ABSORPTIVE LENSES (Sun lenses)
When radiation strikes a lens, it is either transmitted through, reflected off, or absorbed by it. Absorptive lenses, or sunglass lenses, are designed to either prevent certain wavelengths from entering the eye or to reduce the intensity of those wavelengths that are allowed to enter the eye. The filtering capabilities of a lens are determined by its material and the process by which it is formed and treated. All lenses must be treated to become filters. With every pair of sunglass lenses, there is a compromise. There is no one lens that does it all.
ABBE VALUE
The ABBE value of a lens material describes the amount of chromatic aberration: the higher the value, the less amount of aberration, and thus the higher the ABBE value, the higher the optical quality.
LENS SUBSTRATES ABBE VALUE FOR OPTICAL CLARITY
BASE CURVES
The lens base curve defines the shape of a lens and measures its surface curve. Lenses with a lower base curve, such as bases 2 and 4, are quite flat, lenses with a 6 base curve have some wrap, and lenses with an 8 or even 9 base create a full wraparound shape. 10 base curved lenses create the highest wraparound effect.
Dual Lenses:
Dual lenses are a pair of lenses.
Shield Lenses:
Are a single lens spanning from temple to temple and in sunglasses they can range from 4 to 10 in base curvature.
Shield lenses also come in various base curvatures, but a key point of reference for shields is the shape of the lens which are most commonly either spherical or cylindrical.
- Spherical shield lenses curve both vertically and horizontally providing a bubble-like effect. This effect accounts for a larger surface area providing a wider range of peripheral vision and great protection. This type of shield typically provides an optically correct distortion-free vision.
- Cylindrical shield lenses are flat vertically and curved horizontally.
TYPES OF LENSES
Lens technologies are combined to make sunglass lenses with specific properties and functional purposes. The number of combinations is virtually unlimited, the following are some popular lens classifications.
POLARIZED LENSES
Polarized lenses are designed to eliminate reflective glare. When light strikes a horizontal reflecting surface, such as a sheet of glass or water, it vibrates horizontally or right to left. This horizontal light wave is referred to as glare. The polarizing filter eliminates glare by allowing only vertical light waves to pass through.
NON-POLARIZED LENSES
Non-polarized lenses of high quality protect against visible and UV light and can reduce glare, but not horizontal glare. If not in conditions that create horizontal glare, a non-polarized lens is quite efficient and in certain cases the better choice.
PHOTOCHROMIC LENSES
Photochromic lenses lighten and darken in response to changing lighting conditions.
Photochromic lenses are constructed from a variety of lens substrates, including glass and plastic.
When exposed to UV (ultraviolet radiation), mineral photochromic lenses contain silver halide crystals that rapidly expand and transform into clusters of light-absorbing particles. As these particles absorb and expand, the lens darkens. The particles contract and separate when the amount of UV is reduced, causing the lens to lighten.
Another method for plastic lenses is to immerse them in a chemical bath, where the photochromic molecules are absorbed into the lens. When exposed to sunlight/UV, a chemical process occurs in which the molecules effectively move, change shape, and absorb the light, and vice versa.
Film-based photochromic lenses are also available.
To note:
- There are photochromic lenses available today that react to non-UV and UV light.
- Photochromic lenses function even on a cloudy day.
GRADIENT FILTERED LENSES
Lenses where the tint is darker on one portion of the lens, usually at the top, and gets gradually lighter as it moves down the lens.
LENS SUBSTRATES
The building block of any lens starts with its substrate.
MINERAL LENSES
Mineral lenses are made by casting and are composed of roughly 70% sand, 11–13% lime, 14–16% soda, and traces of calcium, potassium, and borax. The highest Abbe Value of any lens material currently in use is found in high-quality mineral lenses, which offer the purest optical clarity possible.
They also offer the highest level of scratch resistance without using additional coatings in a lens.
POLYCARBONATE LENSES
Polycarbonate is a thermoplastic, that in the making of lenses, starts as a solid material in the form of small pellets. Due to their softness, polycarbonate lenses are extremely impact resistant and absorb impact rather than shattering like regular plastic or glass. Polycarbonate lenses can be fabricated to be very thin and light. Additionally, they have built-in UV protection that blocks 100% of UV rays without the need for any additional coatings.
NYLON LENSES
Nylon lens material is a transparent polyamide with a low specific gravity and an ABBE value comparable to mineral lenses. The high Abbe number of nylon lenses can reduce eye strain and fatigue, making them a comfortable option for extended wear. Another main advantage of nylon lenses is their lightweight construction, which makes them incredibly comfortable to wear for extended periods.
Furthermore, nylon lenses are highly flexible durable, and resistant to impact, making them less likely to shatter or crack compared to other types of lenses.
BIO-NYLON LENSES
Bio-Based Nylon lenses are made from an environmentally friendly resin derived from castor oil taking its energy from renewable bio-mass. A mechanical pressing process extracts castor oil from castor seeds, which is then processed into raw material for lens production. The manufacturing process significantly reduces the amount of CO2 produced compared to the production of a standard nylon lens while maintaining the same optical clarity, durability, and strength as traditional Nylon lenses.
CR39 LENSES
CR39 (Columbia Resin #39) lenses are a plastic material made by cast molding and have an ABBE value close to that of a mineral lens thus they provide high optical clarity. CR39 lenses are lighter than mineral lenses and the material takes dyes easily, so they are available in a rainbow of colors. They require the application of UV and scratch-resistance coatings. Due to lower impact resistance, they are intended for fashion and not for sports usage.
TRIVEX LENSES
In comparison to other plastic or glass lenses, Trivex lenses are thin, lightweight, and have an extremely high impact resistance. They are made of a urethane-based monomer and are cast molded in the same way that mineral and CR39 lenses are, resulting in a higher Abbe Value and clearer optics.
LENS COATINGS
The most common sunglass lens coatings include:
UV (ULTRAVIOLET RADIATION) COATING
This coating, which is applied to plastic and glass lenses, prevents the sun’s harmful rays from penetrating the lens. Depending on the lens substrate, some lenses block most UV light by default, but adding a UV-blocking dye increases UV protection to 100 percent. When selecting a sun lens, it is critical to ensure that UV protection is provided.
ANTI-REFLECTIVE COATING
AR, or anti-reflective coating, is a layer of coatings applied to the inside of the lens to absorb bounce-back glare thus providing additional protection from damaging rays from the sun and reducing eyestrain. This is particularly useful when wearing 4 and 6 base frames as glare can hit your lenses from the back.
SCRATCH-RESISTANT COATING
This coating protects the lenses from scratches.
The coating is applied to both sides of the lens, which is especially important for soft lightweight lenses made of high-index plastic polycarbonate.
ANTI-FOG COATING
When the lenses are significantly cooler than the surrounding air temperature, tiny water droplets form on the surface of the lenses, causing fogging.
To eliminate lens fogging, anti-fog coatings are applied to the lens in a variety of ways
Having an interior lens anti-fog coating is particularly useful for lenses used in high wrap frame designs as fog is easily formed because of reduced airflow.
HYDROPHOBIC COATING
Is a chemical coating that repels water, making treated surfaces water-repellent, easy-to-clean, and aids in the repellence of dirt and debris. This can be particularly useful when out and about or performing sports in rainy weather or on a boat in rough seas as water will bead up and not stay on your lens.
OLEOPHOBIC COATING
Is a chemical coating that repels oils and debris, preventing them from adhering to the lens. It also makes cleaning lenses easier. Things like figure prints and dust particles from the air with not adhere to your lens.
HYDRO-OLEOPHOBIC COATING
Is a chemical coating that combines hydrophobic and oleophobic coating properties, so they repel water from fog and mist, and resist oil from figure prints and dust-like air particles.
MIRROR COATINGS
Mirror coatings come in a range of functional and intensity levels. They reflect glare, reducing the amount of light entering the eyes and thus providing additional protection and eye comfort
- Metallized Mirror coatings are a one-layer metal coating applied to the front of a lens that reflects and can absorb light, lowering visual light transmittance and thus darkening the lens. They can be used as a flash mirror with a lighter layer.
- Dielectric Mirror (Multi-Layer) coatings are made of clear oxide multilayers. Dielectric coatings can appear lighter and keep the visual plain on the brighter side depending on the number of layers and the wavelength of reflectance of each. Color and effect options are limitless.
MIRROR COATING APPLICATIONS
- Full Mirror coatings cover the entire front lens surface and reflect glare from the full surface of the lens.
- Gradient Mirror coatings are applied to the lens to reflect glare from above and are darker on the top of the lens, becoming gradually lighter towards the middle.
- Bi-Gradient (Double Gradient) Mirror coatings are applied to the top and bottom of the lens’s exterior, reflecting light from above and below while allowing more light to pass through the center.
- Flash Mirror coatings are metalized coatings that can be used with half or minimal reflectance.
VISIBLE LIGHT TRANSMISSION (VLT)
The amount of light that passes through a sun or optical lens and the amount of light that the eyes are exposed to through the lens is referred to as visible light transmission (VLT). The VLT of a lens is expressed as a percentage ranging from 0 to 100%. The darker the lens, the lower the VLT percentage, and vice versa. The lens VLT range percentages are divided into four categories. These categories provide appropriate guidance and lens use suitability.
