Dictionary of optical terms used:

Add: The addition of bifocals to a prescription or corrective lens.

Astigmatism: The word literally means “without a point”. It denotes a condition of the eye in which waves of light are not focused at one point, as in the normal eye.

Axis: An axis is an imaginary straight line drawn though a body, or a system, around which the body or system groups itself symmetrically. It is also a number (0 – 180) that gives the location of the cylinder power.

Bifocal: Literally, having two foci. The term is specifically applied to lenses made up of two parts, each having a different refracting power, and therefore a different principal focal point; one part, as a rule, being intended for distant vision and the other for reading and close work.

Base: A line dropped from the apex of a triangle perpendicularly to its base.

Corrective Lens: A standard lens made in ˝ diopter increments in minus or plus diopters.

Cylinder: Mathematically this figure of solid geometry is generated by a rectangle being completely rotated around one of its sides. It is a special method to correct lenses for astigmatism.

Diopter: The unit of focalizing power, One diopter is the power to focalize a neutral wave of light at a distance of 1 meter.

Distance: The distance between the principal plane of a lens and its principal focal point.

Far/Near: The distance in space along which there is clear vision of an object, in spite of the disparity of the retinal points. This distance depends upon the width of the pupil, the visual acuity, and the diopterism of the eye.

Far sightedness: (Hyperopia) That condition of ocular refraction in which the posterior principal focus of the eye lies back of the retinal plane, so that neutral light waves instead of focusing on the retina, fall on the retina in diffusion circles of unfocused wave.

Fungus growth:  Fungus growth on lenses in the tropics sometimes has been a problem and these is no evidence that any of the thin film coatings are a deterrent to the mycelia attacking the glass. Attempts to clean glass surfaces after they have been contaminated by mold will fail, as the glass itself will have been damaged and prevention of the mold growth is the only effective cure to the problem. Most fungal damage arises during storage, and silica-gel bags will reduce the ambient humidity and so make attack less likely.

Lens: In general, a lens may be defined as any transparent body having two opposite surfaces polished, at least on of which is curved.

Minus Diopter Lens: A lens having at least one concave surface. Used to correct Myopia or Nearsightedness.

Nearsightedness (Myopia): That condition of the ocular refraction in which the posterior principal focus of the eye lies in front of the retinal plane, so that neutral light waves, entering the eye in a state of rest, instead of focusing on the retina, comes to a focus before they reach it, re reversed, and fall on the retina in diffusion circles of plus waves.

O.D. (Right eye): Abbreviation for Oculus Dexter

O.S. (Left eye): Abbreviation for Oculus Sinister

Plus Diopter Lens: A lens having at least one convex surface. Used to correct Hyperopia or Farsightedness.

P.D. – Pupillary Distance: The distance between the centers of the two pupils.

Prescription Lens:  A lens made for a mask to the exact prescription of an ophthalmologist or optometrist.

Prism: The prism unit of power is the prism-diopter, represented by the delta sign. A prism of one diopter will deviate a ray of light one centimeter for each meter of distance that it travels.

RX: A certified prescription for eyeglasses by an ophthalmologist or optometrist

Sphere: This word is used to designate, shortly, a lens whose surface is a segment of a sphere. It is the power of the lens expressed in diopter

Tempering: A process of hardening the finished lens either by a chemical or thermal process to provide impact and shock resistance to meet safety standards of ANSI, CEN, USFDA and Federal regulations.

Tunnel Vision: Persons who wear far-sighted (+) lenses will experience what is known as tunnel vision. A person that is accustomed to wearing a far-sighted correction in the eyeglasses has a curved front surface in their glasses. When we install this correction into a diving mask we must use a flat surface with the curved surface towards the eyes. This will cause tunnel vision effect. Due to the curvature and thickness, there is some percentage of magnification. This effect in the mask cannot be eliminated.