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Thermal versus Near Infrared

article and photographs by Ben Lincoln with contributions by Dr. David Wilson

Perhaps the most common misunderstanding with regards to multispectral photography concerns the imaging of infrared radiation. Most of us (myself included) learn in school that "infrared" and "heat" are the same thing. Then when we learn about "infrared photography", we naturally assume that this is the same thing as thermal imaging.

Refer for a moment to the electromagnetic spectrum chart from the beginning of A Detailed Introduction. "The infrared" is commonly divided into three distinct bands, the combination of which is much larger than the entire human-visible spectrum.

When photographers refer to "infrared", they are usually describing the near infrared - the part closest to red light - generally between about 750nm and 1000nm in wavelength. This type of infrared energy behaves almost exactly like human-visible light. It can be focused using glass lenses, and the sensors used in digital cameras can easily detect it^[1]. Film cameras can be used for this type of photography as well, provided that special infrared-sensitive film is used. The near infrared is also where most night vision systems and infrared remote controls operate^[2].

Thermal imaging equipment generally operates in the far infrared region - waves as much as 15 times longer than near infrared light - although some types make use of mid infrared (or both far and mid infrared). Not only are specialized sensors needed, but the lenses of these cameras must be made of exotic glass (created from elements like germanium instead of the silicon which makes up everyday glass). These lenses are completely opaque to visible light, and cannot be used for standard photography.

As it is a specialized application, thermal imaging gear is very expensive compared to conventional cameras, and until about 2015 was well beyond the reach of all but the most deep-pocketed hobbyists. By 2015, it was possible to obtain an imager with a resolution of 320x240 pixels for about $4000 on the open market^[3], and less than $1000 for extremely low resolution (80x60 or similar). Some of these lower-resolution models could be hacked into the equivalent of their higher-resolution counterparts if one didn't mind voiding the warranty (see Thermal Imaging Examples). Although this was about the same price a professional photographer would pay for a high-end DSLR body, these devices are much lower resolution - 320x240 pixels is less than even the cheapest bargain-basement webcam delivers in the visible spectrum, and 80x60 is not really useful for most artistic purposes. By late 2017, competition in the marketplace had increased the availability of better hardware at lower prices - for example, FLIR's VUE camera for drones supported 640x512 resolution and was available for about $3200, but this is still a far cry from the tens-of-megapixels resolution of conventional cameras.

FLIR Systems produces several relatively high-resolution models intended for advanced scientific and military use, but these are of the "call for pricing" rarefied-air budget class. They also sell thermal camera modules for smart phones. I think there is fascinating potential for that technology, but the sensor resolution in the current generation is far too low for my purposes.

The most common misconception that I've encountered over the years is the belief that a near infrared imaging system is equivalent to a thermal imager. For example, I've seen misinformed or unethical sellers on eBay offering NIR-related products with accompanying images that were clearly taken from thermal imaging gear (sometimes even with the FLIR watermark).

Dr. David Wilson came across the original version of this article and pointed out that in my attempt to clear things up, I'd actually included some technical inaccuracies from the perspective of the underlying physics. As a result, I've attempted to rewrite the relevant content in order to give three different perspectives^[4] on the key question "is a near infrared imaging system a thermal imager?". Thank you, Dr. Wilson! Any mistakes which survived the rewrite are my own.

The Short Answer

No. Near infrared and what most people would think of as "thermal imaging" equipment is physically different. If one wants to see the world like the alien in Predator, one needs to use a thermal imager. There are plenty of interesting things to see in the near infrared band, but near infrared images look nothing like thermal images unless the photo is exclusively of something that is hot enough to glow incandescently.

The Medium-Length, But Still Simplified Answer

Near infrared equipment and what most people would think of as a "thermal imager" are physically different, and each captures a distinct, non-overlapping part of the electromagnetic spectrum. If one wants to see the world like the alien in Predator, one needs to use a thermal imager. There are plenty of interesting things to see in the near infrared band, but near infrared images generally look nothing like thermal images.

The exception is images of objects which are hot enough to glow incandescently in the near infrared.

All matter with a temperature above absolute zero produces electromagnetic waves - "thermal radiation" - as a result of atomic-level processes. The wavelength of this radiation is extremely long (radio/microwave) for very cold matter, but the minimum wavelength emitted decreases as the temperature rises. For most objects and environments a typical human encounters, thermal radiation includes a significant amount of far- and mid-infrared. It is that far- and/or mid-infrared which a thermal imager captures.

As the temperature of matter rises, the thermal radiation will extend to shorter wavelengths, eventually reaching the near infrared, then the human-visible spectrum, and then to shorter wavelengths. We perceive this as incandescent light. The glow of a hot stove element is caused by the same physics as the surreal glow of a human as captured by a thermal imager.

As a result, if an object is hot enough, it can be argued that a near infrared (or even conventional) camera can function as a "thermal imager", because it is capturing the emissions that result from the heat of the object. However, this is only true for objects that are far too hot to touch, and differentiating between "incandescent light" and "some other source of the same spectral emissions" is much harder than for mid- and far-infrared. Numerous objects produce near infrared, red, orange, yellow, and white light without being as hot as an incandescent object of the same colour. Far fewer produce far- and mid-infrared radiation that would confuse a thermal imager.

The (Hopefully) Technically Correct and Mostly-Complete Answer

Just about everything you probably think you know about heat is a misleading oversimplification based on the assumption that you will spend your life in "normal" environments and not study physics.

Many people (myself included, before I researched this article) believe that "heat" is equivalent to some form of infrared radiation. This is not true, although for most situations humans encounter, treating it as true is a more-or-less accurate simplification.

In reality, heat (in the sense of electromagnetic waves emitted by an object based on its temperature - "thermal radiation") can take the form of many different wavelengths. It is produced by oscillations at the atomic and molecular level. This occurs for any matter above absolute zero in temperature (because at absolute zero, all motion would cease, preventing any oscillations).

The wavelengths produced at a given temperature can be approximated using the black-body radiation model (see previous link for more details). A given temperature of matter will produce EM radiation with wavelengths occupying a curve. For very cold objects, this curve begins in the microwave or radio band. As the temperature rises closer to values that might be encountered outside of a laboratory, shorter wavelengths are also emitted: far infrared, mid infrared, near infrared, light that is visible to humans, and so on.

I used a handy Excel workbook created by Scott Sinex and EasyCalculation.com's Wien's Displacement Law Calculator to derive the data on this page.

Modern thermal imagers typically have a minimum detection temperature of -20 degrees Celsius. This corresponds with the point at which the blackbody radiation curve's emissive power exceeds ((10 watts/meter^2) * (wavelength in micrometers)), as well as the point at which the peak wavelength is about 11,500nm, but either or both of these may be coincidences. Temperatures far lower than this will still produce decent amounts of far infrared at longer wavelengths, so even if one had not already learned that "heat" and "infrared" are not the same thing, it would still be obvious that a "thermal imager" is actually more accurately termed "an imager of thermal radiation produced in a fairly specific range of temperatures".

When the temperature reaches high enough levels, the thermal radiation will be of sufficiently short wavelengths to be imaged using near infrared, or even conventional cameras. This is the incandescence that is familar to anyone who has looked at a hot stove element. It is caused by the same physics as longer-wavelength thermal emissions. I would argue that treating such devices as "thermal imagers" is misleading for most environments that most people will encounter, because essentially none of the unusual/surprising aspects of thermal imagery apply, but I can see arguments on both sides.

In any case, If one wants to see the world like the alien in Predator, one needs to use a thermal imager. There are plenty of interesting things to see in the near infrared band, but near infrared images generally look nothing like thermal images.

Some statistics for various temperatures of interest:

• An object with a temperature of about 200 Kelvin (-73 degrees Celsius, -100 degrees Fahrenheit) produces black-body radiation starting at about 7500nm and extending to longer wavelengths, with a "peak"^[5] at about 14500nm (14.5 micrometers).
• An object with a temperature of about 253 Kelvin (-20 degrees Celsius, -4 degrees Fahrenheit), the aforementioned typical minimum temperature for a thermal imager to detect, produces black-body radiation starting at about 4800nm, with a peak at about 11450nm.
• An object with a temperature of about 273 Kelvin (0 degrees Celsius, 32 degrees Fahrenheit), the freezing point of water under typical conditions, produces black-body radiation starting at about 4200nm, with a peak at about 10600nm.
• An object with a temperature of about 294 Kelvin (21 degrees Celsius, 70 degrees Fahrenheit), "room temperature", produces black-body radiation starting at about 3700nm, with a peak at about 9900nm.
• An object with a temperature of about 310 Kelvin (37 degrees Celsius, 98.6 degrees Fahrenheit), the average human body temperature, produces black-body radiation starting at about 3400nm, with a peak at about 9300nm.
• An object with a temperature of about 373 Kelvin (100 degrees Celsius, 212 degrees Fahrenheit), the boiling point of water at sea level on Earth, produces black-body radiation starting at about 2600nm, with a peak at about 7800nm.
• An object with a temperature of about 600 Kelvin (327 degrees Celius, 620 degrees Fahrenheit), produces black-body radiation starting at about 920nm (with the bulk starting closer to 1400nm), and a peak at about 4800nm. Based on black-body curve calculations and extrapolating from the Draper Point for human-visible light, this is roughly where I would expect to see a typical near infrared camera start capturing an image of incandescence.
• An object with a temperature of about 798 Kelvin (525 degrees Celius, 977 degrees Fahrenheit), produces black-body radiation starting at about 620nm (with the bulk starting closer to 950nm), and a peak at about 3600nm. This is the "Draper Point", the temperature where incandescence becomes visible to human eyes.
• An object with a temperature of about 1000 Kelvin (727 degrees Celius, 1340 degrees Fahrenheit), produces black-body radiation starting at about 420nm (with the bulk starting closer to 620nm), and a peak at about 2900nm. This is typically the lowest value seen on colour temperature scales, a very red light.
• An object with a temperature of about 5800 Kelvin (5527 degrees Celius, 9980 degrees Fahrenheit), the approximate surface temperature of the sun, produces black-body radiation starting in wavelengths shorter than human eyes can see (ultraviolet, etc.), with a peak at about 500nm. This is typically close to the high end of colour temperature scales, and has a pale blue hue.

To illustrate this discussion, here's a set of photos I took of my soldering iron while it was cold:

Next, the same soldering iron after it has heated to operating temperature, as it appears in the near infrared:

It's hot enough to melt solder, but there is no trace of a glow visible in the near infrared, at least in this case. Apparently, it's possible to capture an NIR glow from a soldering iron in a dark room, but I have not succeeded at this to date.

Here is a similar soldering iron warming up as captured using a thermal imager:

As discussed above, the near infrared can be used to detect one particular type of hot object: those that are almost hot enough to begin glowing incandescencently in the NIR band, which occurs at slightly lower temperatures than those at which the same object would glow red.

Here is a closeup of one of the heating elements on my electric stove, with the power turned off:

Here is the same element at low heat in the near infrared. You can see that it is still just as dark as with no power applied.

The same element at medium heat still is not emitting any near infrared light, even though it has visibly expanded from the higher temperature, and the heat being emitting can easily be felt from 20-30cm away.

At the "medium high" setting, finally a change emerges in the near infrared, even though the element is still dark to human eyes. The change is easiest to see in the NIR-R-G false colour rendition.

Halfway in-between "medium high" and high, the element still is not emitting any human-visible light, but there is no mistaking that a significant near infrared glow is present.

At its highest setting, the element is begining to glow a dull red colour, but is putting out a stunning amount of near infrared light.

This last set reinforces my belief that false colour is at least generally preferable to discreet greyscale images. The greyscale near infrared version indicates to the viewer that something is very bright. The false colour NIR-R-G variation intuitively depicts that the stove element is very hot. If you were wearing a set of multispectral goggles, which would you rather see before deciding whether to touch the element or not?

Related Articles:
A Brief Introduction
A Detailed Introduction
Uses of Multispectral Photography
Thermal Imaging Examples

Last updated: 02 January 2018

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