Visible light transmittance (VLT) is a measure of how much visible light passes through a material, expressed as a percentage from 0% to 100%. A window with a VLT of 70% lets through 70% of the visible light that hits its surface and blocks the remaining 30%. The concept applies to anything light travels through: windows, sunglasses, car tint films, camera filters, and even greenhouse panels.
How VLT Is Measured
VLT specifically measures light in the visible spectrum, which spans wavelengths from about 380 to 780 nanometers. That range covers everything your eyes can detect, from violet at the short end to red at the long end. Infrared heat and ultraviolet radiation fall outside this range, so VLT tells you nothing about how much heat or UV a material blocks. A window could have a high VLT (lots of daylight) while still reflecting most solar heat, which is exactly what many modern coatings are designed to do.
The measurement itself is straightforward. Light is directed at a material, and a sensor on the other side records how much comes through. The result is a ratio between 0 and 1, typically converted to a percentage for convenience. A transmittance of 0.85 means 85% of the light passes through. A perfectly clear material would score 1.0 (100%), though in practice even the clearest glass falls short of that because some light always reflects off the surface or gets absorbed.
VLT in Windows and Building Design
For windows, VLT is one of the key performance ratings you’ll see on product labels. In the United States, the National Fenestration Rating Council (NFRC) sets the standard procedure for measuring both visible transmittance and solar heat gain in windows and doors. When you shop for windows, the NFRC label lists VLT alongside other energy ratings so you can compare products directly.
The trade-off with windows is balancing daylight against heat. A higher VLT means a brighter interior and less need for artificial lighting during the day, but it often comes with a higher solar heat gain coefficient (SHGC), meaning more solar energy enters the building and increases cooling costs. Modern low-emissivity coatings try to split these two apart, letting visible light through while reflecting infrared heat. The ideal VLT for a building depends on the climate, the orientation of the windows, and how the space is used. An office with south-facing glass walls in Phoenix needs a very different VLT than a north-facing living room in Seattle.
Semi-transparent photovoltaic panels, which generate electricity while still functioning as windows, illustrate this trade-off clearly. As VLT decreases on these panels, electricity generation increases and cooling loads drop because less solar radiation enters the building. But the view through the window also degrades, which affects occupant satisfaction, task performance, and psychological comfort. Finding the right VLT is a balancing act between energy savings and livability.
VLT Categories for Sunglasses
Sunglass lenses are classified into five categories (0 through 4) based on how much visible light they transmit. These categories help you pick the right lens for the conditions you’ll be in.
- Category 0 (80%–100% VLT): Clear or nearly clear lenses. These are for indoor use, safety glasses, or overcast days. They offer no meaningful glare reduction.
- Category 1 (43%–80% VLT): Light tints suited for cloudy skies, dawn and dusk driving, or people who are sensitive to light indoors.
- Category 2 (18%–43% VLT): Medium tints for general outdoor use in moderate sunlight.
- Category 3 (8%–18% VLT): Dark lenses for bright conditions like beach days or summer driving. This is the most common category for everyday sunglasses.
- Category 4 (3%–8% VLT): Extremely dark lenses designed for high-altitude snow, glaciers, and intense reflected glare. These block so much light that driving with them is illegal in many countries.
If you’re choosing sunglasses for driving, stay within categories 1 through 3. Category 4 lenses restrict your vision too much for safely reading road signs or adapting to tunnels and shadows.
VLT and Car Window Tint
Window tint regulations for vehicles are set at the state or country level, and they almost always reference VLT. The number you see in tint laws tells you the minimum percentage of light the window must let through after the film is applied. A law requiring 35% VLT on front side windows means those windows must transmit at least 35% of visible light.
Regulations vary widely. Arizona, for example, requires a medical exemption for any tint darker than 33% VLT on certain windows. Some states allow as low as 20% on rear side windows while requiring 70% or more on the windshield. Factory-tinted rear windows on SUVs and trucks often have a much lower VLT than aftermarket tint laws would allow on the front, which is why you’ll see vehicles with noticeably darker glass in the back.
When shopping for tint film, keep in mind that the VLT percentage on the product label describes the film alone. Once applied to glass that already blocks some light, the combined VLT drops further. If your car’s factory glass transmits 80% and you apply a 50% VLT film, the result is roughly 40% (0.80 × 0.50), not 50%.
What Affects VLT in Glass
Several physical properties determine how much light a piece of glass transmits. Thickness is the most intuitive: thicker glass absorbs more light on its journey through, so a 12mm pane transmits less than a 3mm pane of the same material. Tints and coatings also reduce VLT by absorbing or reflecting specific wavelengths. Double-pane and triple-pane windows have lower VLT than single-pane equivalents because light must pass through multiple layers of glass and the coatings between them.
The type of coating matters too. Low-e (low-emissivity) coatings are designed to be selective, reflecting infrared wavelengths while allowing most visible light through. A high-quality low-e window might maintain a VLT above 60% while cutting solar heat gain nearly in half compared to uncoated glass. Reflective or mirrored coatings reduce VLT more aggressively and are common in commercial buildings where controlling glare and heat is a priority over maximizing daylight.
Choosing the Right VLT
The “best” VLT depends entirely on what you need the material to do. For home windows in a cold climate, a high VLT (60%+) brings in free solar warmth and daylight, reducing both heating and lighting costs. In hot climates, a moderate VLT (40%–60%) paired with a low SHGC keeps interiors bright without overheating them.
For sunglasses, most people do well with a category 2 or 3 lens for general outdoor use. If you spend time on water or snow, where reflected light is intense, lower VLT lenses in category 3 or 4 will be more comfortable. For car tint, check your state’s legal limits first, then choose the darkest film that keeps you within those limits if privacy and heat rejection are your goals. Remember to account for your factory glass when calculating the final VLT.