Can an iPhone detect infrared?

The honest framing: your iPhone is not an infrared camera and cannot be turned into one by any app. It is a visible-light camera that leaks a little infrared. That leak is sometimes useful, and the only way to know how much your particular phone leaks is to test it.

What infrared-cut filtering is and why it exists

The silicon image sensor in a phone camera is naturally sensitive well past the red end of what your eye can see, into the near infrared. If that light reached the sensor unfiltered, photographs would look wrong: skies would shift, foliage would go pale, skin tones would turn strange, and the colour balance would drift under different lighting. So every consumer camera, including every iPhone camera, has an infrared-cut filter that blocks most near-infrared before it reaches the sensor.

These filters are good, but they are not walls. They attenuate strongly across the near-infrared band rather than eliminating it, and their effectiveness varies with wavelength. A source at 850 nm — a common wavelength for infrared illuminators, and one that also produces a faint red glow visible to the naked eye — sits closer to the visible band and is more likely to leak through than a source at 940 nm, which is fully invisible to the eye and further from the filter's weak edge. So the sources that are hardest for you to see unaided are often also the hardest for the camera.

Front camera versus rear camera

People often find one camera on their phone shows an infrared source clearly while the other shows nothing, and both results are normal. The front and rear camera modules are different hardware: different sensors, different lens stacks, different filter coatings, and different processing pipelines. There is no general rule that one is always better — it varies by model generation.

There is a further wrinkle. On iPhones with Face ID, the front of the phone contains its own infrared emitter and infrared camera as part of the depth-sensing hardware. That is a separate sensor which apps cannot use as a general-purpose infrared viewer; it is not what you are looking through when you open a camera view.

The remote control test

This is the standard way to find out what your specific phone can see, and it takes about thirty seconds. Most television, air-conditioning and set-top-box remotes emit near-infrared from a small LED at the front.

1. Find a remote control with working batteries. A TV or air-conditioning remote is ideal.
2. Open your camera app and point the rear camera at the front tip of the remote, about 10 to 20 cm away.
3. Press and hold any button on the remote.
4. Watch the screen. If your rear camera passes infrared, you will see a faint white, pale lilac or purple flicker at the emitter that is not visible to your naked eye.
5. Switch to the front camera and repeat, at the same distance.
6. Note which camera showed it and how brightly. That is your phone's baseline sensitivity.
7. Repeat in a darkened room. A dim leak that is invisible in daylight is often obvious in the dark, and darkness is the condition you would be inspecting in anyway.

Interpret the result honestly. If neither camera shows the remote at all, your phone is not going to reveal a weaker infrared source across a room. If the rear camera shows it faintly at 15 cm, it will show a much brighter illuminator at two metres in the dark, and nothing subtler. This test sets a realistic ceiling on what the method can do for you.

Why a visible infrared source is not evidence of surveillance

Infrared light is everywhere in an ordinary building. Finding a source means something nearby emits infrared — nothing more. Common sources include:

• Remote controls, and the infrared receiver windows on televisions, set-top boxes, air conditioners and soundbars.
• Motion sensors, presence detectors and some alarm components.
• Automatic taps, soap dispensers and hand dryers.
• Some LED lighting, and dimmed incandescent or halogen bulbs, which radiate strongly in the infrared.
• Heating elements, radiators, kettles and anything else warm.
• Sunlight, and daylight through a window, which contains a great deal of infrared.
• Optical mice, laser sensors and some charging indicators.
• Genuine security cameras with night vision — including declared exterior ones, doorbell cameras, and baby monitors.

Also true in the other direction: plenty of cameras use no infrared illumination at all. A camera without night vision, or with it switched off, emits nothing for this method to find. So the absence of an infrared source tells you very little, and the presence of one usually has a boring explanation you can identify by walking over and looking at the object.

What Findy can help with

• An infrared viewing mode that presents the camera feed in a way intended to make faint, low-contrast light sources easier to notice in a dark room than they are on a standard camera preview.
• Guidance on conditions — full darkness, adjusted eyes, close range, slow sweep — because those matter far more to the result than any processing.
• A place to run the remote-control test and establish what your particular hardware can actually see before you depend on it.

The infrared viewing mode requires the subscription, $39.99 per year in the US.

What Findy cannot determine

• It cannot see what your hardware blocks. If your camera's filter attenuates a 940 nm source below the noise floor, no processing brings it back.
• It cannot detect a camera that does not use infrared illumination, which is many of them.
• It cannot tell an infrared illuminator from a remote-control receiver, a motion sensor, a warm object or a lamp. It shows you a light source; identifying it is your job.
• It cannot measure wavelength, temperature or intensity in any meaningful units. This is not thermal imaging — thermal cameras work in the far infrared, a completely different band that no phone camera sensor responds to at all.
• It cannot compensate for the difference between iPhone models, or between your front and rear camera. Results genuinely vary by hardware, and the app will not pretend otherwise.
• It cannot see a powered-off device, or one whose infrared illuminators only activate in darkness when it is triggered.

Related guides

• How camera lens reflections work — the other optical method, which does not depend on the device being powered on.
• How to check a hotel room — where the dark-room pass fits into an inspection.
• How Findy works — the technical detail on all five methods.
• Full limitations.