7.5 The Geneva Lens Clock, Lens Materials, Progressive Markings & Spectacle Adjustment
Key Takeaways
- The Geneva lens clock is calibrated for an index of 1.53 and must be corrected when used on other materials.
- True surface power equals the clock reading multiplied by the ratio of the actual material index minus one to 0.53.
- Polycarbonate has the lowest Abbe value of common materials, so it produces the most chromatic aberration in high powers.
- Permanent progressive engravings sit 34 mm apart on the horizontal reference line, 17 mm each side of the fitting cross, and never wear off.
- Frame adjustment changes vertex distance, pantoscopic tilt and optical centre height, so it is an optical intervention as much as a comfort one.
The Geneva lens clock
A Geneva lens clock (lens measure) is a mechanical gauge with three pins: two fixed outer pins and a sprung centre pin. Placed against a lens surface, the displacement of the centre pin relative to the outer pins measures the sagittal depth over a fixed chord, from which the instrument's dial reads surface power in dioptres.
The instrument is calibrated for crown glass, refractive index 1.53. It reads correctly only on a material of that index. On any other material the reading is the sagittal depth expressed as though the material were 1.53, so it must be corrected.
Correction formula:
True surface power = clock reading × [(actual index − 1) ÷ 0.53]
Worked example. A lens clock reads +6.00 D on a polycarbonate surface (index 1.586).
True power = 6.00 × [(1.586 − 1) ÷ 0.53] = 6.00 × (0.586 ÷ 0.53) = 6.00 × 1.106 = +6.63 D.
Worked example 2. The same +6.00 reading on CR-39 plastic (index 1.498):
True power = 6.00 × (0.498 ÷ 0.53) = 6.00 × 0.940 = +5.64 D.
The pattern is worth internalising: on a higher-index material the clock under-reads, and on a lower-index material it over-reads.
What the lens clock is used for:
| Task | How |
|---|---|
| Measure base curve | Read the front surface with the outer pins vertical and horizontal |
| Detect surface astigmatism (toric surface) | Rotate the clock on the surface; a changing reading means a toric surface, and the difference is the surface cylinder |
| Verify a warped lens | Readings that vary across the surface |
| Approximate total lens power | Front surface reading plus back surface reading, corrected for index; a rough check only, since it ignores thickness |
| Check a progressive corridor | Readings increase steadily down the corridor |
The lens clock does not replace a lensometer: it measures surface curvature, not back vertex power, and ignores centre thickness.
Lens materials
| Material | Index | Abbe value | Specific gravity | Notes |
|---|---|---|---|---|
| Crown glass | 1.523 | 59 | 2.54 | Excellent optics, scratch resistant, heavy, shatters |
| CR-39 plastic | 1.498 | 58 | 1.32 | The plastic benchmark; good optics, scratches easily |
| Trivex | 1.532 | 43–45 | 1.11 | Lightest; impact resistant; good optics |
| Polycarbonate | 1.586 | 30 | 1.20 | Highest impact resistance; lowest Abbe, so most chromatic aberration |
| Mid-index | 1.60 | 36–42 | 1.30 | Thinner |
| High index | 1.67–1.74 | 32–36 | 1.35–1.47 | Thinnest; more reflection so anti-reflective coating is essential |
Abbe value measures chromatic dispersion — a lower number means more colour fringing. Polycarbonate's Abbe of about 30 is why a high-power polycarbonate lens produces noticeable colour fringes in peripheral gaze, and why a patient in a strong prescription may prefer Trivex or CR-39 despite the extra thickness.
Safety. Polycarbonate and Trivex are the materials of choice for children, monocular patients, sports use and any occupational eye hazard, because of their impact resistance. A monocular patient should be in polycarbonate or Trivex with a safety frame as a standing recommendation.
Reading progressive lens markings
Every progressive lens carries two kinds of marking.
Permanent engravings — faint, laser-etched, and never removable. They sit on the horizontal reference line 34 mm apart, that is 17 mm on each side of the fitting cross or prism reference point — a separation set by international standard (ISO 8980-2) so that it matches the spacing of a focimeter's outer ink markers. To see them, hold the lens at an angle against a dark background under a focused light, breathe gently on the surface, or use a purpose-made progressive identifier with oblique illumination.
- The temporal engraving is usually accompanied by a manufacturer and design code.
- The nasal engraving is usually accompanied by the add power, printed as a number such as 20 for +2.00 D or 275 for +2.75 D.
Removable ink markings — applied at manufacture and wiped off at dispensing: the fitting cross (aligned to the pupil centre), the distance reference circle, the near reference circle, the prism reference point, and the horizontal alignment line. Once wiped off they can be reconstructed from the permanent engravings using the manufacturer's identification chart.
Why this matters clinically. A patient complaining that a progressive "only works in a narrow slot" or "swims" may have a fitting height error. Locating the engravings and reconstructing the fitting cross tells you where the corridor actually sits relative to the pupil. A fitting cross set too low pushes the patient's line of sight into the corridor for distance; too high pushes the distance zone above the pupil.
Frame adjustment as an optical intervention
The blueprint lists making simple spectacle adjustments and repairs under Ophthalmic Patient Services, but the optics belong here, because each adjustment changes what the patient sees.
| Adjustment | Optical consequence |
|---|---|
| Vertex distance (lens closer or further from the eye) | Changes effective power; matters above ±4.00 D. Closer means more effective plus removed from a plus lens and more minus needed in a minus lens |
| Pantoscopic tilt (lower rim closer to the face) | Conventionally 8 to 12 degrees; excessive tilt induces unwanted cylinder and shifts effective optical centre height |
| Face-form / wrap | Excessive wrap induces cylinder and prism, especially in high powers and wrapped sports frames |
| Optical centre height | Mis-set height produces vertical prism by Prentice's rule and is a common cause of asthenopia |
| Segment or fitting cross height | Determines when the patient enters the near zone; too high obstructs distance, too low makes reading uncomfortable |
| Temple length and bend | Affects how the frame sits and therefore all of the above |
| Nose pad position | Changes vertical height of the optical centre and the vertex distance |
Practical adjustment rules:
- Heat plastic frames gently with a warm air frame warmer before bending; never force a cold zyl frame — it will crack.
- Adjust metal frames cold at the temple and bridge, using the correct pliers with protective jaws.
- Check the frame is level by viewing from behind, both temples folded flat.
- Verify the patient's pupils sit at the marked optical centres after adjustment, not before.
- Tighten screws with a properly sized driver and consider thread-locking for repeat loosening.
- Replace worn nose pads; a slipped frame lowers the optical centres and can generate prism.
- Check the frame sits without the lashes touching the lens and without the lower rim resting on the cheek.
After any significant adjustment, re-verify the fit on the patient and document what was changed.
A Geneva lens clock reads +5.00 D on a high-index surface with a refractive index of 1.67. What is the true surface power?
A patient in a high-power polycarbonate lens reports coloured fringes around objects in side gaze. What explains this?
Where on a progressive lens is the add power usually printed?
Rotating a Geneva lens clock on a lens surface produces readings that change from +6.00 to +8.00 D. What does this indicate?
Why is an incorrect optical centre height a clinically significant fitting error?