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object
The figure seen through or imaged by an optical system. It may contain structures, natural or artificial, or it may be the real or virtual image of an object formed by another optical system. In...
object beam
In holography, the wave of light that illuminates the object to be recorded, which diffracts it to the recording medium, where it interferes with the reference beam, generating the hologram.
object conjugate -> object distance
The distance between the object and the cornea, or the first surface of the objective in an optical device.
object displacement
The movement of an object seen through a refracting prism toward the apex of a prism.
object distance
The distance between the object and the cornea, or the first surface of the objective in an optical device.
object space
In an optical system, the space between the object being viewed and the system entrance pupil.
object working distance
Distance between the front vertex of a lens and the object.
objective
The optical element that receives light from the object and forms the first or primary image in telescopes and microscopes. In cameras, the image produced by the objective is the final image. In...
objective aperture -> clear aperture
The limited light-gathering area of an optical system. The area is normally restricted to an edge or outer surface of an individual component. Also known as free aperture or objective aperture. The...
objective grating
A coarse diffraction grating that is used to form a scale of intensities to determine the relative magnitudes of stars. The grating is placed over the objective of a telescope to form a diffraction...
objective prism
1. A prism used in some instruments to bend light 90° before it enters the objective. 2. A dispersing prism located in front of an astronomical telescope objective to produce spectra of all...
oblate crystal -> negative crystal
A uniaxial, birefringent crystal such as calcite or ruby in which the velocity of the extraordinary ray surpasses that of the ordinary ray. This can also be defined as the index of refraction of the...
oblique error
The image error that results from astigmatism, coma, oblique spherical aberration, lateral color and distortion.
oblique illumination
A common technique in microscopy, oblique illumination is one in which the object is illuminated by a light source that is at an oblique angle (on the side) to the optical axis. This technique is...
oblique spherical aberration
coma, fifth order aberration with on-axis focal point variation with incident off axis ray height position
observatory dome
A hemispherical covering that is rotatable about a central axis. There is a slit opening along one side wide enough to allow a telescope to be directed at any vertical angle up to 90°.
obsidian
An acid-resistant, lustrous volcanic glass, usually black or banded.
occluder
A device that completely or partially restricts the amount of light reaching the eye.
ocean color
Refers to the characteristic hue of the ocean according to the presence and concentration of specific minerals or substances, such as chlorophyll. Atmospheric aberrations of color must be corrected...
octave
In optics, an octave typically refers to a range of frequencies or wavelengths that spans a factor of 2. In other words, when dealing with optics, an octave encompasses a doubling of frequency or...
ocular -> eyepiece
Also known as ocular. The lens system used between the final real image in a visual optical system and eye. It acts as an image magnifier. Many types of eyepieces are known; e.g., the Huygens used in...
ocular accommodation
The physical adaption of the eye lens, by means of ciliary muscle contraction, in order to maintain a clear, in focus image of light and other objects and surroundings at a given distance onto the...
ocular accommodation reflex
The eye's involuntary reaction in the process of accommodation. This reflex is most dramatic in response to looking at a distant object after having previously been focused on a near object or vice...
ocular prism
The prism used in a rangefinder to bend the lines of sight through the instruments into the eyepieces.
ocular surface
That surface of a lens located nearest to the eye.
oculometer
An electro-optical infrared tracker consisting of a beamsplitter, an illuminating source, an image dissector tube and an electronics unit, that automatically acquires and tracks particular eye...
off-axis paraboloid and ellipsoid mirror
Off-axis paraboloid and ellipsoid mirrors are specialized optical components used in various optical systems, particularly in applications requiring precise focusing, collimation, or imaging of...
off-axis paraboloidal mirror -> paraboloidal mirror
A concave mirror that has the form of a paraboloid of revolution. The paraboloidal mirror may have only a portion of a paraboloidal surface through which the axis does not pass, and is known as an...
offset prism
A prism or prism assembly that serves to displace the instrument's optical axis.
ohm
The electrical resistance between two points of a conductor when a constant difference of potential of one volt, applied between these points, produces in this conductor a current of one ampere, the...
oil-immersion objective
A form of high-power microscope objective where the space between the object and the first element is filled with an oil having the same index as that element. This form reduces losses, increases the...
oil-on plate
A polished plano-parallel plate that is contacted to an unpolished glass surface to permit see-through analysis of the material. The plate eliminates effects because of the surface quality of the...
oleophobic
Oleophobic is a term used to describe substances or materials that repel or resist oils. The word oleophobic comes from the Greek words "oleo," meaning oil, and "phobos," meaning fear. Oleophobic...
ondoscope
A glow discharge tube placed on an insulating rod to detect the presence of high-frequency radiation in the vicinity of a transmitter. The tube indicates the presence of the radiation by the glow...
opacimeter
An instrument for measuring opacity.
opacity
Opacity refers to the degree to which something is not transparent or translucent, meaning that it doesn't allow light to pass through it easily. It's a measure of how much a material obstructs the...
opal glass
A material consisting of very small colorless particles imbedded in a clear glass matrix. It is available in two forms: flashed opal and pot opal. The flashed variety consists of a rather thin layer...
opal lamp
A tungsten filament lamp that uses an opal glass bulb to diffuse light.
opaque
A term describing a substance that is impervious to light; the characteristic of a substance that has no luminous transmittance.
opaque projector
An opaque projector is a device used for enlarging and projecting images from opaque objects such as printed pages, photographs, or three-dimensional objects onto a screen or surface. Unlike a...
open bubble
A bubble at a polished surface that has been opened by grinding or polishing.
open optical interface
An interface in an optical network that permits an optical signal to pass without changing the optical signal to an electrical signal and therefore does not limit the signal to a specific protocol.
open-dish method
A measurement method for reflectance by gas ionization in which light passes through a vapor before and after reflection. Detection is accomplished with a phosphor-tipped lightpipe that leads to a...
opening
In morphological image processing, a series of erosions followed by the same number of dilations.
ophthalmic
Pertaining to the human eye.
ophthalmic instruments
A family of specialized instruments used by ophthalmologists and optometrists to study a patient's eyes and prescribe spectacles. It includes the ophthalmoscope to observe and photograph the retina;...
ophthalmic photography
The methods and techniques used to obtain medical photographs of the human eye. To photograph the exterior of the eye, conventional still photographic equipment and procedures are used. Similarly, in...
ophthalmology
Ophthalmology is a branch of medicine that focuses on the anatomy, physiology, and diseases of the eyes and visual system. Ophthalmologists are medical doctors who specialize in the diagnosis,...
ophthalmometer
See eye test apparatus; ophthalmic instruments.
ophthalmoscope
Also referred to as a funduscope, an ophthalmoscope is a specialized instrument used by ophthalmologists for observing and photographing the fundus (interior) of the eye which includes the retina,...

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