Infrared light does not have a color because it is invisible to the human eye. This form of energy is classified as electromagnetic radiation, existing just outside the range our biology can perceive. It is positioned on the electromagnetic spectrum immediately adjacent to the longest wavelengths of visible red light, which is the origin of its name, meaning “below red.” Since color is a perception created by the brain’s processing of light, and infrared radiation never reaches the sensory cells, it lacks a visual hue.
Defining Color and the Visible Spectrum
The sensation of color is not an inherent property of light itself but rather a neurological interpretation of a very narrow slice of the electromagnetic spectrum. The human eye is equipped with photoreceptor cells, specifically cones, which are sensitive only to radiation with wavelengths between approximately 380 and 780 nanometers. This small window is known as the visible spectrum.
Within this visible range, different wavelengths correspond to the colors we perceive, from violet at the shorter end to red at the longer end. When light hits an object, some wavelengths are absorbed while others are reflected. The reflected light enters the eye, sending a signal to the brain, which translates the specific wavelength into the experience of a distinct color.
Why Infrared Light Is Invisible
Infrared radiation sits next to the visible spectrum, starting at wavelengths of about 700 to 780 nanometers and extending up to one millimeter. This greater length means infrared light has a lower frequency and less energy per photon compared to visible light. Our eyes did not evolve the biological mechanisms to detect these longer wavelengths.
The photoreceptors in the human retina are designed to react to the energy levels of visible light, but they cannot be excited by the lower energy of infrared photons. The infrared range is broadly categorized into sections like Near-Infrared (NIR) and Far-Infrared (FIR). Thermal imaging, which captures heat energy, typically operates in the Long-Wave Infrared (LWIR) range, which has significantly longer wavelengths, often between 8 and 14 micrometers.
How We Visualize Infrared
The colored images we see labeled as infrared are visual representations of data collected by specialized equipment, not the color of the light itself. Devices like thermal cameras are built with sensors that absorb infrared radiation, which is emitted as heat by all objects above absolute zero. The intensity of this radiation is directly proportional to the object’s temperature.
A thermal camera’s sensor array measures the amount of infrared energy emanating from different points in a scene. The camera’s internal processor converts these energy measurements into electronic data points. This data is translated through “false color” or “pseudocolor” mapping. The camera assigns arbitrary colors to represent the measured temperature range, such as using white or red for the hottest areas and blue or black for the coldest areas.
This assigned color map allows humans to visually interpret invisible heat signatures. Thermal imaging is widely used in applications like home energy audits or night vision surveillance. The colors serve as a key to interpret temperature variations, making the invisible data visible. The colors seen in a thermal image are a technological tool for interpretation, not the inherent color of the radiation.