Spectacle Lens Properties: COA Exam Study Guide

Understanding Spectacle Lens Power

Spectacle lenses correct vision by bending light to focus it properly on the retina. The type and strength of correction depends on the patient's refractive error. As a COA, you need to understand how different lens powers, designs, and coatings address specific visual needs.

Plus and Minus Lenses

Plus Lenses (Convex)

Plus lenses are thicker at the center and thinner at the edges. They converge (focus) light rays, bringing the focal point forward. Plus lenses correct hyperopia (farsightedness), where the eye's focal point falls behind the retina. They are also used for presbyopic reading corrections.

Minus Lenses (Concave)

Minus lenses are thinner at the center and thicker at the edges. They diverge (spread) light rays, pushing the focal point back. Minus lenses correct myopia (nearsightedness), where the eye's focal point falls in front of the retina.

Cylinder and Axis

Patients with astigmatism have corneas that are more curved in one meridian than another, similar to a football rather than a basketball. The cylinder component of a lens corrects this uneven curvature by adding power along one specific meridian.

The axis (measured 0-180 degrees) specifies the orientation of the cylinder correction. Together, sphere, cylinder, and axis define the complete refractive correction. For example, a prescription of -2.00 -1.50 x 090 means -2.00 diopters of sphere, -1.50 diopters of cylinder, oriented at the 90-degree axis.

Prism

Prism power is prescribed to manage binocular vision problems. A prism shifts the apparent position of an image without changing its focus, helping to align the visual axes of both eyes. Prism is measured in prism diopters and specified with a base direction (base up, base down, base in, or base out).

Common applications include:

• Compensating for small deviations in eye alignment (phorias)
• Relieving diplopia (double vision) in patients with strabismus
• Yoked prism for patients with visual field defects

Multifocal Lens Designs

Bifocals

Bifocal lenses have two distinct viewing zones separated by a visible line (segment line). The upper portion contains the distance prescription, and the lower segment provides the near (reading) addition. The most common type is the flat-top (D-segment) bifocal, which has a D-shaped near segment.

Trifocals

Trifocal lenses add a third zone between distance and near for intermediate vision (typically set for arm's length tasks like computer use). Two visible lines separate the three zones.

Progressive Lenses

Progressive addition lenses (PALs) provide a seamless, line-free transition from distance through intermediate to near vision. The power changes gradually through a "corridor" in the center of the lens. Progressive lenses require precise measurements including:

• Monocular PD
• Segment height (fitting cross position relative to the pupil)
• Correct frame alignment with appropriate pantoscopic tilt

Lens Coatings and Treatments

Lens Materials

Common spectacle lens materials include:

• CR-39: standard plastic, good optics, lightweight, but thicker for higher prescriptions
• Polycarbonate: impact-resistant, thinner, built-in UV protection; standard for safety eyewear and children's glasses
• Trivex: similar impact resistance to polycarbonate with better optical clarity
• High-index (1.67, 1.74): thinnest and lightest option for strong prescriptions, but more expensive

Key Takeaways

• Plus lenses converge light (correct hyperopia); minus lenses diverge light (correct myopia)
• Cylinder and axis correct astigmatism by adding power along a specific meridian
• Prism shifts image position to manage binocular alignment issues
• Progressive lenses require precise monocular PD and segment height measurements
• AR coating reduces reflections; polycarbonate provides impact resistance
• High-index materials are thinnest but require AR coating to manage increased surface reflections