## Visible Transmittance (VT) and Light-to-Solar Gain (LSG) ### Overview Visible Transmittance (VT) and the Light-to-Solar Gain (LSG) ratio are critical [[Thermal and Optical Performance Metrics]] used in the evaluation and specification of [[Fundamentals of High Performance Glazing Systems|high-performance glazing systems]]. VT quantifies the daylighting potential and glare control capabilities of a fenestration product, while LSG serves as a key indicator of its spectrally selective performance, balancing visible light transmission with solar heat rejection. These metrics are fundamental for optimizing [[Operational Energy Savings and GHG Reduction Potential|building energy consumption]] and occupant comfort, particularly in the context of [[High Performance Glazing Thermal Coefficients International and Indian Building Code Compliance]]. ### Technical Details #### Visible Transmittance (VT) VT, often denoted as Tvis, represents the fraction of the visible spectrum (electromagnetic radiation with wavelengths approximately between 380 nm and 780 nm) that passes through a [[Glazing System Design for Optimal Performance|glazing system]]. It is a dimensionless value ranging from 0 to 1.0. For calculation, VT is determined as a weighted average of the spectral transmittance of the [[Glass Substrates and Composition|glazing material]], using the CIE (Commission Internationale de l'Éclairage) standard photopic luminous efficiency function V(λ) and a standard illuminant, typically D65 (representing average daylight). For instance, a typical 6 mm clear float glass has a VT of approximately 0.90, meaning 90% of visible light is transmitted. Glazing systems with higher VT values maximize natural daylight penetration, reducing the need for artificial lighting, but can also increase glare if not properly managed. #### Light-to-Solar Gain (LSG) The Light-to-Solar Gain (LSG) ratio is a powerful metric that describes the [[Low-Emissivity Coatings Types and Application|spectrally selective performance]] of a glazing system. It is defined as the ratio of the Visible Transmittance (VT) to the [[Solar Heat Gain Coefficient and Solar Transmittance]] (SHGC): $ LSG = \frac{VT}{SHGC} $ A higher LSG ratio indicates that a gl