Photochromic and Color Lenses

By Liz DeFranco, ABOC, NCLC

Photochromic lenses change from light to dark depending on the amount of ultraviolet light they are exposed to. Early photochromics were strictly glass lenses, but today, you can choose from regular plastic, polycarbonate and high-index glass and plastic.

Each Brand Offers Different Advantages

Thirty-five years after their invention, traditional PhotoGray and PhotoBrown lenses from Corning Medical Optics are still on the market. As their names suggest, the lenses are available in either gray or brown colors that are light enough to wear indoors and darken to a sunglass shade when exposed to ultraviolet light.

The active ingredient that causes the lenses to transform is called silver halide and is mixed evenly throughout the lens. This means the whole lens will change when exposed to light. It also means that if a particularly strong prescription is made, the strongest, thickest part of the lens will be darker than the thinner parts. Also, if there is a large difference in prescription between the two eyes, the lens with the stronger prescription will be darker than the weaker one.

Corning has updated its offerings to include Thin & Dark photochromic glass lenses, which are up to 30 percent lighter in weight than traditional glass lenses and also thinner. These lenses are said to change from clear eyeglass to dark sunglass lenses in 60 seconds. Thin & Dark lenses are also available in gray and brown.

Corning's newest photochromics are SunSensors mid-index gray and brown plastic lenses. A bit lighter in weight and thinner than regular plastic, these lenses have photochromic molecules mixed throughout the material, rather than just in the top layer. However, unlike their glass counterparts, SunSensors change within 60 seconds to a uniform color density throughout the lenses, regardless of the prescription or lens thickness, according to the company.

ColorMatic Extra from Rodenstock is another mid-index plastic photochromic line. A photochromic dye is mixed throughout these gray lenses. ColorMatic Extra lenses can be tinted to alter the color or darken the lenses.

In addition, Hoya Lens makes Sungray Lenses, mid-index plastic photochromics with UV-activated color-changing molecules blended throughout the lens material. They can also be tinted.

Transitions lenses are available for nearly every lens design, refractive index, and prescription. Performance features include advanced variable-tint technology that allows rapid darkening when you go outside (and rapid return to clear when you go in), and 100 percent UV protection. They come in regular, lightweight plastic materials as well as in high-index plastic and polycarbonate. They have a front-surface coating that changes color (to gray or brown) when exposed to ultraviolet light. The changeable coating means that the color darkens evenly regardless of lens prescription or thickness.

Transitions also makes a few niche products including for Drivewear, which features a lens that remains dark behind the windshield of a vehicle. As a general rule, photochromic lenses won't darken behind the windshield because the glass blocks out the UV rays that cause the lenses to change color.

Transhades from KB Co. (a totally different product than Transitions) are plastic polarized lenses that are also photochromic. They come in glass as well as plastic, and in gray and brown. Their polarization properties make them especially comfortable when they are worn in situations in which glare is a problem. Plastic Transhades can be tinted to make them darker or to alter the lens color. The glass lenses are also available in copper, a good driving color.

An example of photochromic lenses. Photos provided by Transitions.

Medical Applications

Beyond the convenience of having lenses that can be worn both indoors and outdoors, some photochromic lenses also have medical applications. Corning's CPF family consists of glass photosensitive lenses with colors in the red family. They are used to enhance the vision of patients who have various ocular pathologies. Doctors who specialize in low vision are familiar with these lenses and can help patients determine which color will be the most helpful.

Another glass photochromic lens that is designed for use by individuals who have beginning-to-moderate light sensitivity because of various ocular conditions is Corning's GlareCutter. This is a darker, more cosmetically pleasing brownish-red that allows a precise level of blue light to "leak" through the lens, thus preserving a more natural color scene for the wearer than the more reddish CPF lenses.

Lenses with Tints

As opposed to the changeable photochromics, another option in colored lenses is a tint, which remains constant at all times. Tints are available on plastic as well as glass lenses and can be had in almost any color of the rainbow. Lighter, fashion tints are used primarily for cosmetic purposes to enhance a wearer's looks. Darker tints allow the wearer to use the lenses as sunglasses.

Typically, fashion tints are applied in light pink, brown or gray, while sunglasses are usually gray or brown. A tint can be solid, when the entire lens is the same color, or gradient, which is a gradual fade from dark to light, usually fading from the top down.

Other colors can be applied to lenses for different purposes:

Yellow, sometimes referred to as a "blue-blocker" because the color keeps blue light from entering the lens, is often the color of choice for target shooters because it decreases haze and makes objects appear sharper, with more contrast.

Green, or its cousin G-15 (the Ray-Ban lens color), is sometimes used as a sunglass, though brown and gray are the most popular sun shades.

Red is a very uncomfortable color to look through, though it does have applications for certain ocular pathologies. However, some people enjoy seeing the world through "rose-colored glasses."

Tints are applied to plastic lens materials through a process of absorption. The lenses are immersed into a warm color bath, and depending on the length of time they sit in the tank, emerge in varying shades of darkness. A lens that requires only a light tint will go into the bath for just a few minutes, while a lens that is meant to be sunglass-dark will stay in for quite some time.

Glass lenses may be manufactured with the color distributed throughout the lens material, or a tint is applied as a coating in a vacuum chamber after fabrication.

[Page updated March 2007]