High index lenses address the challenge of thick, heavy prescription eyewear. The refractive index describes how efficiently a material bends light. Standard plastic lenses, with a refractive index around 1.50, require more material to achieve strong corrective power, resulting in noticeable thickness.
High index materials possess a denser molecular structure, allowing them to bend light more sharply with less volume. This enables a thinner lens profile, reducing weight and thickness for a more comfortable and aesthetically pleasing experience, especially for those with higher prescriptions.
The Prescription Thresholds for High Index Lenses
The decision to choose a high index lens is primarily determined by the strength of the eyeglass prescription, measured in diopters (D). For lower prescriptions, generally up to \(\pm 2.00\) D, a standard 1.50 index plastic lens is perfectly adequate and remains the most cost-effective solution.
The recommendation for high index materials typically begins when the spherical power (SPH) reaches or exceeds \(\pm 4.00\) D. At this level, standard lenses begin to exhibit noticeable thickness, which can create discomfort and a less desirable appearance. For nearsightedness (myopia), represented by a minus sign, the lens is thinnest in the center and thickest at the edges, where the bulk becomes most prominent.
Conversely, for farsightedness (hyperopia), indicated by a plus sign, the lens is thickest in the center, which can magnify the wearer’s eyes. Prescriptions between \(\pm 4.00\) D and \(\pm 6.00\) D should transition to a high index material, such as 1.67, to manage this increasing bulk. This reduction in thickness and weight is particularly beneficial at the edges for minus prescriptions.
Astigmatism correction, noted by the cylinder power (CYL), also contributes to overall lens thickness. If the cylinder power alone is \(\pm 2.00\) D or higher, a high index lens is often recommended, even if the spherical component is low. When the total corrective power, including the cylinder, exceeds the \(\pm 4.00\) D mark, the cosmetic and comfort benefits of a high index material become pronounced.
Understanding Different Index Numbers
The index number dictates how much the lens must curve to achieve a specific power. A higher number indicates a greater ability to bend light, allowing the lens to be manufactured flatter and thinner. Standard plastic is 1.50, while high index options include polycarbonate (1.59), 1.67, and 1.74, suited for progressively stronger prescriptions.
The 1.67 high index material is a frequent choice for moderate-to-high prescriptions, typically ranging from \(\pm 4.00\) D up to \(\pm 7.00\) D. Lenses made from this material can be up to 30% thinner than the standard 1.50 index lens for the same power. This index offers an excellent balance of reduced thickness, lighter weight, and cost-effectiveness.
The 1.74 index represents the thinnest plastic lens material currently available, reserved for the highest corrective needs, generally those with a spherical power above \(\pm 8.00\) D. This material provides the maximum reduction in thickness and weight. For example, a 1.74 lens can be over 40% thinner than a standard 1.50 lens, a difference that is visually significant.
Selecting the appropriate index ensures the lens is thin and optically sound. Choosing an unnecessarily high index for a moderate prescription may not yield a noticeable difference in thickness but will increase the overall cost. The hierarchy of materials responds directly to the increasing power demand of the prescription.
Other Factors Influencing the Choice
Although prescription strength is the primary driver, other variables influence the decision. The size and style of the eyeglass frame play a substantial role in final lens thickness. A larger frame necessitates a wider lens blank, increasing thickness at the periphery, especially for nearsighted prescriptions.
High index materials are often recommended for larger, fashionable frames or semi-rimless/rimless styles, as these expose the lens edges, making reduced thickness aesthetically preferable.
Cost is another significant consideration, as high index lenses are substantially more expensive than standard plastic or polycarbonate options due to specialized materials and manufacturing processes. The 1.74 index, offering the greatest thinning, also carries the highest price tag, which should be weighed against the actual cosmetic benefit for the wearer’s specific power.
There are also optical trade-offs to consider. Higher index materials can exhibit a greater tendency for chromatic aberration, sometimes referred to as color fringing. To mitigate this, and to address the increased surface reflectivity inherent to these materials, an anti-reflective (AR) coating is highly recommended. Finally, while polycarbonate (1.59) is known for its impact resistance, some of the highest index materials, such as 1.74, can be slightly more brittle and less durable against impact than lower index options.