Iseikonic Lenses
1) Introduction
For comfortable binocular vision, the retinal image size in both eyes should be nearly identical. If there is a significant difference, the brain may fail to fuse the images, resulting in aniseikonia. Iseikonic lenses are specially designed spectacle lenses that modify image size to equalize retinal images and restore binocular vision.
2) Definition of Aniseikonia & Iseikonic Lenses
- Aniseikonia: A condition where the perceived image size differs between the two eyes, usually more than 1–2%.
- Iseikonic Lenses: Spectacle lenses designed to alter image magnification so that the image size difference is neutralized, making retinal images nearly equal.
3) Causes of Aniseikonia
- Refractive causes (Anisometropia): Large differences in refractive error (e.g., one eye –6.00 D, other plano) cause different spectacle magnifications.
- Aphakia: After cataract surgery, aphakic eye corrected with high plus spectacles produces much larger image size compared to fellow eye.
- Retinal causes: Retinal stretching or compression (e.g., macular edema, retinal detachment surgery) alters photoreceptor spacing.
- Neurogenic causes: Rare cases due to cortical processing abnormalities.
4) Principle of Iseikonic Lenses
The magnification produced by a spectacle lens depends on several optical parameters:
- Shape factor: Depends on base curve and lens thickness.
- Power factor: Depends on vertex distance and lens power.
By modifying these factors, lens designers can slightly increase or decrease image size, thereby compensating for aniseikonia.
5) Factors Affecting Image Size
- Vertex distance: Moving lens closer or farther from eye changes magnification.
- Base curve: Steeper base curve increases magnification; flatter decreases it.
- Thickness: Greater thickness in plus lens increases magnification.
- Refractive index: Using high-index materials changes magnification properties.
6) Design of Iseikonic Lenses
- Customized design: Lenses are specially calculated to neutralize measured aniseikonia.
- Measurement: Aniseikonia measured using tests such as Aniseikonia Inspector, space eikonometer, or direct comparison tests.
- Adjustments:
- Increase thickness or base curve for eye needing magnification.
- Use flatter base curve or reduce thickness for eye needing minification.
- Final lens pair provides clear images of equal size at both eyes, allowing fusion.
7) Clinical Applications
- Anisometropia: Patients with >2 D difference between eyes benefit from iseikonic correction.
- Aphakia: Spectacle correction produces ~25–30% magnification; iseikonic lenses can reduce size difference, though contact lenses are generally preferred.
- Post-retinal surgery: Equalizing image size after macular or retinal repair.
- Partial aniseikonia: Even 1–2% difference can cause asthenopia; iseikonic lenses help in sensitive patients.
8) Dispensing Considerations
- Careful measurement of PDs and vertex distance essential.
- Frame selection: should allow modification of vertex distance and pantoscopic tilt.
- Explain adaptation period—patients may initially find iseikonic lenses unusual.
- Alternative options: contact lenses or refractive surgery may sometimes be preferable.
9) Advantages & Limitations
Advantages
- Restores binocular fusion and stereopsis.
- Relieves asthenopia and discomfort due to image size difference.
- Useful in cases where contact lenses are not tolerated.
Limitations
- Technically complex to design and dispense.
- Cannot fully correct very large aniseikonia (>5%).
- Cosmetic issues due to increased thickness or curvature in one lens.
- Adaptation can be difficult for some patients.
Spectacle Magnifiers
1) Introduction
Spectacle magnifiers are high-plus ophthalmic lenses worn as spectacles to enlarge near images for patients with reduced vision (e.g., macular degeneration, diabetic retinopathy, optic neuropathies) or for special near tasks. Unlike hand or stand magnifiers, spectacle magnifiers leave the hands free and can provide a wide field of view when used at the correct working distance and with appropriate posture/illumination.
2) Optical Principles & Key Formulas
- Angular magnification (M): Increase in the angular size of an object produced by the device versus the unaided eye.
- Dioptric add (Fadd): High plus power placed before the eye to shift the clear focus to a near working distance.
- Reference distance: In low vision, conventional “magnification” often assumes a 25 cm reference viewing distance.
Useful relationships:
- Working distance (WD): WD (meters) ≈ 1 / Fadd (D) when the accommodation is relaxed.
- Relative distance magnification (RDM): RDM = Original viewing distance / New viewing distance.
- Approximate spectacle magnifier “X” label: M ≈ Fadd / 4 (when referenced to 25 cm). Example: +20 D ≈ 5×.
- Effective power at the eye: Feff = F / (1 − dF), where d is vertex distance in meters (important for very high adds).
3) Types of Spectacle Magnifiers
- Single-vision high-plus readers: Simple, full-field plus lenses (+6 to +20 D or more) worn as glasses. Best for sustained reading at a fixed close distance.
- Aspheric high-plus lenses: Reduce aberrations and spherical aberration at high powers, giving better edge clarity and larger usable field.
- Bilateral high-add bifocals/trifocals: Less common at extreme powers; near segment can be customized. PALs are generally avoided at very high adds due to narrow near zones.
- Microspectacles (spectacle-mounted microscopes): Very high-plus systems, sometimes paired (binocular) or unilateral (monocular) to achieve 6×–12× equivalent, typically with extremely short working distances.
- Galilean or Keplerian telescopic systems with reading caps: Although telescopes are distance magnifiers, adding a high-plus “reading cap” converts them for near tasks; included here for completeness when mounted as spectacles.
4) Equivalent Power & Add Strategy
For practical dispensing, choose an add power that gives a manageable working distance and adequate print size enlargement:
- Reading target: Determine print size required (M units or N-point). Estimate magnification needed from acuity ratio (e.g., 6/24 wants 4× to read 6/6). Start with RDM or add power approximations.
- Pick add power: Example: If the patient can read 1M at 25 cm with effort but needs 2×, select +8 to +10 D (WD ~12.5–10 cm). Fine-tune with actual testing cards.
- Consider effective power: At +16 D and above, vertex distance changes have large effects; keep lenses as close as possible to the cornea (short vertex) and re-check Feff.
- Binocular range: Sustained binocular use is comfortable up to around +12 D (varies); very high adds often require monocular prescribing due to convergence and PD constraints.
5) Binocular vs Monocular Prescribing
- Binocular high-plus readers: Possible when adds are moderate (e.g., +6 to +12 D), the patient can converge at the required near PD, and both eyes have similar acuity.
- Convergence demand: At short WDs, convergence increases. Use base-in prism (each lens) to reduce demand. Rule-of-thumb: BI prism per eye (Δ) ≈ Fadd (D) × (PDfar − PDnear) in meters, or use standard tables (e.g., 2Δ BI per eye per additional +4 D beyond +4 D).
- Monocular prescribing: Preferred if acuity is asymmetric or at very high adds; provides a larger field to the better eye with simpler alignment.
6) Working Distance, Field of View & Illumination
- Working distance (WD): Shorter at higher adds (e.g., +10 D → ~10 cm, +20 D → ~5 cm). Patient posture and ergonomics are critical.
- Field of view (FOV): Increases when the magnifier is closer to the eye and when the lens diameter is larger. Spectacle magnifiers generally give wider FOV than hand magnifiers at the same power because they are “eye-mounted.”
- Depth of field (DoF): Decreases with higher power—head and reading material must be held steady at the focal plane.
- Illumination: Essential in low vision. Recommend bright, glare-controlled lighting (e.g., LED task lamp at 4000–5000 K, directed from the side).
7) Fitting & Dispensing Guidelines
- Refraction & best correction first: Optimize distance refraction and treat ocular surface issues (tear film) before low-vision dispensing.
- Select initial add from task goals: Try +6, +8, +10 D trial lenses to find the lowest power that meets the goal (speed & endurance count, not just threshold reading).
- Frame choice: Choose a small vertical depth frame with good nose support and adjustable pads. Keep vertex distance minimal. Consider strap or cable temples to stabilize posture.
- PD & near centration: Order near PD accurately; for very short WD, near PD can be much smaller than distance PD. Incorrect centration causes prismatic errors and asthenopia.
- Prism incorporation: For binocular high adds, prescribe base-in prism as needed; many labs offer standard BI increments in high-plus readers.
- Lens form: Aspheric high-plus reduces aberrations and thickness. Consider high-index cautiously (it may not significantly reduce thickness at very high plus and can reduce Abbe value).
- Coatings: AR coatings improve contrast perception and reduce veiling reflections. Hard coat + hydrophobic for durability and easy cleaning.
- Ergonomics package: Demonstrate posture, reading stand, and page holder to maintain focal plane and reduce neck strain.
Recumbent Prisms
1) Introduction
Recumbent prisms (also called bed prisms or lazy glasses) are prism spectacles that allow a person lying flat in bed to see forward (toward the ceiling) as if they were looking straight ahead. They are particularly useful for patients confined to bed for long periods, such as after surgery, spinal injury, or chronic illness.
They redirect the line of sight by approximately 90° downward, enabling the patient to read a book or watch television comfortably without lifting the head or straining the neck.
2) Optical Principle
- Based on the prism deviation principle, where light is bent (deviated) toward the base of a prism and the image appears shifted toward the apex.
- A right-angle prism (usually 45°–45°–90° prism) is mounted so that incident light is bent 90°, allowing a horizontal object to be seen while looking vertically.
- Some designs use mirror prisms (reflective surfaces) to achieve the same effect with lighter weight and lower chromatic aberration.
3) Design & Construction
- Material: Optical glass or lightweight plastic (acrylic, polycarbonate) to reduce weight.
- Prism angle: Typically 45° prism (or mirror system) to achieve 90° deviation of gaze.
- Mounting: Incorporated into a spectacle frame worn like glasses. Can be full-field prisms or clip-on attachments.
- Variants:
- Fixed-angle recumbent prisms (standard for bedridden use).
- Variable angle or adjustable designs for different ergonomic applications.
4) Clinical & Practical Applications
- Bedridden patients: Post-surgical recovery, spinal cord injuries, chronic illness—enables reading or watching TV without neck strain.
- Orthopedic or neurological conditions: Patients with cervical spine immobilization, neck brace, or restricted head mobility.
- Ergonomic relief: For individuals with severe back pain or neck stiffness, allowing a reclined posture while reading.
- Therapeutic use: Sometimes used in vision therapy to alter habitual gaze angle.
- Leisure: Healthy individuals also use recumbent prisms as “lazy glasses” for comfortable reading or smartphone use while lying flat.
5) Advantages
- Eliminates need to raise head/neck while lying down.
- Prevents musculoskeletal strain in long-term bedridden patients.
- Improves quality of life for immobile patients by allowing reading, TV, and interaction.
- Hands-free use when mounted in spectacle frames.
- Simple, non-electronic, durable, and relatively inexpensive.
6) Limitations
- Not suitable for walking or general mobility—designed only for recumbent use.
- Reduces field of view slightly due to prism design.
- Some optical distortion and chromatic aberration may be noticeable, especially in glass prisms.
- Weight can be uncomfortable if heavy glass prisms are used (plastic is better).
- Not a substitute for optical correction—does not correct refractive errors; must be combined with Rx if required.
7) Dispensing Considerations
- Frame fit: Stable frame with proper temple support is needed to hold prisms in correct orientation.
- Integration with Rx: Can be combined with distance or near correction lenses if required, though this increases complexity.
- Patient education: Instruct patient not to walk or move around while wearing recumbent prisms—strictly for recumbent tasks.
- Material selection: Acrylic or polycarbonate preferred for weight and safety.
- Hygiene: Easy cleaning is important for long-term bedridden use.
Fresnel Prisms and Lenses
1) Introduction
Fresnel optics were first developed by Augustin-Jean Fresnel in 1822 for lighthouse lenses, allowing large apertures and short focal lengths without the bulk of thick glass. The same concept is applied in ophthalmic optics to create Fresnel prisms and Fresnel lenses that are thin, lightweight, and easy to use.
2) Optical Principle
- A conventional prism or lens is thick because of its continuous curvature or angled surfaces.
- Fresnel design: Breaks the optical surface into a series of narrow concentric zones (or steps), each acting as a small prism or lens section.
- This preserves the optical function (refraction or deviation) while drastically reducing thickness and weight.
- Trade-off: The stepped surface causes some light scatter, reducing image quality.
3) Design & Construction
- Material: Usually made of flexible polyvinyl chloride (PVC) or other lightweight plastics.
- Form: Thin plastic sheets (~1 mm or less) with molded grooves corresponding to prism/lens steps.
- Attachment: Cut to shape and attached to spectacle lens surface (usually back surface) with water adhesion or static cling.
- Power range: Fresnel prisms available from ~1Δ to 40Δ.
4) Fresnel Prisms
Definition: Thin flexible plastic sheets with many narrow prism facets molded across the surface to produce prismatic deviation.
- Advantages:
- Lightweight compared to conventional glass/plastic prisms.
- Allows high prism powers that would otherwise require very thick, heavy lenses.
- Inexpensive and easy to apply/replace.
- Useful for trial/temporary prism correction (e.g., diplopia management).
- Disadvantages:
- Reduced optical quality due to light scatter and reduced contrast.
- Visible groove pattern may reduce cosmesis.
- Requires frequent cleaning; surface scratches easily.
5) Fresnel Lenses
Definition: Lenses constructed using Fresnel’s principle, where the curved surface is replaced with a series of concentric steps, reducing thickness.
- Applications:
- Low-vision aids (handheld magnifiers, sheet magnifiers).
- Head-up displays, overhead projectors, lighthouses, and compact optical systems.
- In ophthalmic practice, mainly used as magnifiers rather than spectacle corrections.
- Advantages: Large aperture lenses possible with thin profile.
- Disadvantages: Loss of image quality, stray light, reduced resolution.
6) Advantages of Fresnel Prisms & Lenses
- Extremely lightweight and thin.
- Inexpensive compared to full-thickness prisms/lenses.
- Allows high powers not feasible with standard optics.
- Easy to cut, fit, replace, and trial.
- Valuable in temporary or diagnostic use.
7) Limitations
- Reduced optical clarity and contrast due to scatter.
- Groove lines visible, which may disturb some patients.
- Not suitable for long-term cosmetic wear in many cases.
- Shorter lifespan—becomes discolored or scratched with use.
8) Clinical Applications
- Diplopia management: Temporary prism correction for patients with strabismus or cranial nerve palsy.
- Prism trial: Before prescribing permanent ground-in prisms, Fresnel prisms can be used to assess tolerance.
- High prism prescriptions: When full-thickness prism lenses would be too heavy or thick.
- Visual field expansion: In hemianopia, Fresnel prisms can shift part of the field to the seeing side.
- Low-vision magnifiers: Fresnel lens sheets provide large-field magnification for reading.
9) Dispensing & Fitting Considerations
- Cut Fresnel prism sheet to the shape of spectacle lens and attach with water or static cling to back surface.
- Check orientation carefully—base direction must match prescription.
- Educate patient about possible blur, reduced contrast, and adaptation issues.
- Recommend for short- to medium-term use; for long-term stable prism needs, prescribe ground-in prisms.
- For magnification Fresnel sheets, instruct on optimal working distance and illumination.
