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If infrared light is not visible to the human eye, why do most tv remote controls and security cameras appear to have a visible red colored LED lit when the infrared light is being emitted?

Is that visible red light present as a convenience (introduced by grace of the component designer?) or as a by-product of emitting actual infrared light?

Is it possible to have infrared light emitted without this visible red color?

enter image description here

dtmland
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    Do you see the infrared with your eyes or with a CCD camera? The CCD camera is sensitive for infrared, but usually the color filters will filter it out from all channels. – Oskar Skog Aug 02 '17 at 04:24
  • @OskarSkog I don't see the infrared light with my eyes - but I can see the little red LEDs just like this picture displays. Is that visible red light present as a convenience (by grace of the component designer) or as a by-product of emitting actual infrared light? – dtmland Aug 02 '17 at 04:27
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    If you see it with your bare eye, then it's a red LED. Looks to be a convenience feature, as I can't see any red light from my remote. (My phone's camera however does see that light.) // It might be possible that it's UV and not IR though. – Oskar Skog Aug 02 '17 at 04:33
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    Your picture looks like an illustration rather an actual photo, with red stuff added so it looks cool. – Dmitry Grigoryev Aug 02 '17 at 16:35
  • @DmitryGrigoryev Thank google image search for the photo. As I mentioned in a comment below, the image may or may not be photoshopped, however, it does represent the general look of such cameras with those red visible leds, according to my observations. – dtmland Aug 02 '17 at 16:48
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    @OskarSkog LEDs are not lasers. They do not emit a single, pure wavelength. If you see a dim red glow from an infrared emitter, you probably are seeing the visible tail of a power spectrum that contains a much brighter, invisible component. – Solomon Slow Aug 02 '17 at 17:05
  • probably also an indication that you are being recorded and the camera is on and not just for show. So having such LED's can serve 2 purpose (illuminator and indicator). – Mayank Aug 03 '17 at 12:41
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    TV remotes do not typically have a visible LED in the front edge position that is aimed at the TV!!! They have it on the button panel (nowadays often under the translucent, soft buttons themselves), as a feedback that the remote control has battery power (it is working). – Kaz Aug 03 '17 at 19:53

10 Answers10

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The simple answer is that they are using near IR. LED manufacturers have a good handle on how to make them so they are affordable.

Their center frequencies may be invisible to the M-1 eyeball (i.e. human eye), but unless they put a filter in front of the LEDs (which cause them to produce less illumination) there will be some of it that you can see.

The effect is minor. Basically, to see it you must look directly at the emitter. You're not going to see it in reflections or scene illumination.

Far-IR is completely invisible. But a whole lot more expensive because the manufacturing process is different.

Near-IR emitters are mass-produced. Far-IR not so much.

IR lasers are another story. They emit on a single frequency, so there is no gaussian curve describing their output in the frequency domain. They are so invisible that they can be dangerous. Working around lab CO\${}_2\$ lasers, for instance, requires removal of all jewelry and controlling the beam. They will not trigger a blink response so you can sustain a lot of damage in a short time and not know it right away.

kbakshi314
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SDsolar
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    There's another factor too: most CCDs are more sensitive to near IR than far, so using near IR for illumination is more efficient. – Jules Aug 02 '17 at 12:09
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    IR laser like those used in CD players DO emit a very little light that can be seen. Those lasers must be near-visible IR, and you see the tail of the optical pump (which has a broad bandwidth), while you DON'T see the huge IR (which is very narrow-bandwidth). – glen_geek Aug 02 '17 at 12:19
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    @glen_geek they are 780 nm, it's pretty visible on the typical powers used, so you can see it. The spectrum is not broad unless you input too small current into this laser to avoid lasing. – Ruslan Aug 02 '17 at 12:45
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    Am I the only person who doesn't know what an *M-1 eyeball* is? Also, this may be of interest [Thorlabs 780nm LED spectrum](https://www.thorlabs.com/images/TabImages/LIU780A_Spectrum.gif) – uhoh Aug 02 '17 at 13:51
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    @uhoh: "Mark 1 eyeball". Original equipment, from the factory, no aftermarket modifications, no external additions. – John R. Strohm Aug 02 '17 at 15:23
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    10.6u CO2 laser light will not penetrate the eye lens, but the burning sensation will be all too real. – Dirk Bruere Aug 02 '17 at 15:37
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    Lasers don't emit a single frequency. They emit a very narrow Gaussian distribution. – Matt Aug 02 '17 at 16:37
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    The mention of a Mark 1 eyeball made me wonder if the original asker happens to have a more advanced model and can see IR wavelengths better than the rest of us. If nothing else, I suppose there might be some manufacturing variations even with standard eyeballs, but how (un)likely that is, I don't know. – ilkkachu Aug 02 '17 at 17:12
  • Green lasers use frequency-doubling optics, (an optical mixer), so they emit 4 frequencies. The original optics produce lots of IR. The M-1 eyeball is most sensitive to green which is why a 5mW green laser is "brighter" than a 5mW red laser. But the bottom line is that green lasers emit a lot of IR along with the green. – SDsolar Aug 02 '17 at 19:23
  • Heh, heh, @ikkachu - maybe so. – SDsolar Aug 02 '17 at 19:32
  • @ilkkachu He may also be an elf, dwarf, or halfling. – Harper - Reinstate Monica Aug 02 '17 at 19:39
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    Even a low wattage IR laser can do interesting things to the eye. When I was much younger, before diode lasers were regulated, I was trying to use a 2 watt communications laser running at an unmeasured number of milliwatts with black and white film (which is more sensitive to IR light than visible) to create a hologram (epic fail). When aligning the laser I didn't realize I had left it powered on. When it swept across my eye I felt nothing, but had a pretty diffraction grating temporarily etched on my cornea that had me seeing rainbows for a week. – pojo-guy Aug 02 '17 at 21:23
  • Why do near and far IR require different manufacture process? – Ooker Aug 03 '17 at 03:06
  • @ilkkachu There's an aftermarket modification available that increases sensitivity in the near ultraviolet range at the cost of ... sensitivity in the near ultraviolet range in terms of damaging the eye. It's not normally available for separate purchase but you may get it offered with other related services. – John Dvorak Aug 03 '17 at 06:58
  • @Ooker, here is a good FAQ on how they are made. https://www.herschel-infrared.com/how-it-works/types-infrared-heater/ Y you might recall how difficult it was for them to achieve blue LEDs because the process is so limited. UV is still pretty much out of reach except for lasers. The bottom line is the bottom line, though. It requires exotic (rare, expensive) materials. LEDs are just silicon. The first LED effect was discovered by accident when they went from Germanium to Silicon (inexpensive) and someone noticed. LWIR is more like antennas. It does not get as hot like you think of IR. – SDsolar Aug 03 '17 at 08:00
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    @uhoh I would never refer to an M-1 eyeball when "Mk. 1" or "Mk-1" etc. unambiguously convey the intended idea. It's not your fault you didn't understand something that vague. – Monty Harder Aug 03 '17 at 19:33
  • Typical IREDs, even some truly invisible types, would still be near-IR.... far-IR is something you see with a thermal camera :) – rackandboneman Aug 03 '17 at 20:54
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    @SDsolar: While some LEDs are now built on Si substrates, that's a fairly recent development. Most IR LEDs are still gallium arsenide on sapphire substrate. The first commercial LEDs did not involve Silicon at all. – Jerry Coffin Aug 03 '17 at 23:27
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    Do note also that there is a fair amount of variation in how far into the IR spectrum people can see. The retina is sensitive to IR, but the cornea and lens filter it out (much like an IR camera actually) with varying degrees of effectiveness. Among the people I work with, it varies from "can't see the red glow looking right at the LEDs" to "can find their way round a room lit only by those LEDs." – Perkins Aug 04 '17 at 00:35
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    @Monty - I see your point, and uhoh's also. I have never seen it written - only used it verbally. I almost wrote M1. Mark 1 would probably have been more clear. – SDsolar Aug 04 '17 at 02:46
  • True enough, @Jerry. But that was the breakthrough that allowed for blue LEDs to be produced and the whole industry seems to be moving in that direction. – SDsolar Aug 04 '17 at 02:59
  • @ilkkachu Sounds like a good question for [biology.se]. – user Aug 04 '17 at 07:20
  • I got some cool sunglasses with my 1 Watt blue laser 450 nmP. I know an accupuncture doctor with a really cool 50W IR pulse laser he says works better than needles. It is password protected for safety. Generally sustained IR is cataractogenic (causes cataracts) and higher energy blue is also bad for eyes which is why iPad 4's can be enabled for warm white at night. Of course we know UV is worse. – Tony Stewart EE75 Aug 13 '17 at 01:02
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The transition from visible wavelengths to invisible is not infinitely abrupt. Your eye's sensitivity falls off in the IR range. But in the near IR, it may not be zero sensitivity.

And the emission spectrum of LED's is not infinitely narrow. So not all of the photons coming off of an LED have the exact same wavelength.

The net effect of these two things is that when near infrared LED's are driven very hard, some photons will come off of them that are visible. To the camera, those LED's are like a super bright spotlight. But to your eye, they are just glowing modestly.

I have also seen cameras where the LED's were not visible at all. So there is some variation there.

user57037
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    I have seen the same with some TV remotes, too. Taken apart you can see they have a plastic filter that helps to block the visible portion. – SDsolar Aug 02 '17 at 05:08
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    @SDsolar, Yes, I have seen those filters. They block most visible light but pass IR. – user57037 Aug 02 '17 at 05:30
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I have never ever noticed that a remote control or any other IR-LED emits any red light. It might glow very, very dark, because a tiny little bit of the light is emitted at higher, visible wavelengths.
Maybe, you are a bit special and can see light deeper into the IR range, that would be interesting.

On the other side, you ask

why do most tv remote controls and security cameras appear to have a visible red colored LED lit when the infrared light is being emitted?

which basically means

Why are there two LEDs, one red, ond IR?

That's just a feedback that the device is working. Remote controls have a visible LED on the top (mine is blue, by the way) and an IR LED pointing forward.

Security cameras indicate that they are on / are recording to the ones in front of the camera, there are even fake cameras with no more electronics than just this LED and its blinking circuit

And on advertisements like your picture, the IR LEDs are often "photoshoped" red.

In reality, camera sensors can see IR light, but it appears blueish white. This is the reason why for example cigarettes sometimes glow blue instead of red on photos. Today, there is a filter in front of the camera sensor, which prevents this. It typically does not block the IR of an LED which is very near to the visible spectrum, but some filters do.

sweber
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    Whether or not that image was photoshopped I cannot confirm. However I can tell you that's exactly what they (many that I have seen) look like in the dark when the infrared is emitting. Are my eyes special in this regard? Well, that could be another question entirely - perhaps for the "Biology" Stack Exchange site. – dtmland Aug 02 '17 at 05:20
  • If you remove the IR filter you can see the light from many remotes. I have worked extensively with the product called TAG. It is not a remote, but you can definitely see the IR LED in TAG with your eye. It is very, very faint, but you can see it. The peak current in the LED is around 2A, but it is pulsed at relatively low duty cycle. In a dark room, with no filter, the light is somewhat visible, and it is definitely an IR LED. It is also true that many products with IR LED's use a filter to block the visible light from the LED. TAG did this. – user57037 Aug 02 '17 at 05:29
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    "is emitted at higher, visible wavelengths." - You mean "higher frequencies" or "lower wavelengths" :) – JimmyB Aug 02 '17 at 12:30
  • "Today there is a filter". Not on a Samsung Galaxy (S6 I think). – Oskar Skog Aug 02 '17 at 12:42
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    Low power IR emitters (like most low-power LEDs) are "potted" in transparent epoxy. As mentioned in other answers, a "near-IR" emitter typically emits a small amount of visible light; but The emitters used in some TV remotes have a dye in the epoxy (looks black or very deep blue) that absorbs all of the visible emission. Other TV remotes put the emitter behind a similarly colored window, and still others have an emitter in a "water clear" package that allows you to see a faint red glow when a button is pressed. – Solomon Slow Aug 02 '17 at 17:14
  • Good point, @james. The potting is more transparent to IR than it is to visible red. But any kind of filtering reduces total output so they only do so much. I haven't seen any IR LEDs with extra-thick potting acrylic to further reduce the visible portion. IR LEDs tend not to be very bright in the first place. – SDsolar Aug 02 '17 at 19:26
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    btw, a lot of video cameras (like the one used to make Paris Hilton's video) use filters on the LED emitter so they are less bright, and used for shorter ranges. They are labeled as being for night-vision. That camera omits the IR filter on the CCD. Most cameras can see some IR (unless they have internal filters) so they make great tools to "see" if your TV remote is working or not. – SDsolar Aug 02 '17 at 19:29
  • Also, an IRED for a TV remote control might be orders of magnitude less bright - and more spectrally pure - than something expressly intended for illumination purposes... LEDs tend to degrade spectrally if you run them hard or even hot! – rackandboneman Aug 03 '17 at 20:50
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Is that visible red light present as a convenience (introduced by grace of the component designer?) or as a by-product of emitting actual infrared light?

It isn't red light at all. It's infrared light which is perceived as red.

The human eye has three types of cones (color sensor cells): S-cones, M-cones, and L-cones. They're roughly equivalent to blue, green, and red color sensors. Here's an approximate set of response curves for the three types:

Human eye spectral response curve, from Wikipedia

L-cones are mostly sensitive to red light in the 560-580 nm range, but will respond weakly up to 1000 nm, which is well into the infrared range. If an infrared emitter is bright enough -- which the LEDs on an infrared camera certainly are! -- it will activate L-cones, making it appear red.

  • Those curves also illustrate why colour blind people with e.g. impaired L-cones (such as myself) have trouble distinguishing red tones, but can still see most of them with their M-cones. – Bart van Heukelom Jul 07 '20 at 19:19
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cheap cameras from china or sourced from a big box store will usually use 840nm-850nm leds driven very hard to produce the illumination (mostly invisible spotlight) for their night vision.

The LED energy output as Light covers something 20nm+ or minus the listed wavelength(center wavelength).

especially in the dark most human eyes(depending on genes)have at least a weak response to something like 900nm. Tests by professionals done as double blind (the test methodolgy not vision lol) have shown some people can reliably detect a little past 1000nm. This doesnt mean it lights the room up. It means when someone in another room switched the ir light on in the test room the person was able to perceive enough of change in their vision that they answered "was it on" correctly over 50% of the time.

your eyes response/signal of brightness to brain trails off like a bell curve with high and low wavelengths and no two people have exactly the same vision (as some spectral charts posted would suggest).

. there is another thing at play as well. Something like a double bounce of the photons inside the eye allows them to trigger a stronger than otherwise would have happened activation. I tried to google and find the paper i happened across last week but i had no luck. maybe someone else can chime in.

practically speaking/applied: the higher you go nm wise the less visible it is, especially at the point its coming from.

If you want ir night vision cams that doesnt scream "here is my camera" or cause a passer by to notice the red orb 10ft off the ground from a distance, look for 940nm ir led illuminators. In pure dark and close to it you may see it but it wont be the obviousness of the 8xx nm or 7xx nm emitters.

most cameras have less sensitivity at 9xxnm but the systems do exist and regular cameras with out ir filters will usually see this much better than your eye. there are some youtube videos comparing 840 and 9xx emitters with average cameras.

its important to note that although the IR light sources are perceived as only glowing faintly, a strong ir source can damage the eyes. So if you buy high powered IR iluminators do not put it next to your eye ball and look at it! You will fry your eyes!

I noticed one commented talked about the price, but its really not that bad and has been following its own moores law so if you looked 6 months ago its worth looking again. At the other end of the spectrum in UV land leds that were a lab experiment 6 years ago and cost 200 till a few weeks ago just dropped to $12 bucks. LED tech is moving fast. anyone quoting a price with out having looked at it that month should refrain from stating it as fact.

J Wuethrich
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In this day and age, cameras can look like anything. Teddy bears, rocks, mud-wasp nests, whatever. But look at the thing. They swerved way out of their way to make it look as much like a camera as they possibly can. The red lights serve that same purpose, to make it look cooler and more intimidating.

I'm not saying they're not using IR LEDs that leak a little visible red. I'm saying they allowed or encouraged that.

Why? Some people who buy cameras want "garish". And those people write checks.

I personally think these ostentatious things make excellent decoys. Perfect world, the mud-wasp nest catches HD video of the crook smashing this thing.

Harper - Reinstate Monica
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Not specific to LEDs, but yes you can generate iR without visible light. Part of the problem with that is that filters designed purposefully to block out visible light can be pricey and require a high power source. The US military uses strobe marker lights that emit a strong visible strobe, but beneath a blackout shield so the light is only visible with night vision or other iR sensitive devices, and is 100% invisible to the naked eye. A strong LED could also have such a coating if needed. The LED itself could simply be bright white (or "clear").

Jesse Williams
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  • Good information, but pertinent … ? – can-ned_food Aug 03 '17 at 16:35
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    "Is that visible red light present as a convenience (introduced by grace of the component designer?) or as a by-product of emitting actual infrared light?" This shows that it is not an inherent byproduct of emitting actual infrared light. – Jesse Williams Aug 03 '17 at 16:36
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Re: 850nm vs. 940nm price points and by inference Moore's law.

As of July2020 one can now purchase 940nm IR fixtures for the same price as 850nm IR emitter fixtures on popular online marketing websites.

For example, one can purchase a "10W" IR floodlight in 850nm or 940nm for about US$28 (note: the "10W" almost certainly refers to the DC power requirements of the LED...and has little to do with the photon output of the LED as usually measured.)

If you're going to be using an IR illuminator to augment a surveillance system, then an 850nm illuminator is usually the better choice as common low-cost surveillance cams are considerably less sensitive at 940nm. Using the 850nm is also usually recommended for cameras that have an IR cutout.

Note that counterfeit items of all sorts (e.g. lithium cells) are a big problem on these websites so you should watch out for 850nm illuminators being labeled as 940nm.

Voltage Spike
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jGecko
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Here are two informative links that should address this issue completely. The second one is the "double bounce of the photons" article (or at least similar to it) that someone discussed in an earlier comment.

I can see the 850nm LEDs with no problem, especially after I have let my eyes adjust by sitting in a dark room for 10 minutes (the longer the better) before I look at the "clear" IR emitter LED of a typical TV remote which does not have a filter in front of the LED. Many consumer devices place the visible light filter in front of the RECEIVER in the TV or other remote-controlled device which helps prevent swamping of the receiver with non-IR photons (KIND of like an RF band-pass filter that helps prevent swamping of the front-end of RF receivers by RF frequencies outside the intended range of received frequencies).

https://www.researchgate.net/post/what_is_the_difference_between_ir_illuminators_with_850_nm_and_940_nm_wavelength

https://source.wustl.edu/2014/12/the-human-eye-can-see-invisible-infrared-light/

jGecko
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I actually was looking at the infrared optic of a mouse, and every time I moved my finger over it, I would see a small amount (a very small amount) of red light. I read the paper included with the mouse, and it is not supposed to emit anything else but infrared. So from experimenting with that, I think humans can see infrared light to a certain point. The flashes of light were red, so I guess infrared really is red. It would make sense too, because it is called infraRED. Although the prefix infra means "below". It is a dark red that I see. Infra means below, so infraRED is "below red", so that must mean that it is a different color. I think infrared does dip into a tiny bit of the visible spectrum, explaining why I see red from the mouse optic.

user14631264
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