Adding a tint to existing prescription lenses for use as sunglasses is possible, but the modification is highly dependent on the lens material and any existing surface treatments. The successful addition of an aftermarket tint relies on a specific chemical process compatible only with certain lens compositions.
Feasibility Based on Lens Type
The ability to successfully add a tint after the lens has been manufactured is primarily determined by its polymer structure. Standard plastic lenses, known chemically as CR-39, are the most receptive material for aftermarket dyeing because they readily allow organic dye molecules to penetrate and bind. However, many modern lens materials resist this process due to their dense composition or protective coatings. High-index plastic, polycarbonate, and Trivex lenses are typically difficult or even impossible to tint uniformly post-manufacture. Furthermore, existing coatings, especially Anti-Reflective (AR) coatings, present a significant barrier to tinting. These coatings must be chemically or thermally stripped from the lens surface before dyeing, a process that often removes the underlying scratch-resistant layer and can potentially damage the lens itself.
The Process of Aftermarket Dyeing
When tinting is feasible, it involves submerging the lenses into an organic dye bath. The dye solution, often made from specialized powders, is mixed with water and heated to a temperature around 200°F (93°C). This elevated temperature causes the plastic material to swell slightly, which allows the dye molecules to permeate the lens surface. The intensity of the final tint is directly controlled by the duration the lens remains immersed in the heated solution. A lighter shade may require just a few minutes, while a darker sunglass tint requires a longer soak time, typically ranging from three to fifteen minutes. Once the desired shade is achieved, the lenses are removed and rinsed with warm water to stop the dyeing process and set the color.
Practical Limitations and Potential Risks
Choosing to tint existing lenses carries several practical risks. One common outcome is uneven color saturation, which can result in a splotchy or streaked appearance instead of a uniform shade. The high heat required for the dye bath also introduces the risk of physical damage to the lenses and the frame if the lenses are not removed. Exposure to temperatures around 200°F can cause some lens materials to warp, or even crack due to thermal stress. Additionally, most optical laboratories will not provide a warranty for lenses they tint after the initial purchase, meaning any damage or unsatisfactory results are at the consumer’s expense.
Alternative Options
For eyeglass wearers whose lenses cannot be tinted or who wish to avoid the associated risks, several practical alternatives exist for adding sun protection. Simple clip-on sunglasses offer an immediate and cost-effective solution, attaching magnetically or via a clasp to the existing frame. A more permanent alternative involves having new, tinted prescription lenses made to fit the current frames. This ensures the tint is applied correctly and uniformly during the manufacturing process, often with a scratch-resistant coating specifically designed to accept the color. Another popular option is choosing photochromic lenses, which contain molecules that react to ultraviolet light, causing the lenses to automatically darken outdoors and fade back to clear indoors. This adaptive technology provides a convenient, light-responsive solution.