The human body is an active biological machine. A fundamental principle of physics dictates that any object above absolute zero must emit electromagnetic radiation in the form of photons. Humans constantly radiate energy outward as a natural byproduct of biological processes, making us light-emitting objects. This emission consists of two distinct forms: one overwhelmingly dominant and tied to our temperature, and another extremely faint one linked to cellular chemistry.
The Dominant Emission: Thermal Infrared Radiation
The vast majority of radiation emitted by the human body exists in the infrared (IR) region of the electromagnetic spectrum. This thermal energy is invisible because its wavelengths are significantly longer than visible red light. Physicists often approximate the body as a “blackbody” because it absorbs and emits energy efficiently.
The wavelength of this radiation depends directly on our core temperature, maintained around 37 degrees Celsius (98.6 degrees Fahrenheit). The peak emission falls within the mid-to-far infrared range, typically 9.3 to 12 micrometers (µm). This long-wave infrared is the heat we feel radiating from a person.
Cellular Metabolism and the Source of Heat
The thermal radiation is a direct result of cellular metabolism, the body’s internal engine. Cells continuously break down nutrients to generate adenosine triphosphate (ATP), the primary energy currency for all biological functions. This process, known as cellular respiration, occurs primarily within the mitochondria.
The conversion of fuel sources like glucose and fatty acids into ATP is not perfectly efficient. During these complex chemical steps, 60 to 70 percent of the initial chemical energy is released as heat rather than being stored in ATP molecules. Heat is also generated when stored ATP is used to perform work, such as powering muscle contraction. The hypothalamus regulates this heat production through thermoregulation, balancing the heat generated by metabolism with the heat lost to the environment to maintain a stable core temperature.
Beyond Heat: The Phenomenon of Biophotons
Beyond the dominant thermal emission, the human body releases an extremely faint form of light called biophotons. Unlike infrared radiation, biophotons are non-thermal and fall within the visible and near-ultraviolet range (approximately 100 to 800 nanometers). This emission is thousands of times weaker than visible light, making it undetectable without highly specialized instruments.
Biophotons originate from specific chemical reactions during oxidative metabolism, not simple heat. They arise when molecules, often involving byproducts like reactive oxygen species (ROS), transition from an excited electronic state back to a lower energy state, generating light. Biophoton emission levels correlate with the body’s circadian rhythm and overall cellular activity, often peaking in the late afternoon or evening. Measuring this ultraweak light offers a non-invasive window into cellular oxidative stress and metabolic health.
How We Measure Human Emissions
The two distinct forms of human light emission require vastly different technologies for detection.
Measuring Thermal Infrared
Thermal infrared radiation is routinely captured using thermal imaging cameras, or thermographs. These devices use specialized sensors to detect the mid-to-long infrared wavelengths emitted by the skin. The camera translates the intensity of the radiation into a visual heat map, or thermogram, where colors represent surface temperatures. This technology is widely used in medical settings to assess blood flow and inflammation, and in security operations to locate people by their heat signature.
Measuring Biophotons
To measure ultraweak biophotons, researchers employ highly sensitive equipment, such as Photomultiplier Tubes (PMTs) or specialized, cooled CCD cameras. These detectors count individual photons and require an almost perfectly dark, electromagnetically shielded environment, often a Faraday cage, to eliminate external noise. Biophoton measurement data is primarily used in research to study cellular communication, assess antioxidant efficacy, and monitor physiological stress levels.