High index lenses offer a solution for individuals who require stronger vision correction but want to avoid the thick, heavy appearance of conventional lenses. The 1.67 material is a popular choice, balancing the aesthetic benefits of a thinner lens with considerations of weight, optical performance, and cost. This index provides a notable reduction in thickness, making it a common recommendation for moderate to strong prescriptions where standard plastic would result in a bulky lens profile.
Understanding High Index Lenses
The number 1.67 refers to the lens material’s refractive index, which measures how efficiently it bends light. A higher index means the material refracts light more sharply, requiring less thickness to achieve the same corrective power. Standard plastic lenses (CR-39) typically have an index of 1.50 and are less efficient at bending light.
Because the 1.67 material is substantially more effective at light refraction, it allows the lens to be manufactured with a shallower curve and a thinner cross-section. For a typical prescription, a 1.67 high index lens can be up to 30% thinner than a standard 1.50 lens. This reduction in volume translates into a lighter lens, which improves comfort for people who wear their glasses throughout the day.
The Prescription Threshold for 1.67
The power of the prescription, measured in Diopters (D), primarily drives the choice of a 1.67 index. This index is generally recommended for prescriptions where the spherical power falls within the range of approximately \(\pm\)4.00 D to \(\pm\)7.00 D. Below this range, the visible difference in thickness compared to lower-index options may not justify the increased cost.
For individuals with nearsightedness (myopia, indicated by a minus sign), a 1.67 lens significantly reduces the noticeable edge thickness, which is a common issue with stronger lenses. In cases of farsightedness (hyperopia, indicated by a plus sign), the 1.67 material minimizes the center thickness of the lens, reducing the visual magnification of the eyes that can occur with high plus powers.
It is also important to consider the cylinder (CYL) value on a prescription, which corrects for astigmatism. Even if the spherical power is slightly below the \(\pm\)4.00 D threshold, a high cylinder correction, often \(\pm\)2.00 D or more, can make the lens substantially thicker and heavier. In these instances, the use of a 1.67 index is frequently suggested to manage the overall bulk and weight of the lens, ensuring a more balanced and cosmetically pleasing result.
Comparing 1.67 to Other Index Options
The 1.67 high index material sits between the common mid-index polycarbonate (1.59) and the ultra-high index 1.74, offering a balanced blend of thinness and optical quality. Compared to polycarbonate, 1.67 lenses are slightly thinner and often exhibit superior optical clarity due to a higher Abbe value. The Abbe value measures a lens material’s ability to disperse light; a higher number indicates less chromatic aberration, or color fringing.
In comparison to the 1.74 index, 1.67 is noticeably thicker. However, the 1.74 index is typically reserved for prescriptions exceeding \(\pm\)7.00 D to \(\pm\)8.00 D, where maximum thinness is necessary. For mid-range prescriptions, the 1.67 often provides a better balance. Its Abbe value (typically around 32) is comparable to 1.74 (around 33) and much higher than polycarbonate, which can offer a visually clearer image for some users. Furthermore, 1.67 lenses are generally more affordable than 1.74, providing a cost-effective solution for significant thickness reduction.
The mechanical properties also differ. The 1.74 lenses can be more brittle and require careful handling, especially in rimless frames. The 1.67 material maintains good durability while achieving substantial thinness, making it a robust choice for daily wear across various frame styles.
Frame Choice and Lifestyle Factors
Factors beyond the written prescription can influence the selection of the 1.67 index. The size and style of the chosen frame play a significant role in the final lens appearance. For instance, selecting a large frame or one with a thin metallic rim will make the edge thickness of a lower-index lens much more noticeable.
In these situations, the 1.67 index minimizes the visible profile of the lens protruding from the frame for aesthetic reasons. For rimless or semi-rimless frames, where the lens edge is exposed or drilled, a high index material like 1.67 is often required for its inherent strength and stability.
While high index lenses cost more than standard plastic, the 1.67 material often offers the best value proposition. It provides a significant reduction in thickness and weight for moderate to strong prescriptions without the substantial price jump associated with the ultra-high 1.74 index. For many users, this index is the optimal sweet spot.