|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. The more common seems to be the near/mid/far split (near/far from the visible spectrum, 380nm..740nm), like:
- Wavelength 740nm to something like 2500nm
- IR LEDs are usually somwhere the 800...1000nm range
- most relevant to optical astronomy(verify)
- 25000–1000000nm(verify), getting close to microwave region
Infrared and cameras
Broadly speaking, there are two main types of IR-related filters:
- IR-cut filters (usually internal to a camera)
- look like bluish (because they also remove a little visible red)
- cuts everything above some transition in 740...800nm
- But it's a transition, where just most is gone. IR remotes (typically in the 840..940nm range) are still 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)
For reference, our eyes see ~400nm to ~700nm, while CCD and CMOS see be ~350nm to 1000nm or so (shapes differ).
In other words, they receive a little UV, and part of near-IR, and if you remove the filter(s) from an optical camera based on one of these filters, they will see.
(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 1000 nm to ~14000nm)
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 filters that makes different pixels sensitive to different colors.
Bayer filters care mostly about the visible range and tend not to filter 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