Extended Optical Dictionary
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- Abbe Number:
The ratio of refractivity to dispersion in an optical medium.
Specifically: (nd-1)/(nF-nC), where n is the index of refraction for the Fraunhofer d, F and C lines, respectively.
An optical defect resulting from design or fabrication error, e.g., coma, distortion,
curvature of field that prevents the
lens from achieving precise focus.
Free of color. Describes an optical system in which chromatic aberration has been
corrected at a minimum of two wavelengths.
A pattern of illumination caused by diffraction at the edge of a circular aperture,
consisting of a central core of light surrounded by concentric rings of gradually decreasing intensity.
Distorted, as in an optical system with different magnification levels or with focal
lengths perpendicular to the optical axis.
Angle of Incidence:
The angle between a ray of light striking a surface and the normal (a line perpendicular
to the surface at that point).
A thin layer of material that, when applied to a lens, increases its transmittance and
reduces its reflectance.
A hole through which light may pass. The aperture stop is that hole in an optical system
limiting the bundle of light able to traverse the system.
Not spherical. To reduce spherical aberration, a lens may be altered slightly so that one
or more surfaces are aspherical.
An aberration in a lens in which the tangential and sagittal (horizontal and vertical)
lines are focused at two different points along the optical axis.
Back Focal Length:
The distance between the last surface of a lens to its back focal plane.
For a filter or thin-film coating, the wavelength range over which transmission is allowed
and possibly maximized.Transmission above or below the bandpass range is restricted by design through absorption
Usually described in terms of transmission level, the bandwidth is the spectral range over
which an interference filter transmits.
An optical device which divides an incident beam into two or more separate and distinct
beams. A beamsplitter may be as simple as an uncoated plano-plano piece of glass inserted in a beam at an angle to
divert a portion of the beam in a different direction. More complex beamsplitters employ coated and cemented right- angle
prisms to separate colors.
The ratio of the curvatures of a lens's two refracting surfaces.
Having two outer surfaces that curve inward.
Having two outer surfaces that curve outward.
The separation of a beam of light into two beams (ordinary and extraordinary) as it passes
through a doubly refracting material or object.
Blocking refers to the filter transmittance outside the bandpass region, and can be
thought of as the degree to which undesired wavelengths are prevented from being transmitted. Filters with deep out-of-band
blocking significantly enhance the signal-to-noise ratio of the system.
The image of a point-source object formed by an optical system on its focal surface. The
precision of the lens and its state of focus determine the size of the blur.
An optical glass containing boric oxide, along with silica and other ingredients, having
relatively high -value and low index of refraction. Some varieties, such as Corning Pyrex® and Schott Tempax®, are
resistant to thermal shock.
A multi-layer antireflection coating with reduced reflection over a broad spectral band.
An optical system containing both reflective and refractive elements.
A periodic structure of thin films comprised of two quarter-wave stack reflectors
separated by a dielectric spacer.Cavities are the building blocks of bandpass filters.
Two different but standard video camera mount configurations. The more prevalent of the
two types is the standard C-mount, which provides a 17.52mm flange focal distance (FFD). The CS-mount typically
provides a 12.5mm FFD.C-mount lenses can be used in CS-mount configurations with a 5mm adapter.
- Charge Coupled Device. A self-scanning semiconductor imaging device which uses
metal-oxide-semiconductor MOS), surface storage and information transfer technologies.
For filters and coatings, the average of the wavelength values at the half-power points of
the transmission band.
The degree to which the optical axis of a lens and the mechanical axis of its mounting
An optical defect in a lens resulting in different wavelengths of light focusing at
different distances from the lens, which can be seen as halos around the image.
The dispersion of white light into its constituent colors. The refractive index of blue
light is higher than that of red light,resulting in a change of image size from one color to the other.
Circle of Least Confusion:
The smallest cross-section of a focused beam of light; the point of best focus for the
The opening in the mount of an optical system that controls the amount of light incident
on a given surface; the entrance pupil of the lens.
Coefficient of Thermal Expansion:
A material property defined as the ratio of the change in length per original length (or
change in volume per original volume) to the incremental change in temperature from a reference. Typically an order of
magnitude higher for metals relative to glasses.
To adjust two or more optical axes with respect to each other; to make rays of light
A beam of light in which all of the rays are parallel to each other.
The aspect of a light source that can be described in terms of hue, brightness and
saturation; the specific property of objects seen as red, yellow or blue as opposed to black, white or gray.
An aberration that occurs in a lens when rays emanating from points not on the optical
axis do not converge, causing the image of a point to appear comet-shaped.
Two or more optical glass elements, sometimes cemented together, shaped to cancel out
aberrations present in either lens alone.
The central angle of a cone of rays converging to or diverging from a point. See Numerical
Two optical elements joined as a pair.
Continuous Wave Irradiation:
Emission of radiant energy (light) in a continuous, rather than pulsed, wave.
The difference in light intensity in an object or image; defined as (Imax - Imin)/(Imax +
Imin), where Imax and Imin are the maximum and minimum intensities.
The bending of light rays toward each other, achieved with a positive (convex) lens.
Describes the surface defects of a lens that are not optically critical and do not impair
its function. Usually described in terms of scratch and dig.
Maximum angle of incidence formed by a ray of light as it passes from a dense to a less
dense medium, e.g., from glass to air, where the critical angle is about 42 degrees. When the critical angle is exceeded,
all the light reflects back to the denser of the two media.
A silicate glass containing oxides of sodium and potassium, used in compound lenses and
spectacles; harder than flint glass, with low index and low dispersion.
Crystalline form of silicon dioxide; very hard with a low expansion coefficient. Transmits
light through the range of 180nm (ultraviolet) to 4.5 micrometers (infrared).
Departure from flatness of a surface. Defined as the reciprocal of the radius of
Dictionary continued ...
C to F
F to M
N to S
T to End