Infrared goggles: the physics of seeing the invisible

We need light to see; in total darkness, we can see nothing. Yet, even in broad daylight, most of the electromagnetic radiation emitted by the Sun remains invisible to us. For example, we can't detect the infrared part of the electromagnetic spectrum because the photoreceptors in our eyes are not sensitive to it. This leads to a natural question: if we can't perceive infrared wavelengths during the day, when they are abundant, how do infrared (IR) goggles allow us to sense them at night, when the Sun, their primary source, has disappeared? The answer lies in the design of the goggles (Fig. 1) and in the fundamental ways light interacts with matter.

Fig. 1. Infrared goggles used for thermal imaging.

Light beyond human vision

The electromagnetic spectrum spans a vast range of wavelengths (Fig. 2), most of which are invisible to the human eye. The Sun emits radiation primarily in the visible, infrared, and ultraviolet regions of the spectrum. Although beyond our visual perception, infrared and ultraviolet radiation behave like visible light in many respects: they propagate through space, carry energy, and interact with matter. When sunlight reaches an object, three main interactions can occur:

• Reflection. Some wavelengths are reflected. The portion reflected in the visible range determines the colours we see.

• Transmission. Some light passes through the material. This interaction, called transmission, typically occurs in optically transparent substances such as glass.

• Absorption. Some radiation is absorbed, transferring electromagnetic energy to the material and converting it into heat.

Fig. 2   Full electromagnetic spectrum, from radio waves to gamma rays. Source: BBC Bitesize

The first two interactions require a continuous flow of radiation from an external source. If the source disappears, there is no light, visible or invisible, for the objects to reflect or transit. Absorption, on the other hand, follows different rules. The absorbed radiation does not immediately return to the surroundings.  Rather, it remains within the material, where it makes atoms and molecules vibrate more vigorously. These microscopic vibrations manifest as thermal energy, which we perceive as heat.

 An object that has absorbed energy becomes a source of its own thermal radiation. It continues to emit electromagnetic waves even in complete darkness, although not necessarily in the visible range. Thermal radiation is so named because its wavelengths depend primarily on the object's temperature. As the temperature rises, the peak emission shifts toward shorter wavelengths. For an emission to move from the invisible infrared to visible red light, temperatures must reach hundreds or even thousands of degrees. Erupted lava, for example, glows red when extremely hot (Fig. 3. But as it cools, its emission shifts back into the invisible infrared.

Fig. 3 Eruption of Arenal Volcano, Costa Rica. Photo by Matthew.landry (Wikimedia Commons, CC BY-SA 3.0).

The Earth's surface does not typically reach temperatures high enough to emit visible light, nor does it cool to the extremely low temperatures found in interstellar space, where microwave radiation dominates. Consequently, objects around us emit primarily infrared radiation. During the day, we rely mainly on reflected visible light to see. But at night, this persistent infrared emission becomes especially important, allowing infrared goggles to provide a different kind of vision.

How infrared goggles work

Even in complete darkness, objects continue to emit infrared radiation, with an intensity that correlates with their temperature. This radiation fills the surroundings with invisible information, a thermal landscape that some nocturnal animals can sense naturally, and that humans access through night-vision devices. The design of infrared goggles begins with a crucial detail: ordinary glass absorbs much of the infrared spectrum, which is one reason windows help retain heat indoors. For this reason, infrared lenses are made from specialised materials that allow infrared radiation to pass through and be focused onto a sensor.

The sensor consists of tiny elements, or pixels, capable of detecting extremely small differences in infrared intensity. These differences are converted into electrical signals. Because the intensity of emitted infrared radiation depends on temperature, these signals can be used to generate a detailed thermal map of the scene.

To create a visible image, the goggles assign brightness levels or colours to the regions of different temperatures. Warmer regions may appear brighter or tinted red, while cooler areas may appear darker or tinted blue. In this way, heat patterns are transformed into images that our eyes and brains can understand. In essence, with night-vision devices, we perceive objects by their heat rather than by the light they reflect.

The key insight is that objects continually exchange information through energy. Infrared technology gives us a new way to access this information, extending our senses beyond their natural limits and unveiling a hidden layer of reality that has been there all along.