**This article presents the glossary of optical lens terms.**

## Aberration, Achromatic, Airy Pattern, and Annulus

Aberration is a defect in the image forming capability of a lens or optical system. Achromatic is free of aberrations pertaining to color or wavelength. Airy Pattern is the diffraction pattern imaging a point source, and is created by a perfect lens with a circular aperture. Annulus is the image bounded by, and encompassing, the area between two concentric circles.

## Aperture, Astigmatism, Bandwidth, and Brightness

Aperture is an opening in an optical system to restrict the amount of light traversing the system. Astigmatism is a lens aberration that leads to off-axis light bundles to focus to an elliptical spot. Bandwidth is the range of wavelengths over which an optical system is engineered to function. Brightness is described as an attribute of a visual perception, according to which an area seems to emit less or more light.

## Bundle, Chromatic, Coma, Complex and Compound Lens

Bundle is a cylindrical or conical cluster of light rays. Chromatic is relating to color or wavelength. Coma is an off-axis lens aberration, due to a disparity of lens focal lengths as a function of aperture zone or annulus. Complex lens is a lens assembly comprising a number of compound lenses. Compound lens is a lens assembly containing several simple lens components.

## Concave and Concentric

Concave is a solid, curved surface resembling the inside surface of a sphere. Concentric is a curved line or surface, featuring a common center of curvature.

## Condenser Lens, Conjugates, Contrast, Convex and Crown Glass

Condenser lens is a lens assembly devised for collecting energy from a light source. Conjugates are a pair of points that are perpetually related to each other. Contrast is the obvious disparity in perceived brightness between two areas within the field of view. Convex is a solid curved surface resembling the outside surface of a sphere. Crown glass is a type of optical glass exhibiting relatively low refractive index and dispersion.

## Diffraction, Diffraction Limited Lens, and Dispersion

Diffraction is the deviation of the direction of propagation of radiation, identified by the wave nature of radiation, and takes place when the radiation passes the edge of an obstacle. A diffraction limited lens has negligible residual aberrations. Dispersion is the difference in the refractive index of a medium, as a function of wavelength.

## Distortion, Doublet, and Effective Focal Length

Distortion is an off-axis lens aberration varying the geometric shape of the image, owing to the difference of focal length as a function of field angle. Doublet is a lens assembly consisting of two simple lens elements. Effective focal length of the lens is the distance between the principle point and the focal point.

## Entrance Pupil, Eyepiece, F/number, and Field of View

Entrance pupil is the limiting aperture spotted when looking into a lens, from the source or object position. Eyepiece is a lens assembly designed to view the image, created by preceding optics within the system. F/number is the ratio of the focal length to the diameter of the axial beam on the entrance pupil, while the object is at infinity. Field of view is part of an extended object that is pictured onto the detector of an optical system.

## Finite Conjugate, Flint, Focus, Fresnel Reflection, and Fresnel Losses

Finite conjugate is a conjugate point at a real, or measurable, distance. Flint is a type of optical glass, having a unique combination of high refractive index and dispersion. Focus is the spot of convergence of the wavefront, originating at a point on the source to create a point image. Fresnel reflection losses are the fraction of the incident radiation, specularly reflected by an interface between two media of different refractive indices.

## Gaussian Optics, Geometric, Glare, Hyperfocal, and Illuminance

Gaussian optics is the theory of elementary properties of lenses and mirrors, considering only rays which lie close to the axis. Geometric in optics is the analysis based on the tracing of light rays in place of wavefront analysis, applied in the field of physical optic. Glare is a random illumination incident on the image plane that is not related to the primary image. Hyperfocal is the distance at which a lens may be focused to generate satisfactory image quality over an extended range of object distances. Illuminance is the light density, or luminous flux per unit area, incident on a surface.

## Image, Infinite Conjugates, Iris, Lateral Color, and Lens

Image is the resemblance of an object generated by a lens or optical system. Infinite conjugates are the object and image points for a lens focused at infinity. Iris is an opening that is adjustable, and limits the amount of light traversing an optical system. Lateral color is an off-axis lens aberration, due to a deviation in lens focal length as a function of wavelength. Lens is an optical component used for converging or diverging an incident wavefront.

## Light, Magnification, Milliradian (mrad), and Modulation Transfer Function

Light is an electro-magnetic radiation consisting of wavelengths in the spectral bandwidth perceived by the human eye. Magnification is the ratio of image to object size in an optical system. Milliradian (mrad) is equal to 0.0573 degrees, or 1/1000th of a radian. Modulation transfer function mathematically represents the ability of an optical system to generate a faithful image of a sine wave target.

## Monochromatic, Nanometer, Numerical Aperture, and Objective Lens

Monochromatic means having a single wavelength or color. Nanometer (nm) is equal to 10^{-9}m. Numerical aperture is a measure of the light collecting power of a lens. Objective lens in an optical system collects light from a source or object and creates an image of it.

## Opaque, Optical Path Difference, Paraxial, and Petzval Curvature

Opaque means non-transparent and incapable of transmitting light. Optical Path Difference (OPD) is the departure from a perfect sphere of a wavefront that is converging toward, or diverging from, a focus. Paraxial is close to, or in the vicinity of, the optical axis. Petzval curvature is the natural tendency for a lens for generating its final image on a curved instead of flat surface.

## Point Source, Point Spread Function, Polychromatic, and Power

Point source is a source of light that subtends a negligible angle in object space. Point spread function is the 3D graphical representation of the image of a point source generated by a lens or optical system. Polychromatic means having many wavelengths. In an optical sense, Lens power is the reciprocal of the lens’ focal length.

## Radian, Reflection, Refraction, and Residual

Radian (rad) is a unit of angular measurement equivalent to the angle subtended at the center of a circle by an arc, having a length equivalent to the radius of that circle. Reflection is a process involving redirection of some energy of the incident light towards the origin of the light because of reflection at the interface. Refraction is a process whereby the direction of radiation is altered, due to variations in its velocity of propagation. In optical design, residual is the aberration that remains even after optimizing the design.

## Resolution, Sagittal, Speed, and Stop

Resolution is the ability to resolve information, or differentiate fine features, within an image. In optics, a sagittal line is one originating radially from the image center. Lens speed represents its light collecting ability, which influences the image brightness. Lens speed is expressed as F/number. Stop is an aperture in an optical system to restrict the amount of light transmitted and/or imaged.

## Stray Light, Tangential, Throughput and Transmittance

Stray light is the light traversing an optical system to the image plane that is not related to the main image. Tangential means contacting the circumference of a circle at one point at right angle to the circle's radius at the point of contact. Throughput, or etendue, means the efficiency of an optical system in terms of collecting and delivering the luminous flux. Transmittance means the fraction of collected light that has traversed an optical system.

## Veiling Glare, Wavefront and Wavelength

Veiling glare is the diffuse of unfocused light at the image plane that contributes to reduced contrast and resolution. When light travels from a point source the wavefront represents that surface, consisting of all points equidistant in time (phase) from the source. Wavelength is the distance in the direction of propagation of a periodic wave between two successive points at which the phase is the same.

## About Oriel Instruments

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

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