What is Infrared Light?

Infrared light lies between the visible and microwave segment of the electromagnetic spectrum. Infrared light has a range of wavelengths that range from red light to violet. "Near infrared" light is closest in wavelength to visible light, while "far infrared" is closer to the microwave region of the electromagnetic spectrum.

Figure 1. Infrared Region of the Electromagnetic Spectrum - Image Courtesy of NASA

Far infrared waves are thermal radiation. In other words, the heat that we feel from sunlight or a fire is far infrared. Far infrared light is even used to heat food sometimes. Special lamps emitting far infrared waves are often used in fast food restaurants. On the other hand, shorter, near infrared waves used by your TV's remote control.

Figure 2. Infrared image of a cat - Image Courtesy of NASA

Seeing Through Infrared

The primary source of infrared radiation is heat or thermal radiation. It can be considered that any object which has a temperature radiates in the infrared. Even objects like an ice cube emit infrared radiation. The warmer the object, the more infrared radiation it emits.

To create an infrared picture or image, special cameras and films are used. These cameras and films detect the differences in temperature, and allocate different brightnesses or colors to them. This yields an image or picture in which our eyes can interpret. Apart from humans and animals, the Earth, the Sun, stars and galaxies also emit infrared radiation.

Figure 3. Normal-spectrum and infrared image of the Earth - Image Courtesy of NASA

The military also used infrared radiation to assist pilots to fly aircrafts at night. An electro-optical thermal imaging device commonly referred to as forward-looking infrared (FLIR) is fitted onto aircrafts which detects far-infrared energy. These far-infrared energy is then converted into an electronic signal to generate a picture for day or night viewing. It is worthy to mention that the infrared wavelength that forward-looking infrared detects is very different to that of night vision. Night vision uses visible light and near infrared ranges.

Source: AZoOptics