Ophthalmic lens design focuses on correcting the refractive errors of the human eye while maintaining visual comfort, image clarity, and minimal distortion across the field of view. Unlike conventional camera or telescope optics, ophthalmic lenses must work in close proximity to a rotating eye, making prismatic effects, field coverage, and off-axis behavior especially important.
This article explains the fundamental principles of ophthalmic lens design, including prismatic effects, far-point imaging, and the optical role of spectacle lenses.
Prismatic Effects in Spectacle Lenses
Spectacle lenses inevitably introduce prismatic effects, especially toward the lens periphery. These effects arise because light passes through regions of the lens that are effectively wedge-shaped away from the optical center.
Ring Scotoma and Diplopia
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Positive (plus) lenses
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Cause an annular blind region in the visual field when the eyes are stationary
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This invisible ring is known as a ring scotoma
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Negative (minus) lenses
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Do not create a blind region
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Instead, objects near the periphery may be seen diplopically (double vision)
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These effects are particularly noticeable in high-power prescriptions and must be carefully managed through lens design and material selection.
The Myopic Eye and the Far Point
A myopic (nearsighted) eye has a finite far point, defined as the furthest distance at which objects can be seen clearly without accommodation.
- Objects beyond the far point are imaged in front of the retina
- As the eye rotates, the far point remains at a constant distance from the eye
- The locus of all far points forms the far point sphere
From an optical perspective:
- The retina and the far point sphere are optical conjugates
- The purpose of an ophthalmic lens is to form the image of distant objects on the far point sphere
Optical Function of an Ophthalmic Lens
The primary function of a spectacle lens is to:
Image objects at infinity (or near distances) onto the eye’s far point sphere, ensuring that light focuses correctly on the retina.
Key system characteristics:
- Aperture stop: the pupil of the eye
- Reference point: the center of rotation of the eye
- Because the eye rotates, the effective aperture is always referenced to this rotation center rather than being fixed in space
This dynamic geometry differentiates ophthalmic lens design from most fixed-aperture optical systems.
Object Plane Considerations
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In standard vision correction, the object plane is assumed to be at infinity
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For near-vision tasks (reading, progressive lenses):
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The object plane is closer
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Lens power distribution and aberration balance change significantly
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Near-vision designs require additional optimization
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As a result, single-vision, bifocal, and progressive lenses are fundamentally different optical designs, even if they share similar materials.
Key Design Parameters in Ophthalmic Lenses
Important parameters that must be balanced include:
- Lens power (diopters)
- Base curve selection
- Prismatic effects
- Peripheral aberrations
- Lens thickness and weight
- Refractive index and dispersion
- Eye-lens distance (vertex distance)
Modern ophthalmic design often uses freeform surfaces to optimize these parameters across the entire visual field.
Materials and Dispersion Considerations
Lens material properties strongly influence performance:
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Refractive index affects:
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Lens thickness
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Weight
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Field curvature
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Dispersion affects:
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Chromatic aberration
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Visual comfort
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Common ophthalmic materials include:
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Optical polymers
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High-index plastics
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Specialty glasses from manufacturers such as Corning
Accurate dispersion data is essential for predicting chromatic performance, particularly in high-power lenses.
Summary: Key Insights in Ophthalmic Lens Design
- Ophthalmic lenses must account for eye rotation
- Prismatic effects dominate peripheral vision behavior
- Positive lenses can create ring scotomas
- Negative lenses may cause diplopic perception
- The far point sphere defines the imaging target
- Material dispersion and index strongly affect comfort
Ophthalmic lens design is a unique branch of optical engineering that combines human vision, biomechanics, and precision optics.
References
- Corning. 2019. Glass Products Data Sheets. Accessed Feb 15, 2015. www.corning.com/worldwide/en/products/advanced-optics/product-materials/specialty-glass-and-glass-ceramics/ophthalmic-glass/glass-products-data-sheets.html.
- Sultanova K., Karasova S., Nikolov I., (2009), Dispersion Properties of Optical Polymers, Acta Physica Polonica A, 116:4, 585-587.