 # Dictionary N onwards

Nanosecond:
A billionth part of a second.
Narrowband Coating:
A coating designed to provide transmittance (or reflectance) over a very restricted band of wavelengths.
Neutral Density:
As applied to thin films, a coating which appears gray to the eye and has a flat absorption curve throughout the visible spectrum. Neutral density filters decrease the intensity of light without changing the relative spectral distribution of energy.
Newton's Rings:
Used to measure the fit of a lens surface against the surface of a test glass. The rings result when two adjacent polished surfaces are placed together with an air space between them and the light beams they reflect interfere.
Nodal Points:
The two points at which the nodal planes appear to intersect with the optical axis, i.e., when a ray is directed at the first nodal point in an optical system, it appears to emerge from a second nodal point on the optical axis with no deviation in its angle.
Numerical Aperture:
Describes the angle in a cone of light emitted by the condenser and accepted by the objective of a microscope; the index of refraction of the medium in which the image lies multiplied by the sine of the half angle of the cone of light.
Object-to-Image Distance:
Also known as the total conjugate distance or track length. Can be finite or infinite depending on application.
Objective:
The optical element which receives light from the object and forms the first or primary image in telescopes,microscopes and other optical systems.
Oblique Ray:
A ray of light that is neither perpendicular nor parallel, but inclined.
Optical Axis:
A line passing through the centers of curvature of a lens or series of lenses in an optical system.
Optical Flat:
A piece of glass with one or both surfaces polished flat. Also known as a test plate, test glass or reference flat.
Optical Interference:
The interaction of two light waves on the total intensity of light.
Orthogonal:
Mutually perpendicular.
Paraxial Image Plane:
Image plane located by using first-order geometric optics. See Gaussian Optics.
Peak Wavelength:
For bandpass filters, the discrete wavelength which has the maximum transmission value in the passband region.
Pinhole:
Generally a small, sharp-edge hole without a lens which can function as an aperture or eye lens.
Plane of Incidence:
The plane that is defined by the incident and reflected rays.
Planetary Polisher:
A polishing machine used in the production of plano parallel elements where both surfaces are polished simultaneously.
Plano Elements:
Lenses or mirrors with perfectly flat surfaces.
Plano-Concave:
A lens with one flat (plano) surface and the other curved inward.
Plano-Convex:
A lens with one flat (plano) surface and the other curved outward.
Poisson's Ratio:
The ratio of the transverse contraction of a bar of material to the elongation per unit length.
Polarization of Light:
The process of affecting light so that its waves vibrate in one plane only; reflection, double refraction, selective absorption and scattering are all ways to polarize light.
Polarized Light:
Light that vibrates in only one plane.
Polychromatic Aberrations:
The separation of an image into planes of distinct color, caused by the variation of the index of refraction of glass, and the focal length of a lens, with the wavelength of light; in a given plane, all colors but one are unfocused.
Polyhedral:
In the shape of a solid formed by plane faces, as in a prism.
Potentiomenter:
An instrument used to measure electromotive forces.
Power Loss:
In applications involving diffraction limited lenses, power loss in the spot is an exponential function of the square of the truncation ratio (Dt/Db): PL= e-2*(Dt/Db)˛.
Primary Reflections:
The principal, intended reflections at optical surfaces, as differentiated from secondary, usually unintended or unwanted reflections occurring in an optical system.
Principal Planes:
Imaginary planes at right angles to the optical axis of a lens; the intersection of a plane and the axis is a principal point.
Prism:
A transparent optical element with at least two polished planes inclined toward each other, from which light reflects or through which it is refracted.
Pulse Modulation:
The process of periodically or intermittently varying the amplitude of a pulse of light.
Q-Switched:
In an optical resonator, the higher the reflectivity of its surfaces, the higher the Q. A Q-switch rapidly changes the Q in the optical resonator of a laser to prevent lasing until a high level of optical gain and energy storage has been reached in the lasing medium; a giant pulse is generated when the Q is rapidly increased.
Quarter Wave Optical Thickness:
Common thin-film term. The QWOT (Quarter Wave Optical Thickness) is the wavelength such that the optical thickness (index 'n'* physical thickness 'd') of a coating evaporant layer is Ľ wavelength. (n*d=/4)
The measurement of the time rate of flow of radiant energy, expressed in watts.
Real Image:
Light rays reproduce an object, called an image, by gathering a beam of light diverging from an object point and transforming it into a beam converging toward or diverging from another point. If the beam is converging, it produces a real image.
Reference Flat:
An optical flat used as a test glass.
Reflection:
The return of light from a surface with no change in wavelength.
Refraction:
The change in direction of a ray of light as it passes through two media through which light travels at different speeds.
Refractive Index:
The ratio between the speed of light through air to the speed of light through another medium; the ratio determines how much a ray of light will bend as it passes through a given medium.
Resolution:
The ability of a lens to image the points, lines and surfaces of an object so they are perceived as discrete entities.
Reticle:
An optical element containing a pattern placed at the image plane of a system. The reticle facilitates system alignment or the measurement of target characteristics.
Reverted Image:
An image in which left and right seem to be reversed.
Ring-Type Auto Iris:
A type of auto iris which utilizes a rotating magnet to move the iris vanes.
Rockwell Hardness:
Resistance of a substance to penetration by a pyramidal stylus pressed in under a specific load; See also Knoop hardness.
Sag Formula:
"Sag" is an abbreviation for "sagitta," the Latin word for "arrow," and refers to the height of a curve from the chord to the highest point.
Sagittal Focus:
The focus of rays lying in the sagittal plane, which is the plane perpendicular to the meridional plane
Scan Lens:
A multi-component objective which is the heart of a graphic arts image recording, printing or engraving system. The balancing of wide angular field, flat image plane and linear relationship between input scan angle and image height make scan lenses ideal for writing characters on film, laser engraving figures or recording characters and figures from texts.
Scan lenses are sometimes also called F- lenses because their image height is proportional to the scan angle () and not the tangent of the angle.
Scan Linearity:
The degree to which the performance of a particular scan lens design follows the equation Scan Length = 2*field coverage = f*2(). The scan linearity of a system can be defined using both the percent position error and percent velocity error techniques.
Seidel Aberrations:
The group of primary aberrations in lenses, including coma, astigmatism, curvature of field, distortion, spherical and chromatic.
Short Pass:
Interference filter type which efficiently passes radiation whose wavelengths are shorter than a specific wavelength, but not longer.
Shutter:
A device for controlling the amount of time a light-sensitive medium is exposed to light.
Single-Layer Coating:
A thin coating that reduces or eliminates reflections at an air-glass surface, such as MgF2.
Slit:
An aperture, typically rectangular in shape, whose length is large compared to its width. Apertures, generally small compared to the light source, may have fixed or adjustable shapes through which radiation enters or exits an instrument.
Snell's Law of Refraction:
Describes the way a ray of light changes direction at a surface between two media that have different indices of refraction. The angle of incidence of the ray is measured from the normal, or perpendicular, to the surface. A ray moving from a low-index medium to a high-index medium bends toward the normal; from high-index to low, it bends away from the normal.
Spatial Filtering:
Enhancing an image by increasing or decreasing its spatial frequencies.
Specific Gravity:
The dimensionless ratio of the mass of an object to the mass of an equal volume of water at 4°C or other specified temperature.
Spectrometry:
Breaking up white light into its constituent wavelengths and measuring them on a calibrated scale.
Spectrophotometry:
Measuring the reflection or transmission of light for each component wavelength in the spectrum of a specimen.
Spherical Aberration:
An optical defect caused when rays of light passing through the curved surface of a lens near its edge converge at a point closer to the lens than those passing through its center (negative aberration); when the outer zone has a longer focal length than the center, the aberration is said to be positive.
Spot Size:
See Blur Circle.
Striae:
An imperfection in optical glass characterized by streaks of transparent material of a different refractive index than the body.
Substrate:
The underlying material to which an optical coating is applied.
Surface Contour:
The outline or profile of a surface.

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