and minimum intensity as a result of interference of spherical waves emitted from every point in the aperture in accordance with the Huygens-Fresnel principle. The intensity distribution in the diffracted beam as registered by an observer is called the diffraction pattern. The light pattern of the diffracted beam does not correspond to the geometric shadow of the aperture. Diffraction phenomena are generally classified into two categories. In Fraunhofer diffraction, the incident light beam is a plane wave (a collimated light beam) and the observation or detection of the light intensity pattern (by placing a photographic screen etc.) is done far away from the aperture so that the waves being received also look like plane waves. Inserting a lens between the aperture and the photographic screen enables the screen to be closer to the aperture. In Fresnel diffraction, the incident light beam and the received light waves are not plane waves but have significant wavefront curvatures. Typically, the light source and the photographic screen are both close to the aperture so that wavefronts are curved. Fraunhofer diffraction is by far the most important.

is an imaging system whose resolution is diffraction limited. Even in the case of perfect stigmatism the actual resolution of an imaging instrument is never perfect. For instance, a parabolic telescope, although perfectly stigmatic for the conjugation between an on-axis distant star and the focal point, exhibits a typical Airy diffraction pattern in the focal plane, with a transverse dimension inversely proportional to the diameter of the entrance pupil, from a signal processing aspect, the Airy pattern is the response function of the apparatus.

The only way to enhance the performance is to enlarge the pupil – hence the realization of so-called “very large” telescopes.

Diffraction limited system is an imaging system whose resolution is diffraction limited. Even in the
case of perfect stigmatism the actual resolution of an imaging instrument is never perfect. For instance, a parabolic telescope, although perfectly stigmatic for the conjugation between an on-axis distant star and the focal point, exhibits a typical Airy diffraction pattern in the focal plane, with a transverse dimension inversely proportional to the diameter of the entrance pupil. From a signal processing aspect, the Airy pattern is the response function of the apparatus. The only way to enhance the performance is to enlarge the pupil – hence the realization of so-called “very large” telescopes.

A bar code verification term. The component of reflected light that emanates in all directions from the reflecting surface (as opposed to the focused light of the scanner laser reflected back to the scanner).

Diffusion coefficient is a measure of the rate at which atoms diffuse. It depends on the nature of the diffusion process and typically it is temperature dependent.                             It is defined as the diffusion flux per unit concentration gradient.