|This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)|
While infrared often refers to near-infrared, it's actually a fairly huge range, split into ranges in a few different ways.
One common seems to be the near/mid/far split (near/far from the visible spectrum, 380nm..740nm), like:
- Wavelength approx 740nm to approx 2500nm
- IR LEDs are usually near-IR, typically somewhere within 700...1000nm
- most relevant to optical astronomy(verify)
- 25000–1000000nm(verify), getting close to microwave region
Infrared and cameras
Infrared around optical cameras mostly deals with the fact that camera sensors can see some amount into near infrared.
For reference, our eyes see ~700nm to 400nm, while CCD and CMOS might see perhaps 1000nm to ~350nm (sensitivity shapes differ).
In other words, they look slightly into near-IR, and slightly into UV-A.
...but far too little to be useful as a thermal imaging camera, because it doesn't even cover a lot of near-IR. Thermographic cameras often sensitive to a larger range, like 14000nm to 1000 nm
Broadly speaking, there are two main things we might call "infrared filters"
- IR-cut filters
- often a filter in front of a camera image sensor
- look bluish from most angles (because they also remove a little visible red)
- cuts everything above some point (well, transition), often somewhere around 740...800nm
- Since it's a transition, bright IR might still be visible. For example, remote controls (typically in the 840..940nm range) may still be (barely) visible, in part because they're actually quite bright and concentrated
- IR-pass, visible-cut filters
- often look near-black
- cut everything below a transition, somewhere around 720..850nm range
- using these on a camera that has an IR-cut will give you almost zero signal (it's much like an audio highpass and lowpass set to about the same frequency)
You now have some options, like
- cutting IR and some red - similar to a regular blue filter
- passing only IR
- passing IR plus blue - happens to be useful for crop analysis(verify)
- passing IR plus all visible
- passing everything (including the little UV)
So for DSLRs, you probably want those in lens filter form.
Actually, there's another filter in color cameras, namely the Bayer filter that basically are per-pixel color filters, making different pixels sensitive to different colors.
Bayer filters care mostly about the visible range and tend not to filter out much outside it. The seem to tend to pass above 800nm partially and roughly equally, so IR comes out looking white anyway(verify).
Two-directional communication is typically half-duplex because a device can easily be blinded or confused by its own signal.
Consumer IR (TV remotes and such)
- Often uses a continuous pulse, not continuous sending. This helps confusion from environment IR. This also means you can pulse the LEDs with more current without destroying them.
- Carrier usually 38kHz. More generally it's somewhere in 33..40kHz or 50..60kHz, often 38kHz, 40kHz, or 36kHz
- In the case of remotes there are hundreds of variant protocols (that is, bit patterns that are specific to brands and devices)
- Universal remotes usually have a lookup table from brand-and-model to one of hundreds specific code sets that the remote supports
- and occasionally the ability to learn codes from an example
- Speed: 2.4 kbit/s to 1 Gbit/s (faster speeds primarily at close range)
- Modulation: baseband, no carrier
- Has a few different layers