Light passing through a window is often confused with direct sunlight. While the light streaming into a room looks bright and feels warm, the physical properties of glass fundamentally alter the solar radiation before it reaches an indoor space. Scientifically, light transmitted through a standard window pane is not considered true direct sunlight because the glass acts as a selective filter, removing specific wavelengths of the electromagnetic spectrum.
The Composition of Outdoor Sunlight
True direct sunlight is a mixture of electromagnetic radiation characterized by different wavelengths that reach the Earth’s surface. The solar spectrum is broadly divided into three main categories: visible light, infrared (IR) radiation, and ultraviolet (UV) radiation. Visible light accounts for approximately 42 to 43 percent of the total energy and is the only part of the spectrum detectable by the human eye. The largest portion of solar energy, around 52 to 55 percent, comes as infrared radiation, which we perceive primarily as heat. The remaining fraction, roughly 3 to 8 percent, is ultraviolet radiation, which is invisible but has the highest energy and is categorized into UV-A and UV-B rays.
UV-A rays are the longer-wavelength ultraviolet component, comprising about 90 to 95 percent of the UV radiation that penetrates the atmosphere and reaches the ground. These rays are known to penetrate deeper into the skin layers and are primarily associated with photoaging and long-term damage. UV-B rays, which make up the smaller portion of 5 to 10 percent, affect the superficial layers of the skin, causing sunburn and triggering the synthesis of Vitamin D.
Wavelength Filtration by Common Glass
Standard architectural window glass, typically made of soda-lime silicate, fundamentally changes the composition of incoming solar energy through selective absorption and transmission. The glass structure is highly effective at blocking the shorter, high-energy UV-B rays, with most panes filtering out about 90 to 97 percent of this radiation. This substantial blockage explains why a person rarely experiences a sunburn when sitting behind a closed window.
In contrast, the longer-wavelength UV-A rays are not blocked as completely and pass through standard glass more easily. While transmission varies based on the glass type and thickness, a considerable amount of UV-A radiation is transmitted, sometimes allowing as much as 74 percent to pass through the pane. This difference in filtration means the light entering the room is missing the majority of the UV-B component.
The other components of the solar spectrum, visible light and infrared radiation, are largely transmitted through the glass with minimal disruption. The high transmission of visible light makes the interior space bright and allows us to see clearly. A significant amount of the infrared radiation also passes through, which is why a sunny spot near a window still feels noticeably warm.
Practical Effects of Indoor Sunlight
The selective filtering action of window glass has several consequences for human health and for the materials inside a building. Since the majority of the skin-burning UV-B rays are blocked, there is no risk of immediate sunburn when sitting near a window. However, the transmission of UV-A rays means that the skin is still exposed to radiation that can penetrate deep into the dermis.
This residual UV-A exposure contributes to cumulative skin damage, accelerating photoaging and leading to wrinkles over time. While UV-B is blocked, Vitamin D synthesis, which requires UV-B exposure, cannot occur behind standard window glass. The filtered indoor sunlight also drives the fading and degradation of materials within the home. This damage to furniture, photographs, and artwork is primarily caused by the combination of transmitted UV-A rays and intense visible light, which possess enough energy to break down chemical bonds in dyes and pigments.
The high transmission of infrared radiation is the primary cause of heat gain experienced in sunlit rooms. This radiant heat significantly warms up interior surfaces and the air, contributing to a noticeable temperature increase near the window. This often places a greater demand on air conditioning systems.