View Full Version : What's the difference between IR and thermal?
They both look for heat, right? What's the difference?
OK, I'll bite: you asked..
All objects emit radiation as a function of temperature. This is known as 'black body radiation'. The emission of energy is very broad and is a strong function of temperature.
Very hot things (like the Sun) radiate significant energy in the visible, near infrared, mid-infrared and long wave infrared spectrum. Cooler things (like us, hogs, birds) are much cooler and radiate both less energy as well as a more limited spectrum.
When you go to Lowes and look at light bulbs, you see references to light color in terms of 2400K or 2600K... this is the equivalent black body temperature in Kelvin, 0K is absolute zero and room temperature is about 290K. The 'hotter' the temperature reference the more to the blue color the lamp appears. This is 'visible' spectrum (what we see).
So, with this basic understanding that all things emit radiation as a function of the constituent material temperature, what does this mean for us?
We (humans) see in the visible spectrum, that is in the radiation wavelength of 350 (violet) to 740 (red) nano-meters (nm) the hotter the temperature, the shorter the wavelength of 'light'.
Dogs and cats see in the near infra red spectrum (cooler temperature, longer wavelength) of 800nm to about 1900nm or 1.9um. This is beneficial WHY?
The stars and reflected light from the moon emit about the SAME total energy in the Near Infra Red (NIR) spectrum as they do in the visible range. This means there is MORE energy to be picked up by their eyes. Good for them, bad for us (prey).
Night vision (and digital cameras without a NIR blocking filter) picks up (i.e. is sensitive to) both visible and NIR energy, and can electrically amplify that energy to result in a formed image (what we see). NVDs use hi voltage and special surfaces to provide this amplification, this is done in the Image Intensifier Tube... when NVDs are referenced to Gen 2, Gen 3 or Filmless, the format of this critical chain is what is being described.
Another wavelength is commonly used for Thermal Cameras that is called Short or Mid Range Infra Red. Mid range Infra Red (MIR) is what you see from airborne bomb impact cameras (typically, not always). The possible resolution is better than the Long Range cameras but at the expense of requiring special cooling of the camera sensor.
Long wave infra red (LWIR) is very common in cameras today (maintenance inspection, fire fighting, security/surveillance, etc.), this energy we feel as HEAT. These cameras are sensitive to the 8000 to 12000nm (8-12um) but have by comparison lower resolution than a mid range IR camera (640x480 pixels vs 2048x2048 pixels) and benefit from cooling but many are uncooled. Un-cooled cameras (LWIR) have very low operating power (2 Watts) and fast turn on times, so are useful for security camera applications where power is a consideration. Mid-Wave IR cameras have high power (for the cooler) in the area of 40 watts and very long stabilization times (minutes).
So, with this information, what is the practical difference?
IR in the OP's question is likely referencing NIR that is used by Night Vision Devices (PVS-14 and the like) and that REQUIRES external reflected illumination to operate. No ambient light with NVDs and all you see is 'sparkles' (that is just noise in the amplification process). That is why we may have IR flashlights on our AR platforms to illuminate the target. NIR lasers also operate in that wavelength so are very effective.
Thermal in the OP's question likely refers to the new breed of Thermal Scopes (FLIR and L3), these are uncooled Long Wave IR cameras with additional software image enhancement (contrast) and a display device (OLED, Organic Light Emitting Diodes) for the user to see.
NIR NVD based systems REQUIRE illumination (1/4 moon is a good metric for practical use) and have limited range (<100 meters). LWIR (thermal scopes) use the energy of the target as the source of illumination and have much longer effective range (300 meters) but lower resolution and higher energy use (shorter battery life).
This is all good stuff, I've personally worked and designed both types of optical systems (cameras) for our DOD/USG folks... Technology has helped our troops and hindered our enemies... It was a good career.
Unreconstructed
10-05-14, 09:49
^ GREAT info! Thanks for taking the time to post.
platoonDaddy
10-05-14, 10:27
Very educational! Dang, that was good.
Thanks
Great post USAF! Thanks for the details.
Yeah, I was talking specifically about optics. So from what I read thermal is the way to go for a weapon sight, as it's more "advanced" and more "effective", not relying on ambient light. It also has a longer range but eats batteries like candy. That said, I'll bet it has a much higher price tag too, especially for a civvie to purchase one.
OK, I'll bite: you asked...
Post of the month - thanks.
montanadave
10-05-14, 21:22
Somebody wanna sticky that post above as I gotta believe that's info worth saving?
Great post USAF! Thanks for the details.
Yeah, I was talking specifically about optics. So from what I read thermal is the way to go for a weapon sight, as it's more "advanced" and more "effective", not relying on ambient light. It also has a longer range but eats batteries like candy. That said, I'll bet it has a much higher price tag too, especially for a civvie to purchase one.
A couple grand versus buying a car.
Sent from my SGH-T999 using Xparent BlueTapatalk 2
Abnak,
Aside from the technical stuff, in application there are other considerations (based on range, resolution and applied tactics):
For example, consider clearing your home with a firearm with only a LASER but no visible light. That is not a good idea as you need to be able to identify the possible target prior to engagement. You use a light to illuminate the target to recognize 'friend or foe'. Additionally, having only a firearm mounted light means you are painting the target with the muzzle (bad form for non-threats), so if it is something you don't want to shoot...., therefore a separate 'support hand' flashlight (White light or IR with NVDs) is a good idea.
Now, for a tactical use of Thermal (Long Wave Infra Red, LWIR), other than hunting hogs (shape of 'blob' identifies that it is not human, and you see no humanoids in the area/background), target recognition is essential (unless it is zombies, but then they may not be very hot, so thermal may not work well....need zombie data). The common Thermal (LWIR) images that are in the glossy product sales ads are commonly at very short range (where resolution is good enough to determine if the person is armed or not). For a rifle, that is an awfully close engagement distance (perhaps not effective).
Realistically LWIR thermal scopes (that us Civilians could afford) will have sufficient resolution for hunting (or zombie apocalypse) to ranges perhaps of a few hundred yards. Home perimeter protection would be very good for scanning for intruders (LWIR is great for this) but still has resolution limits for target recognition.
Night Vision Scopes alone with red dot (NVD compatible, T-1 Aimpoint) or IR LASER and Illumination lights allow for closer identification due to higher resolution than LWIR but NOT the detection capability (in dark or wooded settings) nor engagement range (zombie attack). Can you engage targets at 300 meters, sure, but you have to know that they are not things you don't want to shoot (that means there is enough ambient light or a good narrow IR flashlight to paint the target).
Note that using NVD with a NVD compatible (i.e. low light level red dot, T-1) means that you are using the NVD behind the optic. This could be mounted on the rifle rail or on your dominant side eye with a helmet mount. Both of these cases MEAN that you are painting your suspected target with the muzzle, not a good thing perhaps. Using a LASER on the firearm allows you to scan with the NVD and hand held light source (or bounced light from the firearm IR light), you aim the firearm by looking at the LASER on the validated target. Using this at long range is however problematic as you have to hold over (not really a big deal for minute-of-man <200 meter engagement distances) to compensate for drop at range.
Room clearing, or home perimeter protection may be better suited for a NVD and IR laser/flood flash light. Consider that many (myself included) have a helmet mounted NVD with a rifle mounted IR LASER/Flood. The helmet mount allows for navigation (i.e. tripping around while trying to walk), target identification and possible engagement of 'recognized' threats. A VERY important consideration is that NVDs are VERY easily damaged by bright light sources (including reflected or close range IR LASER painted things, like walls). The damage is a burned spot on the Image Intensifier Tube and is permanent (dark spot). I was demonstrating some of the then new technology Gen 4 (film-less) NVDs to some USG folks and one of them proceeded to look at a close by parking lot light...... burned IIT faster than I could stop them... OUCH!
If I lived on a farm with long distances to defend against zombie hoards, then a thermal scope is the way to go (or as a secondary method), but I think I would first have a NVD (helmet/head mounted) and IR flashlight and IR Laser aiming capability.
Neither solution is a best fit.
First set of gear (and my solution set) is:
NVD (PVS14) and bump helmet for mount combined with IR laser/flood on rifle (with visible green for daytime use, DBAL-D2). Also have visible/IR laser on handgun (CT LGD-426 on P226). White/IR flashlight for searching (Surefire). This pile of gear was perhaps $5k (I have a Gen3 pinnacle NVD...$$$), you can get a lower quality NVD IIT and save a bunch ($2k less).
I will (am) saving up for a FLIR LWIR scope (for hog hunting and techno-nurd gratification), but that is $8k... it will be a while..
For more information on NVDs, and gear Tactical Night Vision Company (TNVC) is very good and the Chappy IR Skills builder DVD is very informative. I'm just a customer of TNVC and have NO financial ties (other than sending them $ for cool gear). They don't sell cheap stuff but you get good gear you can trust.
Hope this helps the non-technical decision side of your question....
Thanks all for the appreciation of the nurd-side stuff....
Great info here. Thanks USAF1980s.
Definitely a candidate for being stickied.
I think USAF's second post should be in that sticky too. First one details the science behind it, second one breaks it down into practicality.
Yeah "ouch!" on the $8K for a FLIR LWIR setup. Guess I better start saving my pennies! I live on 22 mostly-wooded acres in the country so that would likely suit my (potential) needs better.
Nicely explained.
ABNAK, you're not alone in some initial confusion, there's a great deal of folks that use the terms IR and thermal interchangeably....when perhaps they shouldn't. That can be described as semantically and scientifically valid....but are functionally different from an equipment standpoint, to the point that I cringe a little inside whenever somebody describes a thermal optic as "IR," and doesn't draw a line in between how each functions, nor do the pro/con list of each in terms of how light and environmental conditions can alter how successfully they may be employed.
They're not *fingerquotes* wrong, but they're leaving a great deal of room for other folks to misinterpret unless they explain further...as was just admirably done.
For instructional purposes, on our training team, thermals optics were "thermal" devices, and image-intensification optics were "IR" or "I-square" devices, and if we needed to further parse out the IR portion of the electromagnetic spectrum, we could.
That was more of a maintainer thing than a warfighter thing, but going to that depth often bore fruit when we had opportunity to go into how and why IR, thermals, and "day optics" (specifically, magnified day optics) were best utilized in conjunction with one another, as "layers" of a sort. Where the advantages of one type under certain limited-vis conditions could be used to shore up where the performance of another type (or two) still has some use, but is starting to fall off as conditions change.
So why can't they increase the resolution of thermal optics?
A62Rambler
10-06-14, 16:51
USAF1980s
It's posts like yours above that are the reason this site is different than other sites.
So why can't they increase the resolution of thermal optics?
I'm gonna guess here (and might be totally out of my lane), but over time I bet they will.
Then they'll cost $12K. :jester:
MorphCross
10-07-14, 00:23
So why can't they increase the resolution of thermal optics?
I always thought the resolution on Military Helicopter thermal was really good. Maybe there is a limitation on what they want to invest into the smaller commercial thermal market.
Koshinn, (warning to readers... this is long and perhaps painful)
How are Thermal (Long or Mid Wavelength IR) different than say a normal camera (pocket camera or cell phone)?
SIMPLE CAMERA INFO:
Normal visible cameras use lens materials that pass visible (what we can see) through Near Infra Red (NIR, what dogs and NVDs can also see). The lens then focuses the light energy on the camera sensor and this sensor is the primary determiner of obtainable resolution. Visible and NIR sensors are made using the same process as normal Integrated Circuits (ICs) and can and do have VERY small light sensitive surface detectors (areas that convert light photons into charge or voltage, more light = more voltage/charge). Each of these photon receptors are called Pixels. There is a tradeoff in light energy converted into voltage at these pixels fundamentally by the surface area that each pixel has (and is exposed to light from the rear (ocular to us with scopes, image or exit pupil to an imaging lens). More area = more voltage but larger Pixels means the sensor can have lower total horizontal and vertical Pixels for a fixed overall sensor size.
VISIBLE IMAGE SENSORS: (electronic film)
Visible cameras used in cell phones and pocket cameras have an overall image sensor size of perhaps 4 mm on a side (using a square for simplicity, they are usually 4x3 shaped). Larger Pixels allow a more sensitive camera but at the expense of resolution. Larger format (Digital SLRs) use much larger image sensors to mimick 35mm film format but also have higher total Pixel counts. Common Pixel sizes for visible sensors are about 3um (0.000003 meters) on a grid (the sensor area is about 70% of this 'square'). Add color filters and micro lenses above the light sensitive area as well.... not germane to our discussion.
VISIBLE CAMERA SYSTEM:
Construction wise, a visible camera can be as simple as a camera sensor Integrated Circuit mounted on a circuit board with a simple lens (or even pin hole) on the surface. Your cell phone camera is VERY simplistic with a fixed focus and focal length lens mounted above the IC... not much there. And cell phone cameras use VERY small Pixels so they can sell the 120 MegaPixel camera sales-BS as implying better images (but the camera needs flood lights to get enough light on the miniscule Pixel to get any voltage over self-noise). This is why many cell phone type cameras are VERY grainy or noisy images.
THERMAL (LWIR) SENSORS:
LWIR sensors are constructed very differently than visible/NIR Integrated Circuit photon to voltage converters. The conversion of photons at LWIR wavelengths directly to charge/voltage in a Silicon Integrated Circuit is not possible due to material and semiconductor properties. LWIR sensors are commonly done with small resistive blocks (resistors) that change resistance when heated by the photon energy. The more photons per second, the hotter the resistive block gets (Pixel) and the electronics in the image sensor measures this resistance each time an exposure is taken. These sensor resistors are now about 17um in Pixel pitch compared to the <3um for the visible sensors. The 'back side' of the LWIR camera sensor is made using standard IC fabrication techniques (called the Read Out IC, ROIC) and the resisters are plated onto each Pixel location using strange resistive materials (Vanadium Oxide, ViOx is common in LWIR today), this is known as the Focal Plane Array (FPA). You can think of LWIR sensors as being hundreds of thousands of Volt Ohm Meters measuring a bunch of resistors at 30 or 60 times per second... That is what is going on in there. Note that depositing these resistive films is a bit more of a physical process than is used for Silicon IC fabrication, this means there is more variation and yields are much lower (factory scrap costs).
One thing that is very different about LWIR sensors is that the Read Out IC and FPA have to be at a very stable temperature as the temperature of these 'resistors' is changed very slightly by the impinging LIWR radiation (photons). This requires that the whole FPA is located in a sealed near vacuum hermetic container with a very thin (typically Silicon) window to allow the LWIR light to pass. This is a very expensive and sometimes fragile assembly. Why is temperature such a big deal one might ask... It IS what we are measuring from the object through the air, through the lens focused on these tiny resistors. A key parameter of a LWIR sensor is the 'sensitivity' and this is measured in Thousandths of a degree Kelvin. Today's LWIR uncooled sensors are commonly 50mK performance (lower is better) for an f/1 lens (see below discussion on lens parameters). That means that the sensor can tell if a single tiny Pixel has changed by 50/1000 of a degree (K/C). This is really a Signal to Noise reference as noise is always a factor (that is why cooling is required on smaller/higher resolution sensors).
LIMITATION OF LWIR RESOLUTION:
The reduction of Pixel size is the primary reason that LWIR un-cooled sensors are limited to 640x480 or similar formats. Making these resistors smaller yields less resistance change to impacted light and sensitivity over self-noise (these are un-cooled) forms this limit. The only way to increase resolution would be to add power consumption and significant cost by adding an active cooling system. There is no free lunch. 17 um LWIR pixels for un-cooled FPAs are pretty much it (unless FLIR or L3 come up with a new mouse trap).
THE OBJECT-LIGHT PATH-LENS PATH:
The real imaging camera (Visible, NIR, MWIR, LWIR) resolution is really set by the total chain of factors starting with the target or Object (a person for our example here). That person (Object) reflects (or emits in the world of LWIR Thermal) energy into the direction of the imaging device (camera or Thermal scope). The 'energy' from the Object propagates through the air (or space) from the Object to the lens first surface of the camera. This separating distance may have high water vapor, dust, rain that interferes/attenuates or scatters the energy from the Object. We all see the effects of Fog, Rain in our daily lives and have a feel for Visible wavelength propagation impairments. For LWIR, just humidity (water vapor) in the air reduces the energy by both scattering as well as attenuation of that energy (hint, LWIR is impacted by humidity).
Once the light energy impacts the first lens surface it is bent in beneficial ways to take the energy from each part of the Object and focus the energy to the corresponding part of the Imaging Sensor. How much energy is collected is primarily a function of the speed or f number of the lens. A large Objective (front) lens with a relatively short focal length has good light collecting capability (low f number). In most LWIR sensors, a primary specification for sensitivity is referenced to an f/1 lens (i.e. fast lens). On a pair of binoculars, for a given magnification (say 10x) we know that the larger the first lens (Objective lens) the brighter the binoculars will be, same for a camera/scope.
How well any given lens does in the ideal focusing of all light to the image sensor is determined by the shape and type of glass/material used in the lenses as well as many other factors (coatings, surface quality, errors in geometry from the ideal shape, spacing errors, etc.). The end game for a lens is to do this perfectly but there is also a 'best case' physics limit to how well this can be done. Many lens's do pretty well in getting close to this ideal (minimum errors/distortion). The blue edges you see in cheaper binoculars is due to color (chromatic) errors as an example. Blurry objects on the edge of the field of view is also a very common defect in lenses (astigmatism). In the end, both visible and LWIR lenses do a very good job of collecting and focusing energy onto the image sensor (focal plane).
LENS TRADEOFFS:
Visible camera lens's can be made from common glass materials (there are actually 100s of optical glasses) that as a metric are a few dollars per pound (melted blob of glass, not a lens shape yet). LWIR lens's must be made from a VERY limited range of materials (LWIR does NOT propagate through visible glass... see LWIR images of people wearing eye glasses.... alien looking as LWIR does NOT pass through). The most common material for LWIR lens use is Germanium, and this is over $500/lb (last time I did a lens design), it may be higher. Germanium is a dark gray metal semiconductor and is very fragile (brittle). These lenses are then coated with Anti Reflective coatings to aid in LWIR energy transmission. Germanium changes it's optical properties strongly with physical temperature (this is why the FLIR 60mm scope requires a manual focus...simple answer). LWIR lenses are VERY much more expensive than visible simple lenses due to the cost of materials as well as a much smaller volume market.
Bottom Line?
LWIR sensors and lenses will continue to be more expensive and lower resolution than visible systems (cameras). For un-cooled (our civilian world) LWIR, higher resolution and longer focal length (i.e. larger) lenses will demand higher costs. Optical design is a life of trade-offs as is the choice of what LWIR or NIR system we choose.
On LWIR system specifications, these are the key performance parameters:
Resolution: 320x240 or 640x480..... higher resolution is WELL worth the cost delta (if you can afford it), this is still low resolution (compared to visible) so more is really BETTER (i.e. this is NOT the cell phone 200 mega-pixel resolution BS, it is real and you will always be able to tell the difference).
Sensitivity: Most are 50mK for uncooled LWIR today (this is really good, it was much higher just a few years ago)
Lens Focal Length: Given most small (these are small) lens's used for gun scopes are f/1, the trade space is field of view (i.e. longer focal length can see a bit further) vs. cost (longer focal length lenses have more Germanium) trade offs. The auto-focus 35mm lens used in today's LWIR is very good (a-thermalized, no manual focus required) but has a wider field of view than the 60mm version (that requires manual focus on a given target distance, not that big of a deal).
Shock Qualification: The FLIR scopes now on the market are rated for .308 auto-loading (AR) use mounted on the rail. Note that a bolt gun would violate this limitation or a larger caliber (shock impulse). Remember that the FPA is mounted in a vacuum cell, that can't take too much hammering. Note also that NVD Gen3P (Pinnacle) IITs are NOT rated for even .308 AR rail use, the very thin micro-channel film will arc over and destroy the IIT (this is from the internet, I don't claim to have real hard data, but remember, I helmet mount my $4k NVD, guess why).
Energy Use: These LWIR FPAs use power to run all of these 'VOM's' measuring the resistive pixels. More resolution would require a cooler (40 Watts vs. 5 Watts). CR123 primary cells or rechargeables (secondary batteries).... we have to pay to play.
Me, I'm going to save up for a FLIR 640x480, 60mm scope.
DISCLAIMER: I have a long history as an integrator of FLIR core LWIR into cameras I and my team have designed. I'm not disclosing anything other than advertized information on LWIR and FLIR published data. FLIR is a very good company and supports our troops and other folks. I'm a happy 'user' of FLIR OEM products. I have NO financial ties to FLIR.
I've been wanting a FLIR too, or maybe an ATN. I was saving for a long time for a good Night vision scope, but thermal is looking more and more attractive. I used a variety of thermals in Afghanistan and loved them, but they suck some batteries, like 6 double AAs in 90 minutes.
Since everyone has covered the science behind the difference I'll throw my two cents in regarding actually using the different optics.
Night vision is good for actually moving and looking around since it shows more detail and makes moving much easier, for example a PVS 14 is better for patrolling movement, and wearing long term. Depending on the generation and amount of ambient light NVGs will give a better overall picture and higher resolution. This is important when it comes to identifying who or what you are looking at. Longer ranges will of course obscure this to a degree. An example is when wearing NVGs I may not be able to actually make out details of the guys face, but I can reasonably make out uniforms, equipment, weapons etc. For NVGs you aren't limited to a helmet, there are HALO mounts out there.
Thermals are excellent for a post, a non-moving security post, or a stationary op/lp. Thermals can also be used to do scans at night, but it should be noted most are quite bright (In my experience) and this will inhibit your natural night vision, so using swapping between NVD and Thermals can be done, but you pay for it. Thermals in my opinion make better weapon sights, but can be made useless if firing at a fast rate since the heat from the barrel can white out the optic. The Marine Corps for example has an obsession with issuing us machine gunners thermals, which are great until you have to fire the weapon. Of course this won't be as big an issue on a rifle compared to a machine gun, but it can happen. There are thermal monoculars similar in size to the PVS 14, and can be mounted to a helmet/Halo mount, but moving with these is difficult since you can make out the details around you very well. Also without an eye cup the white Also thermals can be hell on the eyes if the display is bright, my old PAs 28s caused headaches after extended use.
Also there is a price difference and it's substantial, but optics like these are tough and extremely capable optics, a thermal http://goo.gl/SSu8tP and here's a night vision http://goo.gl/0AOiZQ
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