The Best Thermal Cameras for Your Phone
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You are looking hot today. Literally: With a thermal camera, I can see that you have a body temperature of over 102 degrees Fahrenheit, so you’ve got a fever. Not only that, but I can see where your hot water pipes are in the wall, determine if your electrical cables are overheating, and locate your dog outside on a moonless night.
All these things, and many more, are possible with thermal cameras that clip onto your phone and display images captured with an infrared light sensor that shows how much heat objects are giving off. Infrared light is normally invisible, but these cameras render it visible through your phone’s screen, allowing you to study the differences in temperature between hot or cold things and their ambient surroundings. These cameras can also help you see how well your home insulation works, showing hot spots where heat is escaping and wasting your money.
All the cameras I tested use a smartphone to do the heavy lifting of processing, showing, and sharing images through apps that are available for free. I tested the USB-C versions for Android phones of the Flir, Seek Thermal, and Uni-T cameras, and the iPhone version of the Perfect Prime camera. All the manufacturers make versions of their cameras for both Android and iPhone. Also, Flir recently released a new mobile camera, the $550 Flir One Edge Pro. I haven't tested it, but this guide will get an update once I have.
Check out our other buying guides, including the Best Android Phones, Best Cheap Phones, Best iPhones, and Best Camera Gear for Your Phone.
Before You Buy
Photograph: FlirA Quick Primer on Thermal Cameras
Thermal cameras capture infrared light, which radiates in waves that have a longer wavelength than visible light, beyond the color your eyes capture as red. Because of a phenomenon called black body radiation or thermal radiation, most things around you give off this infrared radiation. As an object gets hotter, the wavelength of the radiation it gives off grows shorter and shorter until it becomes visible to the human eye as a dull red glow. That’s where the idea of things glowing “red hot” comes from. So if a camera can capture this infrared light, it can see how warm things are even if they haven’t gotten warm enough to noticeably glow red.
As any astronomer will tell you, capturing infrared radiation isn’t as easy as capturing visible light. Normal optical lenses block most infrared light, and infrared camera sensors have an annoying habit of picking up their own heat rather than the things they are looking at. These engineering challenges mean that thermal cameras capture images at much lower resolution than normal cameras: The highest-resolution sensor in our roundup (the Seek Thermal) captures images measuring only 206 by 136 pixels.
Best Overall
The Flir One Gen 3 is the largest of the thermal cameras I looked at; it’s about 2.6 inches wide and 1.3 inches tall, and it fits a lot into that space, including a built-in battery and two cameras. It is also the most complex of the cameras I looked at. This large camera body has rounded edges and two rubber grips on the side and a USB-C plug on the top that fits into the USB-C port (the iOS-ready version has a Lightning plug), which is the only thing holding it in place.
The Flir saves images at a remarkable 1,440 by 1,080 pixel resolution, but that’s a bit of a cheat. Well, perhaps cheat is too strong a word, but some sleight of hand is going on here. The infrared sensor only captures 80- by 60-pixel images. To create the higher resolution image, the device smooths and scales up the lo-res thermal image and combines it with a much-higher-resolution visible light image from a second camera located right next to the infrared one. Yup, this device adds two cameras to the multiple ones your phone already has.
It may be a bit sneaky, but it works. The visible light camera adds a ghostly edge-drawn effect to the image that can be very useful when trying to pinpoint a heat source—such as which side of a window is leaking warm air or which component on a circuit board is overheating—because it provides a visual map.
The downside of this complexity is that it requires more power. To handle this, Flir includes an extra battery in the camera, which you must charge through the USB-C port on the bottom of the device. If you don’t charge it, the camera doesn’t work. You also have to turn the camera on by pressing a button at the bottom of the device once you have plugged it in, then wait about 20 seconds for the Flir One app (available for iOS and Android) to detect the camera. The images you get are great, but it all feels overcomplicated compared to the other cameras, and it is one more device to keep charged.
The problem with this (and the other thermal cameras) is that they plug into the phone's charging port, which requires a tight fit to work. If you have a case on your phone, you may not be able to plug the device in fully. The adjustable plug here is a nice solution, though; twisting the wheel under the connector makes it move up and down, providing an adjustable length to adapt to phone cases of different thicknesses. To use the Seek Thermal and the Prime Perfect cameras, I had to take the case off my phone or use an extension cable because the case blocked the plug from clicking fully into the phone's socket. However, the Flir One Gen 3 worked with the rather chunky Samsung case on my Fold 4.
Best Outdoor Thermal Camera
The Seek Thermal Compact offers the highest resolution of the thermal cameras I tested, capturing 206- by 156-pixel images and a temperature range of –40 to 626 degrees Fahrenheit (–40 to 330 Celsius). That's only a marginally higher resolution than the much cheaper Uni-T camera I’ve written about below. Also, the wider temperature range probably won’t be useful unless you are a refrigeration engineer or like to hang out in volcanoes. The Seek Thermal Compact has a smaller angle of view than the others; it captures a 36-degree-wide view, while the Flir and others capture about a 50-degree wide view. That means it acts more like a zoom lens than a wide-angle lens. That might make the Seek Thermal Compact more useful for some cases (such as hunting, where a tighter view makes it easier to track an animal at a distance) but less useful for others, such as capturing a thermal image of the side of a house to look for hot spots: You have to stand farther back to get the entire house in the frame.
The camera is named the Compact, and it is a neat little package; only an inch and a half wide with a tough plastic case and a focus ring that shields the lens. It is easy to install: You just plug it into the phone's charging port, which holds it firmly in place. Its the only camera I tested with an adjustable focus. Most come with fixed focus, which means taking a picture of anything closer than a foot or so away will give you a blurry image, but the Seek Thermal Compact can focus down to a couple of inches away. Again, that is a mixed blessing: It is useful if you need to move closer to pick out a single overheating component on a circuit board, but focusing on thermal images is sometimes difficult because of the low resolution.
Best Cheap Thermal Camera
The Perfect Prime IR 203 is the cheapest thermal camera I looked at, priced at about $130 for the Android version and $140 for the iPhone model I tested. It’s a decent thermal camera for the money, but the 80 by 60-pixel resolution is lower than the others. The images it produces have little detail. It’s fine to help find a hot spot in your house insulation, but you might struggle to tell a bear from the trees on a dark knight. The camera itself is a little bigger than the Seek Thermal Compact, with a protruding front lens that could get more easily damaged. One nice touch here: The camera comes with a Lightning port extender that makes it possible to use the camera with a phone case. It should work with any iPhone running iOS 12.2 or later.
The companion app, called SenXorProViewer, is available for iOS and Android. It’s pretty decent, with a clear interface that shows useful things like the highest and lowest temperature in the camera’s view. As well as capturing both still images scaled up to 400 by 310 pixels, it captures video at a rather jerky 15 frames per second. The app also includes a couple of unusual modes: average and predictive. The average mode smooths out the sometimes grainy video by averaging several frames. That makes it less prone to noise and other glitches, and it makes the images a little smoother. The predictive mode tries to interpolate between frames to make the motion less jerky and reduce ghosting, but I didn’t find it made much difference in the resulting video.
Another Inexpensive Option
The Uni-T UTI-120M is a small, compact thermal camera that fits onto the bottom of your phone. A small sleeve around the plug means that it works with most phone cases. This 2-inch wide device captures infrared images at a relatively low 120- by 90-pixel resolution, but it has a couple extra modes that work around the low resolution to make the results more acceptable. The blended mode combines thermal images with ones from the phone camera, but this was pretty useless: On my Samsung Fold 4, the thermal camera was at one end of the phone and the built-in visible light camera at the other. With the two cameras at opposite ends of the phone, there was too much of a parallax to combine the images into anything other than a blurry mess, except with distant objects where the parallax is barely noticeable. The picture-in-picture mode is more useful, adding a thumbnail of the visible light image from the phone camera to the corner of the thermal image. It isn’t as effective as the combined images from the Flir camera, but it serves to help you find a heat source in the low-resolution thermal image.
There are some nice touches here: Uni-T includes a short extension cable that lets you use the camera away from the phone or connect it to a phone with a bulky case, although the camera worked fine with a thick case on my Samsung Fold 4. The company also includes a USB-C to mini USB adapter in the Android version so you can use the camera with older Android phones that lack USB-C ports. An included waterproof case makes sure the delicate infrared lens won’t get damaged in your tool bag.
The Thermal Mobile app is also pretty good, although I tested only the Android version. (It's available for iOS too.) The app offers features like tracking high and low temperatures in the image simultaneously: useful if you are testing a heat transfer device like a refrigerator.
