OPTREP GLOSSARY
of Optical Terms
Customers often ask the meaning of optical terms and so the
following abridged glossary is offered. It is difficult to
know where to draw the line, short of a book, but the following
should prove to be the most commonly used terms.
A - B - C - D - E - F - G - K - L - M - N - O - P - R - S - T - V - Z
Abbe prism.
An expensive type of roof prism, more compact than the porro
type and used in, for example, "straight through" binoculars.
Named after Ernst Abbe, the optical researcher.
Aberration.
Imperfect lenses will give chromatic aberration (splitting of
the light) or spherical aberration (curved image) which are
the main types but there are also coma, astigmatism and distortion
types.
Accomodation.
Up to middle age, the eye has the ability to increase the
curvature of the crystalline lens, a process called accommodation,
and thereby increase its overall optical power. Correction
outside of this requires spectacles or contact lenses.
Achromatic (or doublet) lens.
Two lenses (usually cemented together) of different glasses
(e.g: Flint and Crown) to cancel out chromatic aberration for
two primary spectral colours.
Angle of Convergence.
The angle formed between a point object being observed and the
two eyes, giving depth perception. The angle increases as the
point becomes closer.
Angle of Reflection.
The angle formed between a reflected ray of light as it leaves
a surface and the vertical to the surface at that point. Equal
to the angle of incidence of the ray.
Angle of Refraction.
The angle between a deviated ray of light that has passed through
the boundary between two transparent media and the vertical
to the boundary surface at the point of emergence in the second
medium.
Anti-reflection.
Substances or coatings deposited on optical surfaces under a
high vacuum preventing inefficient reflection of light. Coatings
include Magnesium Fluoride, Silicon Dioxide, Titanium Oxide,
etc.
Aperture Stop.
Usually a hole in a thin metal plate, somewhere near the centre
of the optical system. Called an iris if adjustable (as in
cameras). It lets the most light through without allowing the
image to degrade at the edge.
Aphakia.
A condition of the eye after the crystalline lens has been removed
because of cataract opacities. Usually possible to insert an
intraocular lens into the eye to compensate for lens removal.
Aplanatic lens.
A lens usually used to shorten the focal length of an achromatic
doublet lens.
Apochromat.
A lens designed to be colour-corrected for three primary spectral
colours.
Aspheric lens.
Aspheric means non-spherical but, in optics, it relates to a
lens which has one or more of its surfaces not spherical but
of a different curvature. More expensive than spherical lenses
but gives fewer errors.
Astigmatic.
If the lens of the eye becomes non-spherical, or astigmatic,
the vision can be corrected with a suitable toric prescription
lens.
Astronomical telescope.
A telescope without the optics needed to invert the image the
"right" way up since this is unnecessary for astronomy.
Axis.
The optical axis is an imaginary straight line joining the centres
of curvature of a lens with, for example, two curved surfaces
(the surfaces each being part of a sphere). It is a longitudinal
axis of symmetry of a lens.
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Back Vertex of a Lens
(and front vertex) where the axis meets the lens surface/s.
Barrel Distortion
(and pincushion distortion). Image where the edges curve
outwards (image where edges curve inwards).
Bins.
An appalling abbreviation for binoculars!
Binos.
Barely acceptable abbreviation for binoculars.
Binocular Fusion.
Sight with two eyes. Enhances 3-D realism and aids distance
judgement.
Biocular Magnifiers.
Usually military and of lower power than binoculars, typically
for viewing night vision optics.
Bloomed lens.
An old term for anti-reflection coated lens.
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Coating.
Of the light falling on a lens surface, some 4 to 6% would be
lost if the lens was untreated. The lost light is reflected
and plays no part in forming the final image. By coating a
lens or prism surface, this loss is reduced and efficiency is
increased. The coating is a film of various materials (e.g:
magnesium fluoride) deposited in a vacuum process. An electron
beam coating method is used to deposit harder coatings such
as silicon dioxide.
Chromatic Aberration.
Because lenses and prisms deviate blue light more than red light,
a rainbow effect or colour spread will result unless achromatic
combinations of optics are used to correct it.
Coherent Light.
Light made up of photons that have the same wavelength and are
all in phase and thereby very energy efficient. An example
is a laser.
Collimation.
Alignment of the optical axis in an optical system. In a binocular
instrument, alignment of both axes and one with the other to
avoid eye strain.
Coma.
Imperfect optical system can make a star appear like a comet,
with a "tail". This aberration can be reduced by stopping down
the lens.
Concave.
A surface of a lens is concave if it curves inwards.
Condenser Lens.
A positive lens of short focal length, usually used to direct
light into a projector lens or the optical system of an instrument.
Convex.
A surface of a lens is convex if it curves outwards.
Cornea.
The first lens of the eye's optical system, followed by the
aqueous humour and the crystalline lens.
Crown Glass.
One of the most suitable and common types of glass used in optics
and often used in conjunction with flint glass (they are of
different refractive indices) to make up achromatic lenses.
Lighter plastic equivalents of crown glass used for spectacle
lenses are Perspex acrylic and CR39.
Crystalline Lens.
The second lens of the eye, behind the cornea.
Cylindrical Lens.
A lens curved in one plane only or one with a curved bias in
one axis.
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Definition.
General term identifying sharpness of detail.
Depth of Field.
Detail in front of and behind the point of optimum focus that
gives a zone of acceptable sharpness.
Depth of Focus Tolerance (in photography).
How flat the film in the camera must be.
Dioptre
(or diopter). Unit of vergence, giving the refracting power
of a lens as the reciprocal of its focal length, in metres.
Minus indicates diverging and plus indicates converging.
Dioptric Power
(or focal power). The higher the dioptric power the higher
the magnification and the shorter the focal length (and vice
versa).
Distortion.
Barrel distortion and pin cushion distortion are the only types
which can occur in a centred lens system (see Barrel Distortion).
Dove Prism.
Prism using one reflection and two refractions.
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ED (or Fluoro-Crown)
glasses giving very high colour correction.
Electro-Optics.
Instruments using interactions between light and electrical
fields (e.g: an image intensifier).
Eye Relief.
Distance from eyelens outside surface to the top of the eyecup
of an instrument, e.g: binocular. Most comfortable when this
distance is between 10 and 20 mm.
Eyecup
(or eyering). A plastic or rubber ring to help position and
steady the eye correctly. Folded down when spectacles are worn.
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Field Flattening.
Reduction or correction of the curvature of an image by use of
a specific lens element (usually negative).
Field Lens.
Usually a single-element lens at or near the focal plane of
another lens (usually an eyelens) to flatten the field.
Field of View.
The extent of an object which a lens system actually images,
usually expressed in degrees (e.g: 5°) or so many metres wide
at a 1,000 metres distance.
Field Stop.
An aperture in a plate or one of the lenses in a lens system
to control light rays from off-axis points of extended objects.
Flat Field Lens.
A highly-corrected lens that virtually eliminates astigmatism
and curvature to give an almost flat image.
Fluoride (or Fluorite) lens
uses one or more element of Calcium Fluoride made from synthetic
crystals to give very high colour correction. Expensive.
Flint.
One of the most suitable and common types of glass used in optics
and often used in conjunction with Crown glass to make up achromatic
lenses (see Crown).
Focal Length.
Distance from rear nodal point of a lens to the rear principal
focus when lens is focused on infinity. Reciprocal of focal
length is the power of a lens.
Focimeter.
Instrument for measuring lenses. Can be of eyepiece, projection
or automatic electronic type.
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Graticule
(Reticule or reticle). Fine lines (wire or markings on glass)
viewed through an optical instrument to facilitate measuring,
aligning, sighting, etc.
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Keratometer.
Instrument for measuring the shape of the eye in order to specify
the shape of a contact lens.
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Lens Testing Chart.
A test target with various designs to point up lens defects such
as barrel or pin cushion distortion, coma, low resolving power
and aberrations.
Loupe.
A simple magnifier to bring the retinal image of a close object
into focus without appreciable magnification.
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Magnesium Fluoride.
A crystalline compound used to make a robust, thin-layer coating
on a lens surface to reduce surface reflection and increase
efficiency. Commonly has a purple reflection on the lens.
Magnification Power.
Ratio of eye's retinal image size, aided by a magnifying instrument,
compared to largest retinal image obtained with the unaided
eye.
Meniscus Lens.
Lens used to shorten the focal length of a doublet.
Myopia.
When the unaided eye can only focus on close objects.
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Nitrogen Purging.
Filling an instrument (particularly an outdoor one such as a binocular
or riflescope) with zero grade or almost pure nitrogen in order
to exclude all moisture and oxygen and so virtually prevent subsequent
fogging and oxidation of the optics
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Object Lens.
The front lens, in an instrument, closest to the object being
observed.
OG.
Object glass, an old term for the object lens.
Ophthalmoscope.
A hand-held instrument for examining the eye to give an approximation
of the power correction needed.
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Panchratic
telescope in which the power can be changed but refocusing is
needed when this is done.
Parallax.
A change in position of the object, as viewed through an instrument,
if the viewing eye is moved. Parallax correction is especially
important for a riflescope.
Parfocal.
A set of eyepieces (e.g: astronomical) or objectives (e.g: in
a microscope) the focus and image sharpness of which is the
same as they are swapped around.
Photochromic.
Photosensitive material that darkens as the light passing through
it strengthens. Useful in spectacles.
Porro Prism.
Most common type of prism ("triangular") which inverts image
and reverses the direction of the light.
Presbyopia.
Usually in middle age the eye cannot focus on near objects such
as print and so corrective spectacles are needed.
Prism.
There are many designs of polished, geometric glass shapes designed
to "fold the light up" in order to make an instrument more compact.
Prisms give the designer many possibilities and can also be
used to invert the image and give a stereoscopic effect by enabling
the objective lenses to be set wider apart than the eyelenses
(in a binocular for example).
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Radiuscope.
Instrument for measuring short-radius optical devices such as
contact lenses.
Real Image.
An image as focused, for example, on a camera film or a projector
screen (as opposed to a virtual image which we see in a mirror).
Reflection.
Redirection of light by a surface which is highly polished.
Refraction.
Redirection of light by its passing from one medium to another
(e.g: from glass to air).
Refractive Index.
Measure of the refracting power of a medium (e.g: 1.6 for water,
1.46 to 1.95 for optical glass, etc.).
Resolution.
Ability of a lens to resolve fine detail.
Resolving Power.
The ability (of an instrument, the eye, etc.) to resolve the
images of two points. The closer the points are together, the
higher the resolving power required. Resolving power is measured
by the angle subtended between the two points and the eye or
instrument. The eye can resolve two points subtended by an
angle of 100 seconds.
Retina.
The inside, photosensitive surface of the eyeball. It has an
array of detectors sensitive to light and colour.
Retinoscope.
A hand-held instrument for examining the retina.
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Slit Lamp.
An instrument for examining the eye.
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Toric Lens.
A prescription lens to correct the eye's vision.
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Vertex of Surface
(or the Pole). The point on the surface of a lens where the optical
axis enters the surface. For a convex surface, for example, it
is the highest point on the surface.
Vignetting.
Use of a field stop in a position which will cut off the light
at the edges to provide a better image.
Virtual Image.
As seen after refraction by a diverging lens or after reflection
in a mirror. Virtual images are used in instruments such as
microscopes, rangefinders, telescopes, etc.
Visual Acuity.
Measure of visual performance in perceiving detail, ascertained
by letter charts.
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Zoom lens.
System in which the power can be varied while the image stays
in reasonably sharp focus.
A - B - C - D - E - F - G - K - L - M - N - O - P - R - S - T - V - Z
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