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Glossary: S

Sag

The surface profile of a surface. For spherical surfaces, it is calculated from diameter and radius of curvature. For aspheric surfaces, it is calculated from radial distance from the optical axis, curvature, conic constant, and aspheric terms.

  See also Radius of Curvature

Sagittal Plane

The radial component of the spot (or ray bundle) in the z-plane (perpendicular to image axis) at a given image height.

Scheimpflug Condition

Condition wherein both image and object planes are tilted with respect to the optical axis, in order to focus over a depth despite the camera being oblique to the object. 

Seamed Edge

A type of bevel in which the edge of an optical component is cut and conditioned to remove sharp corners. It is less toleranced than a ground edge.

  See also Bevel , Ground Edge

Semi-Plan Objective

A type of microscope objective with a semi-planar design where roughly 80% of the objective's field of view appears flat.

Sensing Area

The size of a camera sensor's active area, specified as the horizontal dimension by the vertical dimension (H x V). It is important in determining the proper lens magnification required to obtain a desired field of view.

  See also Magnification

Sensor Format

Refers to the horizontal nominal inch-based dimension naming scheme for image sensors, not the physical sensor dimensions above 4/3".

Short Wave Infrared (SWIR)

The portion of the electromagnetic spectrum from 900 to 1800nm. Definition may differ by application.

Shortpass Filter

A type of filter where the transmission band is a low wavelength range, typically lower than the region blocked.

  See also Filter , Longpass Filter

Shunt Resistance

The effective resistance of a photodiode. It represents the slope of the I-V curve at the origin (V=0).

  See also Photodiode

Shutter Speed

Corresponds to the "exposure time" of the sensor chip that controls the amount of light incident on it.

Signal Format

The standardization of transmission and reception of analog signals, typically determined by geographical region. Signal parameters include synchronization, number of scanlines, bandwidth, black and blanking levels, frame rate, aspect ratio, etc. Video components of one signal format are not compatible with those of another.

Signal-to-Noise Ratio (SNR)

Comparison of signal power to noise power, including dark current and other unwanted/unpredictable fluctuations. In imaging systems, it is expressed in decibel values for analog systems, and "bit" values for digital systems. Industrial cameras have no less than 46dB SNR, yielding 256 steps of contrast in the image. This corresponds to an 8 bit digital signal (28 = 256). Every 6dB of an analog signal converts to 1 bit when digitized.

Silicon Detector

An electro-optic component that transforms light energy into an electrical current due to the photovoltaic effect.

  See also Spectral Response

Singlet Lens

A lens with only one optical component such as a PCX, DCX, PCV, or DCV lens.

Sliding Focusing Mechanism

Mechanical method of changing the focus of an optical assembly, such as a beam expander or imaging lens, in which optical components are not rotated during translation, which minimizes beam wander and image runout. However, this requires more complex mechanics than rotating focusing mechanisms, which rotate the optical elements during translation, typically increasing the cost of sliding focusing mechanisms. Helicoid barrels are a common type of sliding focusing mechanism.

S-Mount

A mounting thread denoted by M12 x 0.5mm, with no specified flange distance.

  See also CS-Mount , T-Mount , C-Mount

Snell's Law

Physical law governing the refraction (bending) of light rays at an interface between two regions of different index of refraction.

Solid State Laser

A laser using a transparent substance (crystalline or glass) as the active medium, doped to provide the energy states necessary for lasing. The pumping mechanism is the radiation from a powerful light source, such as a flash lamp. 

  See also Gas Laser , Laser

Spatial Filter

An assembly that is used in conjunction with laser applications to eliminate spatial noise. A pinhole is used to select only the central peak of the intensity pattern from a focused spot. The size of the pinhole is selected based upon the wavelength, objective focal length, and input beam diameter.

Spatial Frequency

Method of quantifying object size by a density (e.g. linepairs per millimeter) rather than a size. Allows for more precise way of describing resolution as it considers the necessary space between defects that allows for their detection, as well as describing lens performance by way of a transfer function. 

Spatial Mode

The configurations of energy storage, relative to the structure of a laser resonator, that define the relative intensity distribution of the laser beam.

Spectral Irradiance

Irradiance per unit wavelength interval at a given wavelength, expressed in watts per unit area per unit wavelength interval.

Spectral Response

Measure of a detector's signal during exposure to radiation of a constant power level and varying wavelength.

Spectrograph

An optical instrument for forming the spectrum of a light source and recording it on a film. The dispersing medium may be a prism or a diffraction grating.

  See also Ghost Image

Spectrometer

An optical instrument used to measure and analyze the distribution of radiation in a particular wavelength region.

  See also Ghost Image

Specular

A reflection which comes from a surface at an angle equal to the angle of incidence. An example is a high quality mirror.

Spherical Aberration

A type of optical aberration in which light rays from the outer portion of a lens focus either in front of (undercorrected) or behind (overcorrected) the focus point of the rays from the center portion of the lens. Unlike most other monochromatic aberrations, spherical aberration appears on-axis as well as off-axis.

Sphericity

A manufacturing specification that tolerances the roundness or deviation from being a perfect sphere of a spherical surface such as a ball lens.

  See also Ball Lens

S-Polarization

The linear polarization state which is perpendicular to the plane of incidence.  The plane of incidence is normal to the surface or interface upon which light is incident.  If the light is incident upon the interface at an angle, the light will be oscillating in such a way as to appear to be “skipping” across the surface.

Static Alignment

The degree to which a laser beam is aligned parallel to the axis of the housing.

Stopband

A wavelength interval used to denote a spectral region of energy that is not transmitted by a filter.

  See also Filter

Straight Line Accuracy

For ball bearing stages, it denotes error in the horizontal plane (left and right movement in direction of travel).

Straightness

A measure of linear deviation in a translation stage in the two axes other than the direction of travel. It can be thought of as unintended linear travel in the two axes other than the direction of travel.

Stray Light

Energy outside of the field of view of an optical system that scatters off of the edges of an optical or mechanical component and reaches the sensor in the form of background noise. This can decrease an image's contrast by lightening the background.

Strehl Ratio

The ratio of the illuminance at the peak of an aberrated diffraction pattern to that at the peak of an aberration-free system.

Stress-Optical Coefficient

A proportionality constant used to calculate stress birefringence that depends on glass type, wavelength and temperature.

Surface Accuracy

An indication of the peak-to-valley flatness of a surface given in terms of waves (a multiple or part of a reference wavelength). For example, a specification of 1/4 wave means that the flatness of the surface deviates by one quarter of the wavelength used to test the surface. If the wavelength was 550nm, the flatness would be held to within 140nm in order to produce a surface accuracy of 1/4 wave.  Also given in RMS (root mean squared)  where the peak-to-valley values are squared and then the square root is taken of their mean.

  See also Reference Wavelength

Surface Flatness

A type of surface accuracy specification that measures the deviation of a flat surface such as that of a mirror, window, prism, or plano-lens. This deviation can be measured using an optical flat. The deviations in flatness are often measured in values of waves (λ), which are multiples of the wavelength of the testing source. One fringe corresponds to ½ of a wave. 1λ flatness is considered typical grade, λ/4 flatness is considered to be precision grade, and λ/20 is considered high precision grade.

  See also Optical Flat

Surface Quality

A specification of allowable flaws in the surface on an optic indicated by a hyphenated number (i.e. 60-40). The first number is referred to as the scratch number and quantifies defects of a long nature, such as scratches; the second number is the dig number and indicates round defects such as pits and dents. Scratch-Dig numbers for surface quality conform to the US government standards MIL-PRF-13830B.

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