Did you know the light conditions and their intensity vary throughout the day? It also keeps changing throughout the year as the season turns to the next. The harmful indoor and outdoor lights that you are exposed to are also different. They are blue light or ultraviolet light and its variants. It can affect your vision power depending on the amount of light, the duration, and the type of light that is hitting your eyes.
Modern technology and innovations in eyewear have made it possible to control the amount of light entering your eyes. There are many solutions to shield your eyes from different light conditions. The source of harmful light could be direct sun rays or electronic devices. It could be from deflected surfaces or oncoming traffic.
The good news is every type of light can be shielded, but when to use which type of protective glasses requires a brief understanding of the properties of light. This article explains how light is transmitted and what are options available to protect your eyes to avoid damage and improve vision.
What Is Visible Light Transmission?
If you are unsure which type of lens is suitable for your sunglasses, then consider Visible Light Transmission (VLT) to get started. We also know it as visible light transmittance. VLT is the amount of light that can pass through your lenses. This is expressed in percentages and denotes the opacity of a lens within a sunglasses frame.
A higher percentage means more light can pass through and a lower percentage indicates lesser light. So, if you have a dark lens on your sunglasses, it has low VLT%; and vice versa.
When visible light hits the lens, three responses take place – reflect, absorb, or transmit. We can categorize these into Visible Light Absorption (VLA), Visible Light Transmission (VLT), and Visible Light Reflectance (VLR).
We measure VLT in percentages ranging from 1 to 99. A testing device called a Photometer is used to measure the intensity of light. It calculates the percentage before and after the light passes through a lens.
What Does VLT% Indicate?
Visible Light Transmittance (VLT) is the proportion of visible light (between 390 and 780 nanometers in the solar spectrum) that is passed through the glass. The color and thickness of the glass regulate it. Clear float glass has a VLT of 75% to 92%, while colored glass has a VLT of 14% to 85%.
How much percentage of visible light is required on the lenses depends on your vision needs and usage. Based on the VLT%, we can categorize the darkness of a sunglasses lens into 5 types. Each category has a broader usage for varying applications.
None or Minimal Tint
Typically used for fashion, safety, and nighttime activities such as snowboarding and cycling.
Useful on overcast days. Excellent for shooting, cycling, and golf.
For a normal amount of sunlight
Suitable for brightly reflecting light. An ideal choice for driving, aquatic activities, and snow activities.
Very Dark Tint
Only use in bright light or direct sunlight. This amount of VLT is often intended for high-altitude mountaineers or shop welding.
The higher the VLT percentage, the milder will be the lens tint. Higher VLT% lenses will allow more light to pass through the lens and enter the eye. Lenses with a lower VLT%, on the other hand, will have a darker shade and will prevent more light from entering the eye.
To adjust the VLT of a lens, we apply a darkening tint or a film as a coating. We apply tint coatings for sunglasses by immersing the lens in a colored liquid solution within a heated tint bath.
Visible Light vs. Ultraviolet Light
Notwithstanding the dark coating, the reduced transmission of visible light does not safeguard you from UV rays. This is because visible light isn't the same as UV light. UV has an invisible frequency range, because of which we can't see it.
UV wavelengths range from 10 to 400 nanometers and these are classified as UVA, UVB, and UVC. These are the most harmful light frequencies, posing the foremost risk to your skin and eyes. Hence, UV requires a different sort of screening than visible light to prevent irreversible damage to your eyes.
ALERT: Even an extremely dark sunglass may not improve UV protection.
Another critical element of light transmission is that sunglasses with no side covers or a low base curve allow light to enter from behind or above the frame. Allowing light to travel through a lens can sometimes be beneficial, but only if it is directed in the correct direction.
Mountaineering and cycling sunglasses are specially designed to almost completely hide the eyes. However, standard sunglasses frames designed for daily use do not completely cover. As a result, light reflects into your eyes, causing uncomfortable and distracting reflections. We know this phenomenon as bounce-back, and it can be frustrating and damaging.
An anti-reflective coating can allow "intrusive" sunlight to pass through from behind your lens to prevent this. However, the light passing through the front of your lens is restricted.
Does Lens Color Matter?
Color does matter in everything; for lenses too. The lens color itself does not shield against UV rays or alter the direction of light transmission. But it improves your vision and perception depth. The lens colors should be selected per the VLT% to get maximum benefit.
Green lenses give more contrast than gray lenses and transmit color more accurately than brown lenses. Green lenses reduce glare while enhancing shadows, making them ideal for both sunny and low-light conditions.
Colors associated with a dark tint (8-18% VLT) are more suitable for intense reflected light. They are typically grey or copper. Lenses have a very light tint (43-80% VLT) and are usually yellow, amber, or rose and are chosen for their high color contrast depictions.
If you believe that you are exposed to multiple categories, it would be advantageous to select an interchangeable or photochromic style that can tolerate a wide range of light scenarios.
Photochromic lenses offer a VLT of 30-50% and are adaptable enough to work in almost any situation. The UV intensity of your environment causes these glasses to change automatically from clear to light to dark tint.
However, these lenses should not be worn with a mirror or a hue other than their original color. Any additional or different hue may slow down the transitional tint's reaction time and may prevent the lens from becoming as dark as intended.
If your eyes are strained on a bright sunny day, the VLT% of the lens tint is inadequate. If your lenses are still dark after sunset, you're wearing the wrong pair of sunglasses. Choosing the proper tint for your lenses is not only important for preventing eye strain and irritation, but it is also a required safety precaution. Stay relaxed and confident with a VLT tailored to your needs.