The Preferred wavelengths for Comfort Heating
For many, “Infrared Heating” is Shortwave or Middlewave Infrared: the Tungsten Halogen or Quartz lamps that cast the red light and are popular in outdoor heating and large public spaces. Many specifiers and installers by their own admission are aware there are different categories of infrared, but unaware the differences are important when matching a heater type to the correct function.
What we would like to do in this article is compare shortwave, medium and far infrared specifically as they apply to Comfort Heating – that is – the comfortable heating of people or animals where skin contact is both desirable and inevitable. We think you will find the following eye-opening and understand why we choose to use Far Infrared specifically for Comfort heating.
– Herschel Far Infrared is biologically more significant in comfort heating than shorter (hotter) heat wavelengths because skin content is 80% water which only absorbs heat wavelengths from 3 microns and lower (Far Infrared). Far Infrared is better absorbed by the skin than other wavelengths; is less “transmissive” and is not reflected – this is untrue for shorter wavelengths;
– “Hotter” does not mean “more comfortable” but it does mean “more energy”. Quartz heaters may be “hotter” at 3 microns than Far Infrared heaters, but these intensities make this sort of emitter more suitable to Industrial Drying Applications, not Comfort Heating. Herschel does not recommend the use of Quartz heaters for Comfort Heating. Herschel Far Infrared heaters are more effectively comfortable whilst consuming less energy.
– The skin reflects a lot of shortwave energy and does not absorb it effectively. This is a basic defense mechanism to protect the skin from these very hot wavelengths. Shortwave is nontheless very transmissive (i.e. penetrates deeper). Academic research (citations below) recommends skin and eye protection from these very hot wavelengths. Herschel does not recommend the use of Shortwave in Comfort Heating;
Infrared divides into 3 specific – important – groupings according to wavelength.
Infrared -A is classified as the “hottest” Infrared with temperatures up to 2,700C and wavelengths of 0.7 – 1.4 microns and is also called “Short-wave” or “Near” Infrared;
Infrared – B is infrared with temperatures of 500 – 800C and wavelengths of 1.4 – 3 microns and is also called middlewave or “Medium” Infrared;
Infrared – C is infrared with temperatures of less than 500C and is the final and broadest waveband of 3 microns – 1mm and is also called “Longwave” or Far Infrared.The image shows the spectral output of lamps emitting principally in each of these 3 wavelengths noting that:- all of these lamps output in the different wavelengths in different intensities;
– all of them output in the Far Infrared wavelengths but not all of them output in the shorter wavelengths;
– the shorter the emitter wavelength, the larger proportion of energy is emitted in those shorter wavelengths (i.e. they “peak” earlier).
|These spectral groupings are very important to understand as they cover very different temperatures, they range from light-emitting to “dark”, and have very specifc properties in terms of absorption, transmission and reflection by the materials they touch. Reflected infrared does not convert into heat for the target material (skin, fabrics, objects). Transmitted infrared may convert to heat in the transmitting region – depending on the thickness of the target and how much infrared transmits. However transmission is not absorption, it is “heat passing through” and this is very important to understand – especially for “Comfort”. High transmission implies low absorption and vice versa (high absorption = low transmission).However it is absorbed infrared that converts into lasting heat in the target object. High absorption low transmission properties means the direct radiant heat itself may not penetrate very deeply into the target material, but it can be transmitted onwards by conduction within the material (by the blood and other tissues in the case of skin). Because the skin is 80% water, for Comfort heating and gentle drying purposes, we want an emitter optimised to output in the wavelengths at which water absorbs the best (Robinson, 2014) and; which reflects the least and for which transmission is possible by conduction or other means.|
Infrared absorption by Water
|As the graph shows, of the three infrared bands, water only begins to absorb effectively at 3 microns (regardless of depth) and then again at 6 microns (for thin layers). This is the Far Infrared region.Skin absorbs heat principally because of its 80% water content (Robinson 2014), making Medium/Far and Far Infrared the most biologically significant band for heat absorption and this translates to “more effective heating” by heaters peaking in these bands. Heaters emitting IR-A will still heat and still emit Far infrared, but at lower densities than heaters optimised for Far Infrared output (see above graph) and consequently will be less efficient, whilst consuming more energy.Note the Quartz heater in the diagram above (blue line) also peaks at this optimum wavelength, but its watt density is too high for comfort – making it more suited to industrial drying applications and not comfort heating with drying as a benefit. The lower wavelength Far Infrared emitter (amber line) will perform a more suitable comfort heating role, whilst evaporating water more gently.|
Reflection of Infrared by the Skin
|The skin has evolved to accommodate infrared in many ways to maximise (or reject) its various properties and the skin’s reaction depends on wavelength.The image demonstrates the biologically significant reflection of Infrared-A by the skin, which is high in these shortwavelengths and then decreases after 1.4 microns. Snow, leaves of plants and black paint also reflect shortwave similarly.This means that of infrared waves striking the skin, a large proportion of IR-A (Shortwave) will be reflected and not cause heating. This is a defense mechanism of the skin, because these waves are very hot.Infrared in longer wavelengths consequently assists considerably with absorption and low reflection – two essential properties of heating a material. Conversely Infrared-A is neither absorbed well and is highly reflected.|
Transmission of Infrared into the skin
|IR transmission into the skin is a function of wavelength and here, Infrared A, due to its lower absorption, is the most highly tranmissive into skin. Schroeder et al indicate up to 65% of Near IR is capable of reaching up to 2 microns deep in the dermis and beyond 2 microns the skin is opaque to all types of Infrared. Do not confuse transmission with absorption in terms of heat effectiveness. Because of the very high temperatures of shortwave infrared, this high transmission rate can cause damage through greater penetration of the skin and lower rate of absorption.Voke indicates penetration of Infrared into the eye (note this has nothing to do with colour perception, but of transmission of heat) to be as far as the iris and cornea for Infrareds A, B & C, to the aqueous humour for B and to and retina for Infrared A. Key points to note:- No infrared is capable of reaching the muscle layers. Beyond the subcutaneous layer of the skin, heat transfer is via conduction or diffusion, convection of heated fluids, blood etc, not by direct radiant heating and therefore more naturally favours processes that encourage absorption.- Infrared B and C ony transmit into the skin’s surface micro region, but are more highly absorbed into water and the blood than Infrared A making more effective onward conduction into body fluids and conduction through the tissue and blood.|
– The fact that Infrared A is more transmissive in both the skin and eyes overcomes the natural high reflectivity of the skin which is a defence mechanism. Installers and Operators of shortwave infrared must bear in mind the high temperatures of Infrared A and consider the exposure time and distance from the heater.- Schroeder and others indicate Infrared A as being as harmful as UV light in terms of its ageing effects (Schroeder P, Calles C, Krutmann J. Prevention of infrared-A radiation mediated detrimental effects in human skin. Skin Therapy Letters. 2009 Jun;14(5):4-5.)- Eye damage can also occur from prolonged IR-A exposure and goggles and time limits are recommended (Dr. Janet Voke, Radiation effects on the eye, Part 1 – Infrared radiation effects on ocular tissue, Optometry Today, May 1999).In conclusion:– Herschel Far Infrared emits the biologically most significant Comfort Heating wavelengths because skin content is 80% water which only absorbs heat wavelengths from 3 microns and lower (Far Infrared). Far Infrared is therefore better absorbed by the skin, less transmissive and less reflected;
– “Hotter” does not mean “more comfortable” but it does mean “more energy”. Quartz heaters may be “hotter” at 3 microns than Far Infrared heaters, but these intensities make quarz emitters more suitable to Industrial Drying Applications, not Comfort Heating. Herschel does not recommend the use of Quartz heaters for Comfort Heating. Herschel Far Infrared heaters are more effectively comfortable and consume less energy;
– The skin reflects shortwave energy and does not absorb it effectively. This is a basic defense mechanism to protect the skin from these very hot wavelengths. Shortwave is very transmissive (i.e. penetrates deeper). Academic research (citations below) recommends skin and eye protection from these very hot wavelengths. Herschel does not recommend the use of Shortwave heaters for Comfort Heating;
See the following summary of current research into this topic: “Preferred wavelengths for Comfort Heating“. Dr Gerard McGranaghan, Ceramicx Ltd.
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,
Soyun Cho, Mi Hee Shin, Yeon Kyung Kim, Jo-Eun Seo, Young Mee Lee, Chi-Hyun Park and Jin Ho Chung, Effects of Infrared Radiation and Heat on Human Skin Aging in vivo, Journal of Investigative Dermatology Symposium Proceedings (2009) 14, 15–19;
Schroeder P, Calles C, Krutmann J. Prevention of infrared-A radiation mediated detrimental effects in human skin. Skin Therapy Letters. 2009 Jun;14(5):4-5.
Dr. Janet Voke, Radiation effects on the eye, Part 1 – Infrared radiation effects on ocular tissue, Optometry Today, May 1999