Light Transmission – Definition

Light transmission, or transmittance (T), describes how much light passes through a material. It is expressed as a percentage of the incoming light that makes it through.

In general, the lower the light transmission (also indicated as VLT% – Visible Light Transmission), the darker the spectacle lenses. Eyewear is one of the most common applications of this property.

Types of Materials by Transmission

• Transparent materials – transmit most of the incident light (e.g., clear glass, water).
• Translucent materials – transmit only part of the light (e.g., colored or frosted glass, thin paper, mica).
• Opaque materials – do not transmit light at all (e.g., wood, most metals).

How Light Interacts with Materials

According to basic optical principles, light striking a material is divided into transmitted light and reflected light.

Practical Applications

• eyeglasses and sunglasses
• insulating window glass
• automotive glass with films
• photovoltaic panels
• laboratory optics

Transmission in Eyeglasses

For eyeglasses, light transmission is usually high, typically between 90 and 98%. Their purpose is to correct vision without significantly reducing the amount of transmitted light. Anti-reflective coatings can slightly increase transmission.

Transmission in Sunglasses

In sunglasses, light transmission is a key factor when choosing lenses. Transmission levels are categorized according to the international standard EN ISO 12312-1.

Common sunglasses have transmission values between 8% and 43%, though variants outside this range exist.

• Above 80% – more of a decorative accessory than protection.
• 3–8% – intended for extreme conditions (e.g., high mountains, glaciers); not suitable for driving.

Special Types of Lenses

Photochromic Lenses

Photochromic (self-tinting) lenses react to UV radiation by changing their light transmission.

• Indoors: clear, about 90% transmission.
• Outdoors: darkened, about 10–30% transmission.

Polarized Lenses

Polarized lenses reduce glare from reflective surfaces such as water, snow, or glass, improving visual comfort.