The Manufacturing Process of Photochromic Lenses

Photochromic lenses, also known as light-sensitive lenses, embody the principle of "light-induced reversible color change." Since Corning Incorporated pioneered glass photochromic lenses in the 1960s, the material for these lenses has evolved from heavy glass to lightweight high-molecular resins. The substances responsible for the color change have also transitioned from inorganic "silver salts" (such as silver halides) to organic photochromic molecules (photochromic dyes). These lenses serve a dual purpose: functioning as regular optical glasses indoors and doubling as sunglasses outdoors, thereby securing a significant niche in the market.

So, how are photochromic lenses produced? Depending on the method of incorporating organic photochromic materials into the lens substrate, there are several primary manufacturing processes:

• Bulk Tinting Technology (Casting Molding)

    The hallmark of bulk tinting technology lies in the homogeneous blending of photochromic dyes with the lens's raw material (monomer). Through the casting molding process, the monomer solidifies to yield a formed lens endowed with photochromic capabilities. This technique is typically employed for lenses with refractive indices of 1.5 and 1.56, and it presents relatively lower technical challenges in production. However, due to the varying thickness from the center to the edges of prescription lenses, suboptimal manufacturing processes by lens producers may lead to the "Bull's eye" phenomenon upon wearing. This effect manifests as darker coloration in thicker areas of the lens and lighter shades in thinner regions.

• Film Transition Technology (Spin Coating)

    Film transition technology typically involves coating a pre-formed lens substrate with a photochromic layer that contains photochromic dyes and cross-linked resins. This technique, while more complex, can be applied to lenses of various refractive indices and offers superior performance, making it the mainstream product in the mid-to-high-end market.

• Embedded Photochromic (Imbibition) Technology

    Embedded photochromic technology, as seen in Transitions® products, involves the diffusion of photochromic dyes into a pre-formed lens substrate under heat, with the majority of the dyes residing near the lens surface, akin to a dyeing process. Due to compatibility issues between the photochromic dyes and the lens material, this imbibition method is exclusively utilized for lenses

with a refractive index of 1.5.

• Photochromic Film (Casting Molding)

    Similar to the casting molding process used for polarized sunglasses, the photochromic film requires the replacement of the PVA layer, which is responsible for polarizing light, with a functional layer capable of photochromism. To accommodate various types of lens substrates, the outer protective and reinforcing structures, which safeguard the PVA layer, can be fabricated from materials such as PA, polyurethane (PU), TAC, and PC. Like film transition technology, photochromic films can be applied to lenses of various refractive indices, but the technical complexity is greater, resulting in fewer such lenses available on the market.

    Photochromic lenses are characterized by two pivotal attributes: ultraviolet (UV) light-induced color change and temperature-dependent variation in the speed and depth of the color transition. Specifically, the stronger the UV radiation in the lens's environment, the faster the color change occurs and the deeper the resulting tint. Conversely, higher temperatures accelerate the color change but result in a lighter tint, while lower temperatures slow down the color change but produce a darker tint. These characteristics limit the application scope of photochromic lenses. For instance, during the sweltering summer months, the lenses may not darken as much as desired. Additionally, when used inside a vehicle, the windshield filters out a significant portion of UV rays, leading to suboptimal lens darkening.

• The synthesis of photochromic dyes often yields numerous by-products, and the purification steps are intricate, resulting in a low final yield.
• The difficulty in dye production is generally proportional to the photochromic performance --> dyes with superior performance come at a higher cost.
• To achieve comprehensive excellence in performance, significant development costs are required, especially for combination products such as polarized + photochromic lenses.

Due to the aforementioned objective factors, photochromic lenses tend to be more expensive.