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Comment by smarks | Hacker News Reader

One quick and cost-effective way to detect IR light is by using the front-facing camera on your phone.

Unlike the rear-facing camera, the front-facing camera does not have an IR filter and can easily pick up IR lights in the dark.

To try this at home, grab your TV remote, which uses IR light to send information to your TV and open the front-facing camera on your phone. Next, point the front end of the remote at the camera and press any of the remote buttons. The IR light that is normally not visible will be completely visible on your camera screen.

Well I’ll be darned. This works! (iPhone 7)

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I don't remember seeing a rear-facing camera for which it wouldn't work. Are there extra IR filters installed on newest models, or is that an Apple thing?

The newer iphones block 850nm almost completely. I was shining one at a half watt led just the other day and couldn't see anything. Can't remember the iPhone model.

Many phones are getting better IR filters because sunsets and fireplaces both have large amounts of IR in, and without the filters it's hard to get a decent picture. People rarely use the front facing camera for those things.

I was also surprised. My old Galaxy S6 detected it fine (which is great when DIY-ing a TrackIR). I have to go home, plug it in and see if newer phones are any different.

My S7 handless this as well, and every Android phone I ever tried this on has a rear camera receptive to IR (I usually test it on TV remotes).

It's also a neat trick to check your remote's battery level.

Wonder why the front-facing camera doesn't have an IR filter but the rear one does.

IR is often used for facial recognition (invisible pattern projectors or to better illuminate skin texture). However in colour pictures you get a strong purple cast in some conditions, like bright sunlight, due to excess near infrared radiation. This is not desirable for photography, so most consumer cameras have IR cut filters for this reason. The filter looks like a transparent piece of glass or plastic, but it will shimmer red if you hold it at an angle. Not to be confused with the normal anti reflective coatings on lenses (these often look green/purple).

As the sibling says though, plenty of rear facing cameras are also IR sensitive. I imagine there's some onboard processing to correct the colour balance.

You can't really correct for the chromatic distortion (purple effect) as it is not simply a color correction, the IR light bends differently through the optical elements / lense. (The diffraction coefficient is different for different wavelenghts of light). Thus you have to filter the light out to get rid of the effect.

You can correct it (Lightroom, Photoshop, Darktable, etc include tools for this), but it's a waste of processor power if you can fix it in hardware.

I suppose it depends on the bandpass of the filter. I have several older phones which can take (rear) images of IR leds - we used this as a sanity check at work several times. Images of normal things didn't look noticeably worse for having that sensitivity.

Many visible optical filters leak in the infrared, so perhaps a small amount gets through, but not enough to cause a noticeable colour cast in normal photography.

Vegetation in sunlight is a good test target, too. Leaves strongly reflect NIR.

What kind of light does the FaceID system use? Or does it have a separate camera for the invisible lights?

IR but it has separate hardware. There's an IR dot projector, IR floodlight, and IR camera.

Oh, this is awesome: it works for both of my phone’s cameras! It’s a OnePlus 6 — I wonder how many other phones have this ability in both cameras.

Actually, from a photography aspect, that's more of a bug than a feature because it's creating weird artifacts when photographing a strong IR source.

Nice! So, does this mean that an 'easy' way of defeating non-filtered cameras is to just have a few IR LEDs blasting out? I've no idea on costs or the optical physics here.

Yes: https://www.instructables.com/id/The-Invisible-Face-Mask-/

Many surveillance cameras use an IR cut-off filter for their "day" or color mode though, so this may not work then (or at least, not work as well, depending on the cut-off filter and LED wavelengths).

Well, heck, if that's not a product and market fit, I don't know what is?!

Worked for me as well, iPhone X. Very cool!

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