Opticians Glossary of Terms Questions and Answers
"A" measurement
The horizontal dimension of the boxing system rectangle that encloses a lens or lens opening.
Abbe value
The most commonly used number for identifying the amount of chromatic aberration for a given lens material. The higher...
Opticians Glossary of Terms Questions
and Answers
"A" measurement - answer The horizontal dimension of the boxing system rectangle
that encloses a lens or lens opening.
Abbe value - answer The most commonly used number for identifying the amount of
chromatic aberration for a given lens material. The higher the Abbe value, the less
chromatic aberration present in the lens. Abbe value is the reciprocal of dispersive
power and is symbolized by the Greek letter nu, or v. Synonyms: nu value,
constringence.
Aberration - answer the resulting degradation of an image that occurs when a point
source of light does not result in a single-point image after going through the lens or
lens system.
Absolute refractive index - answer the ratio of the speed of light in a vacuum to the
speed of light in another medium. "formula = speed of light in a vaccum / speed of light
in medium = absolute index of refraction".
Actual power - answer Synonym for true power.
Near addition - answer The power that a lens segment has in addition to that power
already present in the main portion of the lens.
Horizontal alignment - answer A lack of deviation of the two datum lines in a pair of
spectacles from a single horizontal plane (neither lens higher than the other when
viewed from the front).
Standard alignment - answer An impersonal standard, independent of facial shape,
for the alignment of spectacle frames. ie: "Four Point Touch"
Vertical alignment - answer Lack of deviation of lenses from the vertical plane (one
being neither further forward nor backward than the other).
Vertex power allowance - answer The amount by which the front surface curvature of
a lens must be flattened to compensate for a thickness-related gain in power.
Amethyst Contrast Enhancer (ACE) - answer A selectively absorbing glass
developed by Schott said to enhance contrast and be advantageous for target and trap
shooting, hunting, computer terminal viewing, skiing, and bird watching. The lens allows
highest transmission in the blue, green, and red regions of the spectrum.
,Apical angle - answer The angle formed by the junction of two nonparallel prism
surfaces.
Crest angle - answer The angle from the top of the nose between the eyes to the tip
compared with the vertical plane of the face. (Because the line from the top of the nose
to the tip is not always straight, the area where the bridge of the frame rests will become
the frame of reference.)
frontal angle - answer (1) The angle with which each side of the nose deviates from
the vertical. (2) The angular amount the nosepad face deviates from the vertical when
the frame is viewed from the front.
Pantoscopic angle - answer (1) In standard alignment, that angle by which the frame
front deviates from the vertical (lover rims farther inward than upper rims) when the
spectacles are held with the temples horizontal. (2) In fitting, the angle that the frame
front makes with the frontal plane of the wearer's face when the lower rims are closer to
the face than the upper rims.
Antonym: retroscopic angle; synonym: pantoscopic tilt.
Retroscopic angle - answer The angle that the frame front makes with the frontal
plane of the wearer's face when the lower rims are farther from the face than the upper
rims.
Antonym: pantosopic angle; synonym: retroscopic tilt.
Splay angle - answer (1) The angle formed by the side of the nose with a straight
anterior-posterior surface that would bisect it vertically (also called transverse angle)/ (2)
The angle the face of a nosepad makes with a plane perpendicular to that of the frame
front when viewed from above.
Temple fold angle - answer The angle formed when a temple is folded to a closed
position.
Vertical angle - answer When viewed from the side, that angle formed between the
plane of the lenses and the long axis of the adjustable nosepads.
Angle of deviation - answer The difference between the angle of incidence and the
angle of refraction.
Anisometropia - answer A condition in which one eye differs significantly in refractive
power form the other.
ANSI - answer American National Standards Institute
Aperture - answer (1) An opening or hole that admits only a portion of light from a
given source or sources. (2) The central, optically correct portion of a lenticular lens.
, Lens aperture - answer The portion of the spectacle frame that accepts the lens.
Synonym: lens opening.
Apex - answer The junction point at which the two nonparallel surfaces of a prism
meet.
Aphakic - answer A person or eye whose crystalline lens has been removed.
AR - answer Antireflection coating
Arm - answer In a semirimless mounting, the metal reinforcement that follows the
upper posterior surface of a spectacle lens and joins the centerpiece to the endpiece.
Synonyms: bar, browbar.
Guard arm - answer A synonym for pad arm.
Pad arm - answer A metal piece that connects an adjustable nosepad to the front of
a frame.
Aspheric - answer A nonspherical surface. An aspheric lens surface generally
decreases in power peripherally to correct for aberrations found in the periphery of a
lens.
Full-field aspheric - answer An aspheric lens that begins its asphericity where the
small central spheric region leaves off. It continues in its asphericity to the edge of the
lens blank.
Astigmatism - answer The condition in which light focuses as two line images at
different distances along the optic axis instead of at one single point. These two-line foci
are at right angles to one another.
Oblique astigmatism - answer The lens aberration that occurs when rays from an off-
axis point pass through the lens and light focuses as two line images instead of a single
point.
Synonyms: radial astigmatism, marginal astigmatism.
ASTM - answer American Society for Testing and Materials.
Axis of a cylinder - answer An imaginary reference line used to specify cylinder or
spherocylinder lens orientation and corresponding to the meridian of least refractive
power.
Optical axis - answer The line that passes through the center of a lens on which the
radii of curvature of the front and back surfaces fall.
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