Daylighting, or the use of natural daylight to illuminate the interior spaces of a building, is an important consideration in modern building design. It has been shown to positively impact the health, well-being, and productivity of occupants while reducing energy consumption. Visual Light Transmittance (VLT) is a crucial factor in determining the performance of glazing systems used in daylighting, and understanding its significance is essential for architects, designers, and building professionals. In this blog article, we will delve into the concept of Visual Light Transmittance, its importance, and how it affects daylighting in buildings.

What is Visual Light Transmittance (VLT)? Visual Light Transmittance (VLT) is a measure of the amount of visible light that is transmitted through a glazing system, expressed as a percentage. It represents the portion of visible light that passes through the glass and enters the interior space, contributing to the natural daylighting of a building. VLT is typically measured in the visible light spectrum, which ranges from 380 to 780 nanometers (nm).

Importance of Visual Light Transmittance in Daylighting Visual Light Transmittance plays a crucial role in the performance of glazing systems used in daylighting for several reasons:

1. Daylight availability: VLT determines the amount of visible light that can penetrate through the glazing and enter the building’s interior, contributing to the overall daylight availability. A higher VLT means more visible light passes through the glazing, resulting in brighter and more naturally lit spaces.
2. Visual comfort: VLT affects the visual comfort of building occupants. Glazing with a high VLT allows for a balanced distribution of daylight, reducing glare and creating visually comfortable spaces that promote well-being and productivity.
3. Energy efficiency: VLT impacts the energy efficiency of a building as it determines the amount of natural daylight that can be effectively utilized, reducing the reliance on artificial lighting during daylight hours and lowering energy consumption.
4. Aesthetics: VLT also affects the aesthetics of a building, as it determines the amount of natural light that enters the interior spaces, enhancing the visual appeal of the building and creating visually pleasing environments.

Calculating Visual Light Transmittance Visual Light Transmittance can be calculated using the following formula:

VLT = (Illuminance transmitted through the glazing / Incident illuminance on the glazing) x 100%

The illuminance is typically measured in lux, which is the unit of measurement for the intensity of light on a surface. Incident illuminance refers to the intensity of light falling on the glazing from the exterior, and transmitted illuminance refers to the intensity of light passing through the glazing and entering the interior space.

Factors Affecting Visual Light Transmittance Several factors can affect the Visual Light Transmittance of a glazing system, including:

1. Type of glazing: Different types of glazing, such as clear glass, tinted glass, or coated glass, have different VLT values. Clear glass typically has a higher VLT, allowing more visible light to pass through, while tinted or coated glass may have lower VLT values, reducing the amount of visible light transmitted.
2. Thickness of glazing: The thickness of the glazing can also impact the VLT, with thicker glazing typically resulting in lower VLT values due to increased light absorption.
3. Coatings and treatments: Coatings and treatments applied to the glazing, such as low-e coatings, can also affect the VLT. Low-e coatings are designed to reduce heat gain or heat loss, but they can also affect the amount of visible light transmitted.

Here’s a table showcasing common types of glass used in windows, along with their corresponding Visual Light Transmittance (VLT) and U-values:

Type of Glass	Visual Light Transmittance (VLT)	U-value (W/m^2K)
Clear Glass	80-90%	5.8-6.2
Tinted Glass	30-70%	2.6-3.1
Low-E Glass	40-70%	1.8-2.0
Reflective Glass	20-30%	2.5-2.7
Insulated Glass	70-80%	1.5-2.0

Note: The values mentioned in the table are approximate and may vary depending on the specific manufacturer, product, and glazing configuration. U-value measures the heat transfer through a glass pane, with lower values indicating better insulation properties.

It’s important to note that the selection of glass for windows depends on various factors such as climate, orientation, energy efficiency goals, and design preferences. It’s crucial to consult with a qualified professional or refer to the manufacturer’s specifications to determine the exact VLT and U-values for specific glass products. Additionally, building codes and regulations may also impact the permissible VLT and U-values for windows in different regions.

The requirements for VLT in GRIHA are based on the type of building and its location. Generally, GRIHA recommends the following VLT requirements for different types of buildings:

1. Residential Buildings:

• Living spaces: VLT of at least 40%
• Other spaces (except bathrooms and service areas): VLT of at least 30%

2. Commercial Buildings:

• Workspaces: VLT of at least 40%
• Other spaces (except bathrooms and service areas): VLT of at least 30%

3. Institutional Buildings:

• Classrooms, libraries, and common spaces: VLT of at least 30%
• Other spaces (except bathrooms and service areas): VLT of at least 20%

It’s important to note that these requirements are indicative and may vary depending on factors such as building orientation, climate zone, and specific project requirements. It’s recommended to consult the latest GRIHA guidelines and work with a qualified professional to ensure compliance with the applicable VLT requirements. Additionally, it’s essential to consider other aspects of window design, such as shading, glare control, and daylighting, to optimize visual comfort and energy performance in buildings.

As per the ECBC, the requirements for VLT of glass in windows depend on the orientation of the building and the climate zone. The ECBC classifies buildings into three climate zones in India: Hot and Dry (HD), Warm and Humid (WH), and Composite (C).

The following are the recommended VLT requirements for windows in different climate zones as per ECBC:

1. Hot and Dry (HD) Climate Zone:

• Living spaces: VLT of at least 30%
• Other spaces (except bathrooms and service areas): VLT of at least 20%

2. Warm and Humid (WH) Climate Zone:

• Living spaces: VLT of at least 40%
• Other spaces (except bathrooms and service areas): VLT of at least 30%

3. Composite (C) Climate Zone:

• Living spaces: VLT of at least 30%
• Other spaces (except bathrooms and service areas): VLT of at least 20%

It’s important to note that these requirements are indicative and may vary depending on the specific project requirements and compliance path chosen (e.g., prescriptive, trade-off, or whole-building performance). It’s recommended to refer to the latest ECBC guidelines and work with a qualified professional to ensure compliance with the applicable VLT requirements, along with other energy performance requirements of the ECBC. Additionally, it’s essential to consider other aspects of window design, such as shading, glare control, and daylighting, to optimize visual comfort and energy performance in buildings.