Near Infrared
Written by John Carl Villanueva
Near infrared or NIR is a subdivision in the infrared band somewhere between the 800 nm and 2,500 nm wavelengths. Its applications range from fiber optic telecommunication, to astronomical spectroscopy, and remote monitoring.
The infrared band, on the other hand, is a set of electromagnetic waves between microwaves and visible light. The reason why it's called infrared is because if you look at the electromagnetic spectrum, infrared or IR is found right beside the outermost red in the visible light band. Gives you an idea where the ultraviolet or UV band is found, eh? That's right, it's right beside the outermost violet in the visible light band.
Let's have a closer look at the applications mentioned above.
NIR in fiber optic telecommunication – The main reason why optic fibers are more preferred for communication than metal wires is because they are less susceptible to loss. Optic fibers, as we know, carry light to transmit signals. However, not all wavelengths of light are created equal. For optic fibers using silica (there are actually only a few that are not), it is the NIR wavelengths that provide the smallest degree of attenuation.
NIR in remote monitoring – Because plant life reflects practically all wavelengths of of the infrared band, and because our cameras capture only wavelengths that are within or very near the visible light band, plants look completely white when near infrared photography is applied. Making use of this phenomenon, agriculturists take infrared aerial photographs of crops to analyze yields and monitor pest infestation.
NIR in astronomy – This is perhaps the application that you are most interested in. First of all, let's discuss a brief background on infrared imaging in general.
Have you ever wondered where the phrase "red hot" comes from? Why not "blue hot" or "yellow hot"? You see, our skin is most sensitive to electromagnetic waves in the infrared region. So when we feel the heat of the sun or a flame, we're actually feeling the IR wavelengths more. That doesn't mean that other wavelengths don't carry heat, it's just that we're most sensitive to the red ones.
Now, there are hot objects and there are hotter objects, and we are able to distinguish one from the other. So you can imagine that each IR wavelength carries a certain degree of 'hotness'. So what has all these got to do with astronomy? Well IR detectors are like our skins. They are able to distinguish between different IR wavelengths.
That is, detectors that are designed specifically for NIRs are able to capture images of objects in space that have temperatures between 740 K and 5,200 K. These are basically cooler red stars and red giants. Other objects with temperatures outside that range will be emitting perhaps mid-infrared or far-infrared, and hence will be invisible.
Another reason why you would want to view objects via near infrared detectors is that near infrared waves can pass through dust whereas visible light can't. Hence, you'll be able to capture images of objects that otherwise would have been hidden from view.
Universe Today has some interesting related content that you might want to read. Want to know about the balloon experiment that solved the mystery of far infrared background? There's also one that talks about how the Cassini found a new infrared aurora.
There's also some interesting reads at Physics World like the High Performance Near Infrared Camera or the Near Infrared Universal Optical Coupler
Or if you just want your ears to go to work, here are two more at Astronomy Cast:
Filed under: Astronomy
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- Podcast: Infrared Astronomy
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