What is Infrared Heat?

Winter sun-bath. Skiers bathing in the sun's infrared.
Skiers bathing in the sun’s infrared

Infrared is radiated heat: the feeling of warmth from the sun on your face; the heat from a coal fire, or a toaster. It is even the same form of heat emitted by your own body. It is the most basic form of heating known to man.

Used by cavemen to heat themselves by fires; by Romans in their hypocausts, by log burners and tile stoves, Infrared heating has been favoured for millennia because like the heat of the sun on your surrounding environment – even during winter – Infrared heats objects, which then radiate back and keep the environment warm around you. Radiant heat does not heat air – which holds little heat and rapidly disperses.

Infrared waves travel through the air and when they touch a surface, heat energy is released regardless of the surrounding air temperature. That heat energy excites the molecules in the object it meets which being to vibrate and gain energy (and warm up). Water absorbs Far Infrared specifically well, and as our skin is 80% water, we are perfectly adapted to Far Infrared (Robinson, 2014). Far Infrared – unsurprisingly – the same band of infrared that the human body itself emits.

All objects (including people) absorb and emit infrared and whether one is absorbing or emitting depends on the difference in temperatures between objects in an environment. If objects in an environment are warmer than you are, you will warm up from them. If you are warmer than objects in an environment you will radiate out to them and feel cold. (This Infrared emission is why police Infrared cameras can see fugitives trying to escape detection). But it is also why we can still feel cold in centrally heated rooms, which only heat the air and don’t heat the objects within a room.

If you are in a centrally heated room at 21°C with your back to an outside wall at 17°C, you will be radiating heat out to that outside wall and you will therefore feel cold: regardless of the room’s “comfortable” air temperature. This underlies a fundamental difference between infrared and “convection” heating.

An experiment at the John B. Pierce Laboratory, USA, clarified the different human perceptions of heat:

“Test persons in a room with a temperature of 50°C (122°F) of warm air and cooled walls froze deplorably; when in a room with a cool air temperature of 10°C (50°F) and warm walls, they broke into an unpleasant sweat.”
(source: Techn. Info “Strahlungsenergie – die Ur-Energie, neu entdeckt, TT Technotherm GmbH, Nürnberg).

Feeling warm has nothing to do with air temperature. It is all about absorption of infrared from our surroundings (warming up) or stopping ourselves losing radiation (cooling down) to a “colder” outside.

But in the last 60 years, we have forgotten about radiant heating: not because a better technology replaced it, but because fossil fuels that powered central heating made it so cheap just to heat air.

Why do you say “Far” Infrared? What does it mean?

Because Infrared Heat covers the whole spectrum of radiated heat (ranging from very intense heat from a light bulb at 2600°C to heat you’d feel from a glowing coal at 600°C, to a rock, warmed to 20°C by the sun), 3 correspondingly very different categories of Infrared have been defined, exactly fitting the above examples. These are “Near” Infrared (Also called Shortwave or IR-A); “Medium” Infrared (also called Middlewave or IR-B) and “Far” Infrared (also called “Longwave” or IR-C).

And exactly along the above categories, from a point of view of Comfort Heating, the only acceptable waveband is Far Infrared (Humans reach optimal “Comfort” at around 21°C).

  • Shortwave is too harsh. The body has developed certain protection mechanisms over millennia to avoid it (not looking directly at the sun; seeking shade from direct sunlight (Voke: 1999); the skin, indeed also reflects up to 35% of received shortwave and does not efficiently absorb the heat – indeed shortwave is transmissive into the skin producing discomfort (a signal to get out of its way). It is not well absorbed. (Schroeder et al 2008). Heat lamps emitting shortwave do not make appropriate Comfort heaters.
  • Middlewave is better absorbed by skin and less reflected than shortwave, but in terms of Comfort heating of humans, emitters that output Middlewave heat are still strong (e.g. 600°C) and better adapted to industrial heating and drying processes than to provision of comfort.
  • Longwave or Far Infrared on the other hand is the waveband at which water begins to absorb heat specifically well for the least input energy of the above 3 wavelengths.  Far Infrared heat is optimally absorbed by the skin surface, where the warmth is readily absorbed by conduction into the tissue and blood and transported around the body.  This is why Far Infrared is used in heating cabins and baby incubators and why Herschel specifically uses Far Infrared heaters.

See also “Preferred wavelengths for Comfort Heating“.

What Makes Herschel Panels Far Infrared?

The wavelength of Infrared produced by Herschel Heaters specifically relates to their surface temperature and surface area, which we require to operate at 1 kilowatt of heat energy per square metre (or 0.1 watts per square cm). This “watt density” as it is called projects Far Infrared within a 2.5 to 3 metre distance from the heater (which spreads out radially from the panel to cover an arc up to 5 metres by 5 metres depending on panel power). This coverage is perfect for domestic or office “comfort” heating.

However, this “watt density” is also key to the panel’s energy efficiency. As we said, human skin reacts optimally to Far Infrared heat, because of its 80% water content. You do not need to apply any more energy than 0.1 watts per cm2 of panel surface area in order to produce “Comfort” within the distances shown above.  Early Infrared panels achieved this with thinly stretched, tightly coiled wire densely placed over the panel surface area, but more modern panels with carbon fibre and carbon crystal elements can achieve this watt density even more efficiently (being very close to 100% energy consumed to energy output at the panel surface).



Professor Anthony Robinson, Department of Mechanical and Manufacturing Engineering, Trinity College, Dublin 2.

Peter Schroeder, Judith Haendeler, Jean Krutmann, The role of near infrared radiation in photoaging of the skin, Experimental Gerontology, Volume 43, Issue 7, July 2008, Pages 629-632, ISSN 0531-5565

Dr. Janet Voke, Radiation effects on the eye, Part 1 – Infrared radiation effects on ocular tissue,Optometry Today, May 1999