Difference between revisions of "Infrared notes"

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While infrared often refers to near-infrared, it's actually a fairly huge range, split into ranges in a few different ways[http://en.wikipedia.org/wiki/Infrared#Different_regions_in_the_infrared].  
 
While infrared often refers to near-infrared, it's actually a fairly huge range, split into ranges in a few different ways[http://en.wikipedia.org/wiki/Infrared#Different_regions_in_the_infrared].  
The more common seems to be the near/mid/far split {{comment|(near/far from the visible spectrum, 380nm..740nm)}}, like:
+
 
 +
One common seems to be the near/mid/far split {{comment|(near/far from the visible spectrum, 380nm..740nm)}}, like:
  
 
* Near-Infrared
 
* Near-Infrared
** Wavelength 740nm to something like 2500nm
+
** Wavelength approx 740nm to approx  2500nm
** IR LEDs are usually somwhere the 800...1000nm range
+
** IR LEDs are usually near-IR, typically somewhere within 700...1000nm
 
** most relevant to optical astronomy{{verify}}
 
** most relevant to optical astronomy{{verify}}
  
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==Infrared and cameras==
 
==Infrared and cameras==
  
Broadly speaking, there are two main types of IR-related filters:
+
Infrared around optical cameras mostly deals with the fact that camera sensors can see some amount into near infrared.
* IR-cut filters
+
** present in many cameras, because most sensors receive some near-IR.
+
** Good ones make sure the contribution is negligible, but they vary in how much they cut. Simple/cheap setups may not yet block much in the 740...800nm region, which is why you can use some cameras to see what your IR remotes are doing (LEDs typically in the 840..940nm range).
+
  
* IR-pass, visible-cut filters
+
For reference, our eyes see ~700nm to 400nm,  
** which often look black
+
while CCD and CMOS might see perhaps 1000nm to ~350nm (sensitivity shapes differ).  
** Often start passing at 750..850nm
+
+
  
http://dpfwiw.com/ir.htm#ir_filters
+
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
  
For unfiltered and not-so-strictly-filtered cameras,  
+
 
you can get quick-and-dirty IR by adding an IR-pass,-visible-cut filter:
+
 
The result is that only the fairly near-IR light gets through.
+
Broadly speaking, there are two main things we might call "infrared filters"
This is what a lot of the see-through IR hacks do.
+
 
May be quite grainy because there's not much energy getting through, so can be quite grainy.
+
* '''IR-cut filters'''
A good source of IR can help. Lightbulbs are decent (actually give off about as much IR as visible light), CFLs not so much.
+
:: 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 [https://en.wikipedia.org/wiki/Bayer_filter 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}}.
 +
 
 +
 
 +
<!--
 +
 
 +
For some DIY, like FTIR projects,
 +
 
 +
note that there are ready-made solutions, such as the Raspberry's NoIR camera
 +
 
 +
 
 +
If you want to do this yourself,
 +
you may want to replace a webcam's IR-cut with a IR-pass filter,
 +
and probably with thin plastic rather than glass.
 +
 
 +
[[File:IR-cut from webcam.jpg|200px|thumb|right|two IR-cut filters from webcams. Looking through them looks blue, they reflect red.]]
 +
While in DSLRs the IR-cut is typically one of a few layers mounted on top of the sensor (so that not every lens has to have it),
 +
in webcams, the IR-cut filter may well be on the back of the lens assembly
 +
 
 +
 
 +
 
 +
You can get IR-pass filters as foil.
 +
 
 +
Apparently floppy disks are a halfway decent DIY alternative,  
 +
though they block a bunch of eveything (including IR) and their transmission band seems fairly narrow{{verify}}).
 +
 
 +
 
 +
To have a camera see mostly IR, you want to remove IR-cut,
 +
and possibly add something that cuts optical but passes
 +
 
 +
 
 +
 
 +
 
 +
 
 +
Indoor, a good source of IR can help.
 +
Lightbulbs are decent (actually give off about as much IR as visible light), CFLs not so much.
 
Candles are also great IR sources.  
 
Candles are also great IR sources.  
There are also cosntructions with a lot of IR LEDs.  
+
There are also constructions with a lot of IR LEDs.  
  
  
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In general, you can get better IR sensitivity by ''also'' taking out the IR-cut filter.
 
In general, you can get better IR sensitivity by ''also'' taking out the IR-cut filter.
 
Depending on the camera, this may be a pretty delicate operation, and tends to be fairly permanent.
 
Depending on the camera, this may be a pretty delicate operation, and tends to be fairly permanent.
 
  
  
 
Hacks:
 
Hacks:
No, it won't be as good as a heat camera (one of those false-color things).
 
  
 
* In webcams the IR-cut tends to be a glass in the screwable lense.
 
* In webcams the IR-cut tends to be a glass in the screwable lense.
  
 
* For DSLRs the IR-cut this is typically a layer on top of the sensor
 
* For DSLRs the IR-cut this is typically a layer on top of the sensor
 +
-->
  
 +
See also:
 +
* http://dpfwiw.com/ir.htm#ir_filters
  
 +
* https://kolarivision.com/articles/choosing-a-filter/
  
 
==Communication==
 
==Communication==

Latest revision as of 16:39, 9 October 2021

This is for beginners and very much by a beginner.

It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book.


Some basics and reference: Volts, amps, energy, power · Ground · batteries · resistors · changing voltage · transistors · fuses · diodes · varistors · capacitors · inductors · transformers · baluns · amplifier notes · frequency generation · skin effect


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IO: Input and output pins · wired local IO · wired local-ish IO · ·  Various wireless · 802.11 (WiFi) · cell phone

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Noise stuff: Stray signals and noise · sound-related noise names · electronic non-coupled noise names · electronic coupled noise · ground loop · strategies to avoid coupled noise · Sampling, reproduction, and transmission distortions


Audio notes: See avnotes

Microcontroller and computer platforms Arduino and AVR notes · ESP series notes · STM32 series notes · · · ·


Less sorted: Ground · device voltage and impedance, audio and otherwise · electricity and humans · power supply considerations · Common terms, useful basics, soldering · PLL · pulse modulation · signal reflection · resource metering · SDR · Project boxes · vacuum tubes · Unsorted stuff

See also Category:Electronics.


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[1].

One common seems to be the near/mid/far split (near/far from the visible spectrum, 380nm..740nm), like:

  • Near-Infrared
    • Wavelength approx 740nm to approx 2500nm
    • IR LEDs are usually near-IR, typically somewhere within 700...1000nm
    • most relevant to optical astronomy(verify)
  • Mid-Infrared
    • 2500–25000nm
  • Far-Infrared
    • 25000–1000000nm(verify), getting close to microwave region


See also:


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).


See also:

Communication

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

http://en.wikipedia.org/wiki/Consumer_IR


IrDA

  • Speed: 2.4 kbit/s to 1 Gbit/s (faster speeds primarily at close range)
  • Modulation: baseband, no carrier
  • Has a few different layers

http://en.wikipedia.org/wiki/Infrared_Data_Association