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Photochromic lenses, also known as “light-adjusting lenses.” According to the principle of reversible photochromic reaction, the lenses can quickly darken under light and UV light, blocking strong light and absorbing UV light, and absorbing visible light neutrally; back in the dark, they can quickly return to their colorless and transparent state, ensuring the lens’ translucency. Therefore, photochromic lenses are suitable for indoor and outdoor use simultaneously, preventing sunlight, ultraviolet light, and glare from harming the eyes.

Classification of photochromic lenses and principles

Photochromic lenses are divided into two types: basic photochromic lenses and coating photochromic lenses, depending on the part of the lens that changes color.

The basic photochromic lenses are made by adding silver halide chemicals to the basic material of the lenses and by using the ionic reaction of silver halide, which decomposes into silver and halogen when stimulated by strong light and then combines into silver halide when the light becomes weaker, and the color becomes lighter.

Coating photochromic lenses are specially treated in the lens coating process. For example, spiro-pyran compounds are applied to the surface of the lenses with a high-speed spin coating, using the reversal of the molecular structure itself to pass or block the light, depending on the intensity of the light and UV rays.

The photochromic lens darkens after the sun’s rays come in. When the illumination fades, it becomes bright again. This is possible because of the crystallization of silver halide.

Under normal conditions, it keeps the lens clear. When exposed to sunlight, the silver in the crystals separates, and the free silver forms small aggregates inside the lens. These small aggregates of silver are irregularly interlaced and cannot transmit light but only absorb it, with the result that the lens becomes darker. In the case of darkening, the crystals are formed again, and the lens returns to its bright state.

Photochromic lenses are generally more expensive because of the production process, especially in optical stores, and most supermarkets do not have these types of lenses in their eyeglass counters. Online glasses stores can give you another way to buy them, which can be cheaper for the same quality. Cheap glasses today don’t mean low quality because photochromic lenses themselves are not inherently low quality.

Characteristics of the photochromic lens

(A) Speed of color change

The speed of color change is an essential reference factor in purchasing photochromic lenses. The faster the color change of the lenses, the better. For example, from a dim interior to a bright exterior, the color changes faster to prevent damage to the eyes from the strong light/UV rays in time.

Generally speaking, coating technology is faster than basic color change technology. For example, in the new coating color change technology, the photochromic factor uses spiro-pyran compounds, which have better light responsiveness and use the molecular structure of their reversal of opening and closing to achieve the effect of passing or blocking light, so it is faster in color change.

(B) Uniformity of color change

Uniformity of color change refers to the uniformity of the color of the lens during the process of changing from light to dark or dark to light. The more uniform the color change, the better the photochromic lens.

With conventional basic lenses, the photochromic factor is on the basic lens material, and the color change is affected by the thickness of the different areas of the lens. Since the center of the lens is thin, and the surrounding area is thick, the color change in the center of the lens is slower than in the surrounding area, and the panda-eye effect occurs. In the case of the coating photochromic lenses, the use of high-speed spin coating technology and the uniform spin coating of the color-changing film makes the color change more uniform.

(C) Service life

Generally, the service life of photochromic lenses is about 1-2 years, but many companies are trying to extend the service life of color-changing lenses.

After the spin-coating of the color-changing layer, the coating-changing lenses are also treated with an enhanced coating. The color-changing substance used – spiro-pyran compounds- has better light stability, so the color-changing function lasts longer than two years.