Thermal energy is a fundamental aspect of our environment, constantly emitted by all objects with a temperature above absolute zero. This energy, often perceived as heat, travels in the form of electromagnetic radiation. A common question arises regarding this invisible energy: can it pass through glass, a material transparent to visible light? Understanding how different forms of radiation interact with various materials helps clarify this intriguing phenomenon.
Understanding Thermal Radiation
Thermal radiation refers to the electromagnetic waves emitted by the thermal motion of particles within matter. While our eyes detect visible light, thermal cameras are designed to sense a different part of the electromagnetic spectrum known as infrared (IR) radiation. The electromagnetic spectrum encompasses a broad range of wavelengths, from radio waves to gamma rays, with infrared falling between visible light and microwaves.
The infrared spectrum is further divided into several bands: near-infrared (NIR), mid-wave infrared (MWIR), and long-wave infrared (LWIR). Thermal imaging cameras primarily operate by detecting long-wave infrared radiation, which typically spans wavelengths from 8 to 14 micrometers. This specific range is chosen because it corresponds to the radiant heat emitted by objects at typical ambient temperatures, including the human body.
Glass and Infrared Radiation
Standard glass, commonly used in windows, interacts differently with various parts of the electromagnetic spectrum. While it allows visible light to pass through with minimal obstruction, its behavior changes significantly when confronted with infrared radiation. Glass is largely opaque to mid-wave and, more importantly, long-wave infrared radiation.
The primary mechanism by which glass blocks LWIR is absorption. When LWIR radiation strikes a glass surface, the glass molecules absorb this energy. This absorption causes the glass to heat up, and it then re-emits its own thermal radiation based on its temperature. Consequently, the thermal energy from objects behind the glass does not pass through directly; instead, the thermal camera detects the temperature of the glass itself.
Practical Implications for Thermal Imaging
The opaque nature of standard glass to long-wave infrared radiation has significant practical implications for thermal imaging technology. Thermal cameras cannot “see through” windows or other glass barriers. When a thermal camera is pointed at a glass surface, it will primarily capture the temperature of that glass, not the objects or environment beyond it.
This limitation means that in scenarios such as security surveillance, a thermal camera placed indoors cannot detect a person standing outside a window. Similarly, during home energy audits, thermal imagers cannot directly assess heat loss or gain through a window pane from the opposite side; they would only show the surface temperature of the glass. Instead of a clear image of what is behind the glass, a thermal camera might display a blurry reflection of objects in front of the glass or simply the glass’s own temperature signature.
Materials That Transmit Thermal Radiation
While standard glass impedes long-wave infrared radiation, several specialized materials are engineered to be transparent to these wavelengths. Germanium is widely used in thermal camera lenses and infrared windows due to its effective transmission of infrared radiation.
Other materials capable of transmitting LWIR include silicon, zinc selenide, zinc sulfide, calcium fluoride, and magnesium fluoride. Certain specialized plastics, such as thin films of polyethylene, can also allow long-wave infrared to pass through.