Instrumentation

The lensometer is the most important instrument for opticians. It measures and verifies the prescription of finished lenses.

Parts of the Lensometer:

- Eyepiece — Adjustable ocular for focusing the reticle to the examiner's eye. Always adjust FIRST.

- Power Drum/Wheel — Rotates to bring target lines into focus. Reads sphere and cylinder power.

- Axis Wheel — Rotates the target to align with the cylinder axis of the lens.

- Lens Platform/Stage — Where the lens rests for measurement. The back surface faces the examiner.

- Reticle (Crosshairs) — The central target used for centering and reading prism.

- Prism Compensator — Used to measure prescribed prism.

- Marking Device — Three dots that mark the optical center and axis on the lens.

Reading Lens Power:

1. Adjust the eyepiece until the reticle crosshairs are sharp (before placing a lens).

2. Place the lens on the stage with the back surface facing you.

3. Turn the power drum until the first set of lines comes into sharp focus — this is the sphere power.

4. Continue turning until the second set of lines focuses — the difference is the cylinder power.

5. Read the axis from the axis wheel when the sphere lines are focused.

For minus cylinder notation: Read the less minus (or more plus) power first as sphere.

For plus cylinder notation: Read the more minus (or less plus) power first as sphere.

Detecting Prism:

- If the target is centered on the reticle crosshairs = no prism at that point.

- If the target is displaced from center = prism is present.

- Each ring on the reticle = approximately 1 prism diopter.

- The direction of displacement indicates the base direction.

Measuring Add Power (Multifocals):

1. Read the distance power through the upper portion of the lens.

2. Move the lens so you read through the bifocal segment (or progressive near zone).

3. Add power = near reading minus distance reading.

Pupillometer:

A device specifically designed to measure interpupillary distance (PD) accurately.

Advantages over PD Ruler:

- More precise (±0.5mm vs ±1mm with a ruler)

- Measures monocular PDs directly

- Eliminates parallax error

- Provides distance and near PD

Using a Pupillometer:

1. Patient looks into the device at a distant target (for distance PD) or near target (for near PD).

2. The examiner adjusts the measurement markers to align with each pupil center.

3. Both monocular PDs (OD and OS) are read from the scale.

PD Ruler Method:

1. Stand about 40cm (16 inches) from the patient.

2. Hold the ruler against the patient's brow.

3. Close your right eye, align zero with the patient's right pupil center.

4. Open your right eye, close your left eye, read the measurement at the patient's left pupil.

Corneal Reflex Method:

- Shine a penlight at the patient's nose from 40cm.

- Measure the distance between the corneal light reflexes.

- More accurate for patients with strabismus.

Digital PD Devices:

Modern digital pupillometers use cameras and software for automated PD measurement. Many also measure fitting heights for progressive lenses.

Purpose: Measures the curvature of the central cornea (K-readings).

Uses in Opticianry:

- Contact lens fitting (base curve selection)

- Detecting corneal astigmatism

- Monitoring corneal changes over time

Principle: The keratometer measures the size of a reflected image from the corneal surface. A steeper cornea (higher power) produces a smaller reflected image.

K-Readings:

- Expressed as radius of curvature (mm) or diopters of corneal power.

- Two readings are taken — one for each principal meridian.

- The difference between the two readings indicates corneal astigmatism.

- Average corneal radius: approximately 7.80mm (43.25 D).

Typical K-Reading Format:

- 43.00 D @ 180 / 44.50 D @ 090

- This indicates 1.50 D of corneal astigmatism.

Note: Opticians generally do not perform keratometry in most states, but understanding the instrument is part of ABO exam knowledge.

Automatic Edger:

- Cuts and shapes uncut lens blanks to fit specific frames.

- Uses a pattern (physical or digital) of the frame shape.

- Modern edgers use digital frame tracings for precise cuts.

- Can produce various edge types: flat, bevel, groove (for semi-rimless), drill points (for rimless).

Lens Blocker:

- Attaches a block (chuck) to the lens before edging.

- The block must be positioned at the optical center (or at the correct decentration point).

- Proper blocking ensures the OC aligns with the patient's pupil after edging.

Layout/Marking:

- Before blocking, the lens is marked with the optical center, axis line, and any prism reference points.

- The lensometer marking device places three dots: one at the OC and two indicating the axis.

Generator (Surfacing):

- Used in the surfacing lab to grind the back surface of a lens to the prescribed power.

- Not typically used by dispensing opticians (done at the lab).

Hand Edger:

- A manual grinding wheel used for minor lens modifications.

- Used for small adjustments that the automatic edger cannot handle.

Frame Warmer/Heater:

- Used to heat plastic frames before adjustment.

- Temperature: approximately 250-275°F (120-135°C).

- Types: salt pan (glass beads), hot air, infrared.

- Never apply heat directly to lenses.

- Metal frames do not require heating for adjustment.

Thickness Gauge (Caliper):

- Measures lens center and edge thickness.

- Important for verifying minimum thickness requirements.

- Digital calipers provide accuracy to 0.01mm.

Geneva Lens Clock (Lens Measure):

- Has three pins — two fixed outer pins and one movable center pin.

- Measures the surface curvature (power) of individual lens surfaces.

- Calibrated for a refractive index of 1.523 (crown glass).

- For other materials, a conversion factor is needed.

- Reading: Nominal Lens Formula = Front surface + Back surface = Approximate total power.

Radiuscope:

- Measures the base curve (radius of curvature) of contact lenses.

- Can also verify spectacle lens curves.

- Uses reflected light to determine surface curvature.

UV Meter:

- Measures ultraviolet light transmission through a lens.

- Verifies that UV coatings or built-in UV protection meet standards.

- Polycarbonate and Trivex should block 100% UV.

Polariscope:

- Detects stress patterns in lenses.

- Used to verify that heat-treated (tempered) glass lenses have proper stress patterns for impact resistance.