Many biological creatures are enabled with the ability to “see” the heat – reptiles, for example, effectively use this ability to detect the size of the heat-emitting object and its range. Other biological entities, including mammals, are enabled with the ability to perceive heat by a process known as Thermoception or Thermoreception. As humans, we are capable of sensing temperatures above or below our body temperature. It is part of our Somatosensory System that lets one feel or see sensations (such as pressure, pain, or warmth) that can occur anywhere in the body, in contrast to one localized at a sensory organ (such as sight, balance, or taste). In the case of sensing heat in humans, thermoreceptors carry the information about temperature changes whereas mechanoreceptors can convey perceptions of touch, pressure, etc.
The discovery of infrared (which literally means “beneath red”) was made in the 1800s by Sir William Herschel, who placed four thermometers positioned according to the solar spectrum (dispersed by a prism), one just outside the red color. That thermometer registered the highest temperature whereupon research into the characteristics of IR was born. IR light lies between the visible and microwave portions of the electromagnetic spectrum and has a range of wavelengths, just like visible light has wavelengths ranging from red light to violet. Near infrared (NIR) light commonly used in night vision goggles and in fiberoptic communications is the closest in wavelength to visible light with wavelengths of 0.75–1.4µm. Short-wavelength infrared (SWIR) with wavelengths ranging from 1.4–3µm is most often seen in long-distance telecommunications hardware. Mid-wavelength infrared (MWIR) with wavelengths 3–8µm finds applications in the heads of ‘heat seeking’ missiles as well as in scientific IR imaging. For many electronics thermal management applications, it is the Long-wavelength infrared (LWIR) 8–15µm wavelength known as the “thermal imaging” region in which sensors can obtain a completely passive image of objects only slightly higher in temperature than room temperature Far infrared (FIR) with wavelength range 15–1000µm finds applications in astronomy such as imaging gas clouds in faraway galaxies, etc.
There is a metric known as minimum resolvable temperature difference (MRTD) which serves to measure the performance of IR cameras. It is the camera’s ability to ‘see’ the minimum resolvable level of thermal sensitivity that the camera’s operator can see. MRTD is measured as per ASTM Standard E1213. Commonly used IR cameras have MRTDs of tens of milliKelvins. But here is where the MRTD capabilities of man-made IR detectors and IR-capable biologic entities diverge –the resolution of MRTDs in the animal kingdom is far inferior to those we make by several orders of magnitude –so much so that many animals prefer colder background temperature in order to detect a warmer body.
So aside from a few animals that can sense infrared light, our ability to see electromagnetic radiation is limited to only the visible spectrum. However, recent research on engineered rodents to see infrared light by implanting sensors in their visual cortex may one day make its way to human beings! The laboratory of Dr. Miguel Nicolelis of Duke University is well known for pioneering studies in neuronal population coding, Brain Machine Interfaces (BMI) and neuroprosthetics in human patients and non-human primates. Dr. Nicolelis is also the author of a book “The Relativistic Brain: How It Works and Why It Cannot Be Simulated by a Turing Machine.”
The Duke University research team surgically implanted a single infrared-detecting electrode into an area of the rat’s brain that processes touch (somatosensory cortex). The other end of the sensor positioned outside the rat’s head was exposed to the environment including sources of infrared light. After sensing IR, the sensor sent electrical messages to the rats’ brains that seemed to give them a physical sensation. The team also inserted three additional electrodes, spaced out equally so that the rats could have 360 degrees of infrared perception. When they were primed to perform the same reward task, they learned it in just 4 days, compared with 40 days with the single implant.
The researchers also began redirecting infrared traffic -instead of the somatosensory cortex the electrodes were placed into the rats’ visual cortex. The rats soon after started receiving “visual” stimulus of infrared learning the task in a single day. The research team speculates that the visual cortex adjusted so well because the wavelength of infrared light is very close to that of visible light. The quick learning curve in rats suggests that human adults may also have more malleable brains than we know at present.
It is worthwhile to pause here and assess the implications of seeing IR in humans. In fact, such a capability would impact in ways we don’t yet fully comprehend. If our eyes are capable of seeing the entire electromagnetic spectrum, we will see everything… or nothing at all! The Duke team is definitely not suggesting that the human eye’s visible spectrum will one day include NIR –in fact the day we see the faint infrared light coming from our television remotes will not occur anytime soon! But, it not too farfetched to think that one day our ‘seeing’ capability may evolve to sense heat even though the heat source is too far away for our somatosensory system to react.
Photons absorbed by the retina of human eyes are converted into biochemical energy by photosynthetic pigments which begin the process of converting light into vision. In standard vision, each of a large number of photosynthetic pigments absorbs a single photon. By altering the pulsing light sources, it is possible for two photons to be absorbed by a single photosynthetic pigment, a well-known technique used in two-photon microscopy. A most promising combination for such photons will be to use those from the visible spectrum we are born with and some from the spectrum in the NIR band, as this publication suggests.
Aside from the scientific curiosity, as thermal management professionals, this topic gives us many reasons to pause and to ask some questions! First off, how is this an electronics thermal management topic? Well, to begin with, it is not too uncommon to encounter accidents in laboratories where optoelectronic components including those with light sources in the NIR band are tested. From my personal experience, I can narrate one unfortunate incident where an 850nm VCSEL was firing/lasing and a technician looked into it directly without realizing it had been powered on. There was some permanent damage to his left eye but his vision was not completely lost. One could argue that the incident has more to do with lapses in safety procedures than the need for enhanced vision capabilities in humans including ‘seeing’ NIR. The cost of IR / laser safety glasses remain in the affordable range but it is the practice of wearing one that needs to be enforced rigorously. Nevertheless, a backup capability enabled by the sixth sense of ‘seeing’ NIR could one day warn us to wear eye protection or similar mishaps that are not caught by our five senses.
A more prudent case can be made for enhancing human vision in those suffering with macular degeneration, a topic we have previously addressed in Electronics Cooling (see Thermal Management in Body-Embedded Electronics). Similar to the two-photon microscope, newer types of Ophthalmoscopy can benefit from using a combination of visible and NIR spectra of light. This could yield information on how healthy retina responds to pulsed and combined light sources and compare the data thereof to those with macular degeneration.
The Duke University research team says their findings are quite encouraging for researchers trying to develop sensory prosthetic devices that could one day augment human senses in addition to those we are born with. The somatosensory cortex could be augmented with infrared cortex in a manner that does not overwhelm the existing senses, perhaps integrate with both vision and touch. It is therefore conceivable that one day a blind person with retinal implant can walk the city streets equipped with NIR transponders without needing walking sticks. Such autonomy has potential to individualize and improve the quality of life for visually impaired persons.
