So far we have only discussed thermal infrared – the elegant balance between vibrating atoms and vibrating electro-magnetic waves. Colour occurs in the infrared just as in the visible range and infrared cameras are available to measure the spectrum to distinguish different materials. For instance, some minerals emit less infrared at longer wavelengths so images with pseudo-colour contrast can be generated. Most natural objects, such as vegetation, have no colour and emit thermal infrared to a maximum intensity determined by the temperature. The spectrum is broad from 3 to over 100 microns with a soft peak at 10 microns for normal temperatures. Most cameras are tuned to the infrared windows 3-5 microns and 8-12 microns and the optics are bloomed for the centre of these bands. It is not possible to tune optics for the whole infrared spectrum so it is surprising that in nature we find infrared structures tuned for specific wavelengths. Even more surprising the tuned wavelength is often outside the normal atmospheric windows.
The radiating antenna of the house mouse, Mus musculus, has a distal termination that can be interpreted as a first order diffraction grating and we can determine the tuned wavelength absolutely without need for an effective refractive index.
The angle of emission is not known precisely so a likely range is 15-17 microns. The possibility of a second order grating i.e. two wavelengths between the teeth of the grating (tuning for 8.1 microns) can be tested. The Leonardo Condor camera images in both the 3.7-5 microns and 7.7-10 microns band on alternate frames. The images can be presented side-by-side or a pseudo-colour image showing spectral differences. Condor has not shown anything unusual in a wide range of animals including mice and bats. The cat images shown below are identical which indicates that the emission is purely thermal infrared and there is no excess emission at 8.1 microns. This removes the unlikely possibility that the grating is 2nd order reinforcing the prediction of 15-17 micron emission.
The hair shaft in Mus musculus is a 2nd order fibre Bragg grating and is also tuned to around 16 microns. The common shrew has a radiating antenna that is tuned throughout its length to 16 microns. There is therefore strong evidence that this is the critical wavelength involved in the thermoregulation in small mammals. The problem is:
Infrared emission at 16 microns has never been reported
This strange value is well outside the normal range of thermal infrared. The only other known source of infrared is chemiluminescence but this is a very under-researched field. In the visible, chemiluminescence is the emission of light (luminescence) as the result of a chemical reaction and is well known from everyday examples, such as: glow sticks and bioluminescence in nature. This process is due to light emission from excited, vibrating atoms following a chemical reaction. Infrared emission is associated with vibrating molecules following a chemical reaction. Molecules involving carbon, oxygen, nitrogen and hydrogen have resonances in the infrared. As an example, infrared emission from excited carbon dioxide has important vibration resonances around 15-16 microns. A well known application of radiation from gases is the CO2 laser which usually operate at 10.6 micron and are the most efficient gas lasers.
The most likely source is from a blood reaction catalysed in the dermal papilla. It will be necessary to employ a spectrometer to measure the emission spectra of small mammals to confirm the model. The involvement of biochemists and blood chemistry specialists will be needed to complete the science of infrared thermoregulation.
We do not have any thermal cameras that are sensitive to 16 micron. There is an indirect way of confirming it.
In February 1800 William Herschel performed his classic experiment to show that a thermometer became warmer when placed beyond the red part of a solar spectrum. He concluded that there was invisible energy at a longer wavelength than our eyes can see. 100 years later this was explained by the thermal radiation laws and was simply part of the blackbody spectrum for the suns chromosphere at 5700 degrees centigrade. These laws and the laws of thermodynamics prevent the harvesting of thermal infrared to generate usable energy. So if an animal is radiating thermal infrared that energy cannot warm up another object to radiate more brightly. If an animal is radiating far-infrared bioluminescence then it is allowed to warm up another object to radiate more strongly.
A very rough experiment was set up to test this. Debbie Harwood of Hampshire bat group and a carer for injured bats made available a soprano pipistrelle bat that was recovering from a cat attack and was being flown to test for suitability for release. After a series of exercise flights which lasted for 20 minutes in total the bat was filmed. Standard gauge printer paper was placed close to the fur but not pressed into the fur so the only significant heating was from radiation and not conduction. The paper is opaque in the infrared but glowed brightly and constantly, due to radiation from the fur of the bat. Later photographic analysis showed that the paper emission was brighter than the bats own emission. It is not allowed by the radiation laws to increase brightness using thermal infrared so this experiment supports the theory for far-infrared bioluminescent emission.
