IR illuminating circuit

Hello, below i will attach a circuit of a IR illuminator from

Also might i add that this circuit is meant to fit around a cmos security camera.

The way the circuit works is a 100k POT and a LDR act as a potential voltage divider.
As the intensity of the light falling on the LDR decreases, the current wants to take the path of least resistivity, so it gets pulled high and activates the transistors, which activate the relay, which activates the IR LEDs.

I get that all objects emit some Infrared light and humans cannot see IR light.
In low light imaging (which is different from active illumination) works by taking what light is there and amplifies it to a screen (usually the green screen), which makes sense to me.

What i don't get is with active illumination, how do IR LEDs help the camera see at night? Does it reflect off objects and bounce back to the camera? I have noticed in the past when i look through my phone camera at an IR led, you can see it as a purplish light. Can it almost be thought of as humans use light to see at night, a camera uses ir LEDs as a light in the dark because cameras receive light different from humans?

IR LED camera.jpg

IR LED camera.jpg

Video cameras receive most visible light and some IR if they do not have an IR filter.
Humans emit IR but not much so amplification is often needed. IR lights provide additional illumination but are "visible" if someone is looking for them.

This is an interesting question, as I install CCTV and IR blasters a fair bit and have never put the thought into the science behind this phenomenon.
After a bit of googling, it seems as though InfraRed light has longer wavelengths starting around 700 nanometers and reaching up to 1 millimeter.
Our human vision is good for a range of 380nm which is the colour Violet up to 740nm which is red. I am guessing we can see some lower frequency Infra Red light which is why we can see the LEDs lighting up on the cameras, as well as a dull purple glow from some of the roadside speed cameras.

Silicon sensors in cameras can capture wavelengths up to 1300nm, which allows them to receive the reflected infra red light just like we do with the lower wavelength light we use to see.

I hope that answers you question. Please see below for a couple of links I found some of this info on:

silicon switching wavelengths in cameras - BingZhioNZjsyYt2oiaMDbAaA2CTmGa7IGPmrlw%2159QvTtiXK6zNtWUSm*mjl4Y5R6V9MAEGX&plvar=0

IR is electromagnetic radiation, part of a huge spectrum. It works exactly like the radiation that we call visible light. It's just that its frequencies are outside the range that humans can detect. But it's not outside the range that camera sensors can detect.

The trick is that the camera does a little frequency shifting to convert the IR image into a visible image that suits our pathetically small range of vision. Probably most commonly seen as thermal imaging (heat radiates mostly in the IR region).


Absolute yes to your question. The CCTV cameras are sensitive to IR light in exactly the same way that they and our eyes are sensitive to visible light. The IR LEDs emit IR in exactly the same way that an LED in the visible spectrum emits its particular colour of light. IR behaves in exactly the same way as visible light. However, our eyes are not sensitive to IR so we can not tell when we are being illuminated by IR, so excellent for covert surveillance.

More about IR
As has already been said here, IR or infra red light is simply redder than red light and behaves in exactly the same way as light because it is light, or more correctly termed electromagnetic radiation. We perceive IR light as heat radiation. If you put your hand in front of strong IR source, you can feel the heat.

IR was first noted by Newton in 1600s although he did not understand what he was looking at. In his experiments with visible light he set up a rainbow spectrum with a glass prism (cutting edge technology of the time). He knew there was some sort of link between light and heat so he put a thermometer (more cutting edge technology) in each of the different colours.

As a good experimental scientist, he put a thermometer just off to the side of the red light to act as an experimental control to measure the room temperature. However, he found that the control thermometer, apparently in the dark part of the spectrum, showed an increase in temperature, whist all the rest in the visible light showed no increase of temperature. Newton could not explain this and thinking his experiment had failed, simply noted it in his lab book and went on to think about other things.

Some animals can see in IR. Some snakes use IR to detect warm mice at night.

Health and safety
IR is nowhere as harmful as UV, ultra violet, (more violet then violet light), but it can still be harmful if powerful enough. Don't look directly into IR arrays like the one the OP shows for any considerable time. Looking directly at the Sun is accepted as a very silly thing to do. It is the IR that burns your retina. I had to work with IR of typical CCTV illumination powers, a few years ago so I used a credit card sized detector that glowed when exposed to IR. Very helpful.

Its probably worth making explicit that near-IR, as in the security camera is very close to red of
visible light, and generally can be focused with ordinary lenses and reflected with ordinary mirrors.
Some animals can see it I believe. Such security cameras cannot see in the dark, but to them
near-IR is light.

The kind of IR (far IR) associated with thermal cameras is very very different - the wave length is about 6 to 15
times longer than red light, and ordinary lenses and mirrors will not work, as glass and plastic tend to
absorb such radiation efficiently. Lenses have to be made out of silicon, germanium, calcium fluoride
or other exotic materials and are very expensive. A front-side or metal mirror is needed to reflect
far-IR. Clean uncoated metal surfaces cannot have their temperature measured by a thermal camera
or IR thermometer because they don't emit, just reflect the surroundings. This is why heat sinks have a black
coating, to emit heat.

Thermal cameras can see whenever there are temperatures differences, as they sense the heat radiation
directly emitted by the objects and need no illumination.

There are a few creatures that have far-IR eyes, such as the pit-viper, and can thus detect prey in the
dark. Predator-vision for real.

Thanks, everyone, I am really impressed with the comments.
I took everything you guys have said, and ask this question.

If I was in a dark room with my phone camera and had let's say i had an IR Flashlight and turned it on. Would that let my phone camera or any camera really see in the dark better?

Some phone cameras have an IR filter on them- my iPhone SE has a filter on the rear camera so that it doesn't pick up IR, but there is no filter on the front camera so that does see IR.

Some phone cameras have an IR filter on them- my iPhone SE has a filter on the rear camera so that it doesn’t pick up IR, but there is no filter on the front camera so that does see IR.

Thank you for the reply. What is the purpose of an IR filter, if humans cant see IR?
Thanks again!

The purpose of an IR filter on a camera would be to better capture a scene as viewed by humans rather than having the image skewed by IR.

The purpose of an IR filter on a camera would be to better capture a scene as viewed by humans rather than having the image skewed by IR.

Make sense. Thanks. I noticed after learning more about IR, that my home security camera makes a click noise when i turn off the light. it removes the IR cut filter to allow more light to come through then it also activates its IR LEDS.

There would be little point in activating IR LEDs and NOT removing the IR filter. If the IR filter was left in place then much more IR lighting would be required.

I am glad to see that you are asking questions and figuring things out!

The purpose of an IR filter on a camera would be to better capture a scene as viewed by humans rather than having the image skewed by IR.

The focus for visible light and IR are not the same. Adding IR to a visible light picture will make it look grainy.

For several years I have used trail cameras to try to catch a picture of an occasional cougar that leave their tracks. No luck, yet. But have lots of night IR illuminated pictures of deer, coyotes, rabbits, mice, stray cats and dogs.

Most of those animals will look up toward the camera when the IR LEDS turn on, and then go back to their business. ALL of the IR pictures have a grainy, slightly out of focus look, but daylight photos are perfect.


Graininess is purely due to low photon count, not focus issues.

What is the purpose of an IR filter, if humans cant see IR?

Vegetation is usually very pale in near-IR, so if the camera picks up IR as
red light, vegetation will have a very poor hue, shifting from green towards
the red end of the spectrum.

The IR filter can just be a more selective red filter over the red pixels.

Photos purely in near-IR give vegetation a ghostly appearance, and blue sky
shows up black, overall the effect is more like moonlight than daylight.

As Mark T stated, the graininess is probably more related to the quality of the image sensor and the level of sensitivity or amplification required to gain up the incoming light level.

This one comes back to my small amount of time with digital photography. With my basic Canon 550d camera, if I have low light and I push the ISO (basically light sensitivity) to 800 then I will get a fairly grainie image. If I were to move up to something like a 1Dx with a lot higher quality and higher priced image sensor then I can probably push the ISO to 12,000 or more and get a similar level of gaininess, but I will have a much brighter image from the amount of amplification that has taken place over the ISO of 800.

Likewise, CCTV and other lower quality field cameras would have image sensors that are of a good enough quality to get really good images in daylight when there is heaps of natural light from the sun bouncing around everywhere, but then to capture images in darkness with only a small number of IR LEDs reflecting off whatever surface they can then these small cameras would need to push the ISO levels up to huge numbers to view as much light reflected back as possible. This graininess would therefore be digital noise cause by the quality of the image sensor as well as any video processing noise that is then also getting amplified.