1. How tinted lenses reduce the visible light you see
On a sunny day, wearing dark sunglasses makes glare feel softer. This is because tinted lenses reduce the overall amount of "visible light" entering your eyes. The amount of light that passes through is measured by a number called visible light transmittance.
For sunglasses, this is measured using the method in JIS T8141 (shading protective equipment), with an allowed range of within ±7 of the displayed value [3]. A smaller number means a darker lens. The question that arises here is whether invisible UV rays are also weakened in the same way.
2. Why a sunglasses lens's darkness does not show how much UV passes through
In a product test by Kanagawa Prefecture examining commercially available sunglasses, UV transmittance was found to be unrelated to lens darkness. All products tested met their displayed values [4]. There is also an allowed range of within ±10 of the displayed value for UV transmittance, measured by JIS methods [3]. However, these results apply only to the products tested and do not guarantee all products.
On September 9, 2026, six organizations including the Japan Ophthalmological Society published a statement. They pointed out that the effectiveness of blocking UV rays is determined by the presence of UV-cut performance, not by the color or darkness of the lens [5]. The subject of the statement appears to be the basis for recommending constant wear at school.
When you understand that lens darkness measures visible light and UV-cut measures UV rays as separate rulers, you can see why "dark means safe" is not a reliable rule.
3. What can happen when dark lenses have no UV protection
According to an explanation from an ophthalmologist introduced by Weathernews, dark lenses are meant to reduce glare by decreasing visible light, and this is unrelated to the effect of blocking UV rays [6].
Furthermore, it is stated that if you wear dark lenses without UV-cut, your eyes may feel dark, causing the pupil (the hole in the eye) to open wide. This could allow more UV rays to enter the eye. This explanation specifically concerns dark lenses that do not have UV-cut.
Because of this, looking at the UV transmittance display is meaningful, rather than choosing based on color darkness alone.
4. How polarized lenses reduce glare from water and wet roads
Glare from water surfaces or wet roads occurs because reflected light contains many components that vibrate in a horizontal direction [1].
Polarized lenses have a fine structure that selectively reduces the polarized components of reflected light, decreasing discomfort from glare and distortion of vision [2]. The principle is that light passing through a vertical gap cannot pass through a horizontal gap. This property is also the basis for camera PL filters [1].
Tinted lenses weaken light overall. Polarized lenses reduce reflected light by selecting its direction. UV-cut blocks a different type of light called UV rays. While these three functions can overlap, they each have a different role.
5. Why you should check the UV transmittance number instead of lens darkness
You can see lens darkness with your eyes, but you cannot see how easily UV rays pass through. The clue is the displayed UV transmittance. The allowed range for visible light transmittance is within ±7 of the displayed value, and for UV transmittance, it is within ±10, meaning some deviation between the displayed and actual values is permitted [3].
In the Kanagawa Prefecture test, some products had visible light transmittance deviations that exceeded the allowed range [4]. If you do not understand what the display means, please check with the retailer or the instructions.
Do not make decisions about eye health on your own; refer to explanations from experts such as ophthalmologists.
6. How to check the visible light and UV transmittance numbers on your own glasses
What you can do today: Look at the tags on sunglasses or glasses you have at home and search for the numbers for visible light transmittance and UV transmittance. When comparing dark items with light items, see if the two numbers appear together or if only one is written. Be careful not to look directly at the sun.
When you want to know more: The Consumer Affairs Agency's miscellaneous goods quality labeling guide explains the rules for sunglasses displays [3]. The Kanagawa Prefecture product test shows what was actually measured [4].