Your Driving Sunglasses Might Be Too Dark

In this article, we explored why Serengeti’s photochromic lenses don’t activate fully behind a windshield. While that might sound like a flaw on paper, it actually highlights a common problem most people never realize:

Many sunglasses are too dark for driving.

Let’s break it down.

The VLT Problem: Glass on Glass

Sunglasses are rated by Visible Light Transmission (VLT)—that’s the percentage of light that passes through the lens. The lower the number, the darker the lens.

Typical category 3 sunglasses (which is most of what’s sold) have a VLT of 10–15%. That’s great when you're in direct sun, hiking, fishing, or on the beach—but once you step behind a car windshield, things change fast.

Why?

Because windshields already block light, often reducing it by 30–35% before it even reaches your eyes. Combine that with your sunglasses, and you’re now down to just 10% or less of visible light reaching your eyes. That’s darker than most people realize—and in many conditions, it’s too dark for comfort or even safety.

This is what a typical car looks like with tinted windows in the back, with only a slight tint in the front. (This is why I don't wear sunglasses in the back seat normally - it's too dark.)

That slight tint is deceiving, though. Even clear lenses like regular eyeglasses or untinted windows block around 9% of light. Standard car windows in the US and Canada are usually tinted to 30%, which you generally wouldn't notice but it's apparent when you compare like this:

What does this mean for total light coming in, together with sunglasses?

If there were 100 units of light coming from the sun, only 70 are getting to the sunglasses once the window does its part. If the sunglasses are a 15% VLT, which is standard for dark sunglasses, they will block 85% of the 70 units coming through, so only 10.5 units are making it to your eyes.

10.5% vs 15% when standing outside the car. That is, of course, 30% darker. That’s a pretty big difference when it comes to contrast, clarity, and comfort, especially in shaded or overcast conditions. It makes the category 3 lenses closer to a category 4 lens.

When sunglasses are too dark while driving, it can affect:

• Depth perception

• Contrast sensitivity

• Reaction time

• Visibility in shaded or variable light conditions

On overcast days, shaded roads, tunnels, or when driving at dusk, overly dark lenses can quickly become a liability.

Serengeti Lenses: A Smarter Match for Driving

Here’s where Serengeti lenses become uniquely valuable.

Serengeti lenses combine photochromic, Spectral Control (contrast enhancement), and polarization (optional) in one package. But as we covered in this article, their photochromic activation is significantly reduced behind a windshield.

Instead of seeing that as a negative, it makes perfect sense for driving. Because their lenses don’t darken fully behind glass, Serengeti’s driving lenses typically hover around 20% VLT under most conditions in a car. That’s noticeably brighter than standard sunglasses, but often just right behind a windshield.

Other brands also have great lenses with higher VLT, but I wanted to highlight Serengeti because they really thrive here. Some others worth looking at are Oakley Prizm Ruby, Maui Jim Blue Hawaii in MauiPure material, Maui Jim HT, and Dita Lancier Land.

Most people never think about the VLT of their sunglasses or the tint of their car windows. But once you combine both, you’re often getting double-dimmed—and that has real consequences on how well you can see.

Let Me Show You

Let's start with a mixed day, sometimes sunny when the clouds part, sometimes darker. Without sunglasses, you can see, but you will be squinting heavily. For much of the US, EU, and Canada, this is what you will see most on a typical highway drive.

Now let's put on a brown lens with a 20% VLT. The colors are vivid, bright, and very easy on the eyes. Remember, the windshield is helping out with a 70% VLT as well, so it feels very comfortable, not too dark.

Now, if you have a brown lens with a 10% VLT, it will be too dark. Clearer colors and more contrast and definition than with nothing, but not enough light to give proper information about the road and the distance.

Now let's see a very bright, clear day. It is not just a matter of squinting here, it is nearly impossible to open your eyes. It is hazardous. And that's with the slight tint of the windshield.

Here, I used a 15% VLT on a grey lens like Randolph American Gray. The view is now comfortable and safe, you can easily make out all details, and the colors are nice and not washed out.

If in this situation you wanted to wear a 10% VLT lens like Costa 580 Grey, it would be even more comfortable, but not remarkably so. I would probably use a lens like this in this case, but the point is that you don't have to. The tinted window allows relatively lighter lenses (but still dark ones) to be comfortably worn in bright conditions.

Should You Still Use Darker Lenses for Driving?

It depends. On very bright days or if your car has untinted front windows, a darker lens might still work fine. But for many people, especially those who find their sunglasses too dark for early morning, late afternoon, or cloudy days, switching to a higher-VLT lens can be a game-changer.

If you are light sensitive, you may want a darker tint as well. But not as many people are as light sensitive as they think; they are just wearing poor shades.

Conclusion - Your Driving Sunglasses Might Be Too Dark

When it comes to driving, it’s easy to assume that darker is better—but that’s not always the case. Once you factor in the light filtering from your windshield and front windows, many standard sunglasses end up being too dark for optimal vision on the road. That can impact your ability to see clearly, react quickly, and stay comfortable, especially in everyday driving conditions that aren’t full, blinding sun.

That’s why lenses like Serengeti’s, which maintain a moderate VLT behind glass, are so well suited for driving. They give you enough protection from bright light, but still allow plenty of visibility, contrast, and clarity when the sun ducks behind the clouds or you head into a tunnel.

Ultimately, it’s not just about how dark your sunglasses are—it’s about how they work with your environment. And once you start thinking about VLT in that context, you’ll understand why so many people unknowingly drive with lenses that are too dark for their needs.