Why you can trust ZDNET : ZDNET independently tests and researches products to bring you our best recommendations and advice. When you buy through our links, we may earn a commission. Our process

'ZDNET Recommends': What exactly does it mean?

ZDNET's recommendations are based on many hours of testing, research, and comparison shopping. We gather data from the best available sources, including vendor and retailer listings as well as other relevant and independent reviews sites. And we pore over customer reviews to find out what matters to real people who already own and use the products and services we’re assessing.

When you click through from our site to a retailer and buy a product or service, we may earn affiliate commissions. This helps support our work, but does not affect what we cover or how, and it does not affect the price you pay. Neither ZDNET nor the author are compensated for these independent reviews. Indeed, we follow strict guidelines that ensure our editorial content is never influenced by advertisers.

ZDNET's editorial team writes on behalf of you, our reader. Our goal is to deliver the most accurate information and the most knowledgeable advice possible in order to help you make smarter buying decisions on tech gear and a wide array of products and services. Our editors thoroughly review and fact-check every article to ensure that our content meets the highest standards. If we have made an error or published misleading information, we will correct or clarify the article. If you see inaccuracies in our content, please report the mistake via this form.


Flir One review: Thermal imaging on your smartphone

Written by Rupert Goodwins, Contributor

Flir One

8.0 / 5

pros and cons

  • Includes infrared and visual-light cameras
  • Available for iOS and Android devices
  • Delivers absolute temperature estimates
  • Affordable for a thermal imaging camera
  • Expensive for a smartphone add-on
  • Fiddly on-off button
  • Android version has fixed orientation

Anything promising superhero powers on your mobile phone must be an epic game or heroic hyperbole -- but with the Flir One, it's close to the truth.

When 19th-century physicists started uncovering the invisible universe of radiation, radio waves and X-rays, it was fashionable to see this as proof of spirits, ghosts and the afterlife. That hasn't panned out, but while we're now used to hearing voices from space and seeing around the world, there's something undeniably spooky about the Flir One, a thermal imager that opens an affordable window into the world of the long infrared.


The £199.99 (inc. VAT) Flir One is available for iOS and Android devices.

Image: Flir

Costing £199.99 (inc. VAT), the Flir One is a small device that clips onto the Micro-USB port of your Android device, or the Lightning port of your iOS tablet or smartphone. It takes pictures, still or moving, in long-wave infrared, using a second, visual-light camera to simultaneously provide an overlay that lets you easily identify what you're looking at. It can also produce an absolute temperature estimate of a spot on the image -- again shown as an overlay.


If you can stand the heat, get your Flir out in the kitchen -- a good source of calibration and experimentation for the infrared explorer.

Image: Rupert Goodwins/ZDNet

The images themselves are garishly coloured, with the default palette ranging from dark blue for the coolest parts of the image to bright yellow or white for the hottest. Intermediate levels are shown as shades of red or orange. This is immediately recognisable from any number of movie scenes, science documentaries or police chase programmes. where people and vehicles are tracked in darkness by their infrared radiation -- and indeed Flir has been in business since 1978 selling many of the professional and military IR imaging systems behind those pictures.


Three ceramic coffee mugs, one filled with iced water, one with hot water and one fresh from the kettle, show the range and precision of the Flir's Lepton sensor.

Image: Rupert Goodwins/ZDNet

The company's engineering pedigree comes through in the quality of the hardware and of the software, which is simple to use, robust and well-designed. The app integrates smoothly with Android and iOS's default image storage and sharing options, making it instantly usable for anyone who knows their mobile. It has some neat tricks: you can reveal the visible-light image behind the infrared by rolling the one over the other through swiping, while an efficient edge-detection system overlays the shape of objects onto the infrared image, considerably aiding identification of what you're looking at without obscuring the details of its thermal shape. Many different palettes are selectable, each highlighting different aspects of the thermal image, and PC and Mac software is available that allows a much finer-grained image analysis and manipulation.

The hardware unit has its own rechargeable battery that runs for about an hour before needing to be charged through its own Micro-USB port. It doesn't also charge the phone when that's in use, but you can run the unit while charging, giving you the option of much longer periods of portable operation if you take a power pack along. The only control is a small on-off button with its own status LED, which is a bit fiddly to operate. The only other criticism is that the unit doesn't feel particularly solidly attached in use, at least on Android phones, but given the huge range of different form factors it has to work with it's probably copes as well as it can.

Another difference between the two platforms is that the iOS model can swivel to point in either direction -- Flir suggests you can take a 'thermie' (an IR version of a selfie), but the Android version is fixed in whatever direction your USB port demands. You can use a Micro-USB extension cable to operate the imager away from your phone, which is a useful option if your USB connector is awkwardly placed or makes the Flir One point towards you, but this is fiddly and needs practice.

Top ZDNET Reviews

Raspberry Pi 4

Top ZDNET Reviews

Raspberry Pi 4

Raspberry Pi 400

Top ZDNET Reviews

Raspberry Pi 400

Samsung Galaxy Xcover Pro

Top ZDNET Reviews

Samsung Galaxy Xcover Pro

reMarkable 2

Top ZDNET Reviews

reMarkable 2


Flir supplies a lanyard-hung carrying clip, which works quite well if you're out and about and want to also use your phone without encumbrance.

How it works

Although visible light and heat are both electromagnetic radiation, Thermal Imagers use a completely different technique to visible light sensors. Photons of visible light have higher energy than those in the infrared, and the light is a shorter wavelength -- energy and wavelength being inversely proportional.

Visible light can be absorbed by electrons within atoms, directly generating an electrical current that can be amplified and digitised. Infrared photons don't interact with electrons in that way, instead affecting the electronic bonds between atoms in molecules and increasing the motion of those atoms. That shows up as heating -- very little of it.


A simplified architectural diagram of the Lepton camera module.

Images: Flir

Thus, Flir's Lepton sensor chip at the heart of the Flir One has an array of 80-by-60 pixels made from 17-micron vanadium oxide bars, suspended on silicon supports above the chip surface. These change resistance when heated, which is what the rest of the chip senses and turns into digital image data at 14 bits/pixel and 8 frames/second. The system has a resolution of 50mK, or 0.05 degrees C.

This is a vastly simplified picture. The physics of infrared detection by such sensors -- called microbolometers -- is complicated. The sensor itself is uncooled and emits infrared photons at a higher local level than those it's trying to detect. In practice, this means the Lepton pauses every thirty seconds or so to move a shutter across the sensor in order to calibrate itself.

Extracting the required signal needs a lot of processing and filtering, which is handled by a separate custom SoC bonded to the Lepton package. The Lepton itself can be bought on an Arduino shield and other experimenter/design-friendly packages.

In use

The Flir One does exactly what it claims, and does it very well. In a couple of weeks of use both indoors and out it adapted to the wide range of conditions and temperature ranges that a Scottish summer provides. Outdoors, it revealed which parked cars had recently been driven, and which areas of roofing, walls and windows were losing most heat. It can't see through solid surfaces, of course, but it can show people and animals at night or behind broken cover such as bushes or grass.

Indoors, it has a variety of uses, including finding breaks in door seals on fridges or cookers, spotting overheating cabling and providing an instant census of what electronic devices and power supplies are on in a room -- potentially useful in an office environment for quickly revealing who's got an illicit wi-fi hotspot (literally) stashed under their desk.


The on-board power regulator and an interestingly uneven heatmap beneath the CPU fan show the main sources of thermal energy in this Dell 7020 mini-tower.

Image: Rupert Goodwins/ZDNet

The most traditional use for thermal imaging in IT, though, is in finding if components are getting too hot. This can be when configuring or testing a motherboard, especially when checking CPU operation, or spotting blocked fans and filters. It can also check when inaccessible wall- or ceiling-mounted devices are failing or have failed.

Slightly more spookily, it will detect residual heat left by flesh contact on surfaces. Bare feet on wooden floors leave footsteps that persist for many seconds, and even quick dabs of the finger onto a cold surface can be detected afterwards -- some security experts advise that you should rest your palm on an ATM or card payment point's keypad after entering your PIN, to wipe out this trace from hovering miscreants.


After half an hour running the Prime95 CPU stress test, even Intel's lowest-power processor makes its presence clear.

Image: Rupert Goodwins/ZDNet


I can't deny it, I've wanted a thermal imager for decades, and the Flir One is the first that I've been able to afford. Whether it's justifiable as an expense that will pay back is questionable, although if I was still maintaining or repairing electronic equipment I'd expect it to cut diagnostic times drastically in some cases.

As a piece of desirable technology, though, it has impeccable credentials. Anything that genuinely extends the senses is going to provide a new way of looking at the world, and if you have an ounce of wonder or imagination left in your soul then the Flir One will be a satisfying and unique addition to your digital experiences.

Read more reviews