Lens Material
This common plastic lens material has a refractive index of 1.49
What is CR‑39?
Unequal refractive power between eyes is called this.
What is anisometropia?
Measurement between the innermost edge of the right lens and the innermost edge of the left lens
What is dstance between lenses
This bifocal design has a round near segment.
What is a R seg?
This optical appliance is prescribed to enhance near vision for low vision patients.
What is a magnifier?
This lens material is most impact‑resistant and often used for safety eyewear.
What is polycarbonate?
Unequal retinal image sizes perceived by the brain is called this.
What is aniseikonia?
This measurement is taken from the lower eyelid margin to the top of a bifocal segment.
What is segment height?
This bifocal design provides the widest near vision zone.
What is an executive bifocal?
This protective eyewear is used in industrial settings to prevent ocular injury.
What are safety glasses?
This material combines impact resistance with superior optical clarity compared to polycarbonate.
What is Trivex?
Helps with convergency insufficiency
What is base‑in used for near?
The horizontal distance between lens centres in a frame.
What is frame PD?
This bifocal design has a D‑shaped near segment.
What is a D‑seg?
This aberration occurs when different wavelengths of light are refracted unequally.
What is monochromatic aberration?
A measure of the change in the speed of light as it passes from a vacuum (or air as an approximation) into the material.
What is refractive index?
The distance from the apex of the cornea for the back surface of the lens.
What is back vertex distance?
This tilt of the frame front relative to vertical improves near vision comfort.
What is pantoscopic tilt?
This lens type provides a continuous change in power from distance to near.
What is a PPL/PAL lens?
This aberration results in different magnification across the visual field, often seen in high prescriptions.
What is Distortion?
This lens design reduces spherical aberrations and thickness in high prescriptions.
Incorrect PD measurements can lead to this optical effect, causing discomfort or visual distortion.
What is induced prism?
This tint is commonly used for driving and outdoor sports due to its ability to block horizontal glare.
What are polarised lenses?
This measurement is critical for fitting progressive lenses accurately.
What is fitting cross height?
This tint darkens in response to UV light and returns to clear indoors.
What are photochromic lenses?