The Eyewear Glossary: Every Term Explained
Reference
The Eyewear Glossary: Every Term Explained
Acetate, accommodation, base curve, pantoscopic tilt. A plain-English reference for every term you will meet on a product page, a prescription or an optician's counter.
In this article
- How to use this glossary
- Frame anatomy
- Frame materials
- Lens technology
- Prescription notation
- Measurements and fit
- Clinical terms you may encounter
- Frequently asked questions
How to use this glossary
Eyewear carries vocabulary from three separate worlds: manufacturing, optics and clinical practice. A product page borrows from all three at once, which is why a straightforward purchase can feel opaque.
Terms below are grouped by where you will meet them: on the frame, on the lens, on your prescription, and in measurements.
Frame anatomy
| Term | What it means |
|---|---|
| Temple | The arm running from the hinge to behind the ear |
| Temple tip | The curved end section resting behind the ear |
| Bridge | The section spanning the nose between the two lenses |
| Rim | The frame material surrounding each lens |
| End piece | The short extension where the front meets the hinge |
| Hinge | The joint allowing the temple to fold |
| Nose pad | Small pads contacting the nose, adjustable on metal frames |
| Pad arm | The small metal arm holding a nose pad |
| Rimless | No rim, lenses mounted directly to bridge and temples |
| Half rim | Rim on the upper edge only, lower edge held by wire or groove |
| Wrap | A frame curving markedly around the face |
More detail on how these parts affect comfort is in our guide to the anatomy of sunglasses.
Frame materials
| Term | What it means |
|---|---|
| Acetate | Cellulose-based plastic, rich colour depth, holds adjustment |
| CP | Cellulose propionate, injection moulded, lighter than acetate |
| TR90 | Flexible thermoplastic, very light, resists impact |
| Monel | Common nickel alloy used in metal frames |
| Titanium | Light, strong and corrosion resistant metal |
| Stainless steel | Durable metal with good corrosion resistance |
| Bio-acetate | Acetate using plant-derived plasticisers |
Lens technology
| Term | What it means |
|---|---|
| UV400 | Blocks all wavelengths up to 400 nanometres |
| Polarized | Filters horizontally reflected light to cut glare |
| Photochromic | Darkens in response to ultraviolet light |
| Gradient | Tint fading from darker top to lighter bottom |
| Mirrored | Reflective outer coating reducing transmitted light |
| Lens category | Darkness class from 0 to 4 |
| Anti-reflective | Coating reducing surface reflections |
| Hydrophobic | Coating repelling water and reducing smearing |
| Oleophobic | Coating resisting oils and fingerprints |
| Polycarbonate | Impact-resistant lens material, index 1.59 |
| CR-39 | Standard plastic lens material, index 1.50 |
| Refractive index | How strongly a material bends light |
| Abbe value | Measure of colour dispersion, higher is better |
| Base curve | The curvature of the front lens surface |
The lens index guide covers refractive index in detail, and lens categories covers the darkness scale.
Prescription notation
| Term | What it means |
|---|---|
| OD | Oculus dexter, the right eye |
| OS | Oculus sinister, the left eye |
| OU | Both eyes |
| SPH | Sphere, the main correcting power |
| CYL | Cylinder, the astigmatism correction |
| AXIS | Orientation of the cylinder, from 1 to 180 degrees |
| ADD | Additional near power for reading |
| PD | Pupillary distance, between pupil centres |
| Prism | Correction for eye alignment |
| Dioptre | The unit of lens power |
How to read a full prescription slip is covered in this guide.
Measurements and fit
| Term | What it means |
|---|---|
| Lens width | Horizontal width of one lens in millimetres |
| Bridge width | Distance between the two lenses |
| Temple length | Length of the arm, commonly 135 to 150 mm |
| Lens height | Vertical depth of the lens |
| Total width | Full width of the frame front |
| Pantoscopic tilt | Forward tilt of the lens plane |
| Vertex distance | Gap between lens and eye surface |
| Face form | Horizontal curvature of the frame front |
| Asian fit | Design with raised nose pads and adjusted tilt |
The three-number string printed inside the temple is explained in how to read the numbers on your sunglasses.
Clinical terms you may encounter
| Term | What it means |
|---|---|
| Myopia | Short sight, distant objects appear blurred |
| Hyperopia | Long sight, near objects harder to focus |
| Astigmatism | Uneven corneal curvature causing distortion |
| Presbyopia | Age-related loss of near focusing ability |
| Accommodation | The eye's ability to change focus |
| Photophobia | Discomfort or pain in bright light |
| Photokeratitis | Ultraviolet burn to the cornea |
| Cataract | Clouding of the eye's natural lens |
| Dry eye | Insufficient or poor quality tear film |
Important
Clinical terms here are given for general understanding only. Diagnosis belongs with a qualified optometrist or ophthalmologist.Frequently asked questions
Q1 What do the three numbers on sunglasses mean?
They are lens width, bridge width and temple length in millimetres, usually printed inside the temple arm as something like 52-18-140.
Q2 What is the difference between OD and OS?
OD is oculus dexter, the right eye. OS is oculus sinister, the left eye. OU refers to both eyes together.
Q3 What does base curve mean?
It describes the curvature of the front surface of a lens. Higher base curves are flatter to the face's contour and are used in wrap styles.
Q4 What is Abbe value?
A measure of how much a lens material disperses light into its component colours. Higher values mean less colour fringing at the edges of vision.
Q5 What does Asian fit mean?
A frame design with raised nose pads, adjusted pantoscopic tilt and often a wider bridge, intended for lower nose bridges and higher cheekbones.
Q6 What is vertex distance?
The distance between the back of the lens and the front of the eye. It affects effective lens power, which matters most in stronger prescriptions.
Key takeaways
Eyewear vocabulary comes from three separate fields
Frame measurements are always in millimetres
SPH, CYL and AXIS describe your correction
UV400 and lens category describe different things
Refractive index affects thickness, not protection
Clinical terms need a professional for diagnosis
