From: pcdhSpamMeSenseless@electrooptical.net
Michal Wlodarczyk wrote:
> On Thu, 8 Jan 2026 00:55:40 -0000 (UTC), Phil Hobbs wrote:
>
>> Michal Wlodarczyk wrote:
>>> Hello,
>>>
>>> I am considering a hobby-level project to build a simple and low-cost
>>> open-path two (or three) wavelength absorbance meter for atmospheric
>>> measurements targeting nitrogen oxides (NOx), which exhibit
>>> yellow/brown color, lying in the visible range.
>>>
>>> The basic idea is an active system with a transmitter/receiver unit on
>>> one side and a retroreflector placed approximately 10–100 m away. I am
>>> currently considering two wavelengths, one 405 nm and second not
>>> critical but maybe 530 nm, to allow differential absorbance
>>> measurements.
>>>
>>> For the receiver, I am thinking of using a PIN photodiode placed at the
>>> focus of a relatively large lens to increase beam diameter and make
>>> system less prone to air instability, het haze etc.
>>>
>>> The main uncertainty concerns the transmitter side. One idea is to use
>>> high-power LEDs and couple the emitted light into the same optical axis
>>> as the by reflecting it off plain glass plates (microscope slides?)
>>> placed in front of the photodiode. LEDs are inexpensive and can provide
>>> substantial optical power, so emission strength is not the primary
>>> concern. Probably LEDs are far better than laser diodes in this setup -
>>> high power, no speckles and paradoxically not being point source.
>>>
>>> However, I am worried about stray reflections, surface scratches, dust
>>> on the glass plates and lens and general backscatter potentially
>>> overwhelming the weak signal returning from the distant retroreflector.
>>> A polarizer in front of the photodiode might help, but I am unsure
>>> whether this would be sufficient in practice.
>>> In principle, at these path lengths it might be possible to
>>> discriminate the returning signal based on time-of-flight, but I am not
>>> sure whether this is practically achievable with LEDs (or lasers) and a
>>> simple photodiode-based receiver.
>>>
>>> I would appreciate any thoughts, references, or alternative optical
>>> layouts.
>>>
>>> Best regards,
>>> Michal Wlodarczyk
>>>
>> Fun project.
>>
>> You really have to do some calculations to see if you have any chance of
>> making it work, though. I expect that a much shorter path that you can
>> shield from stray light would be easier.
>>
>> If you go to the hitran database (search for that), you can get
>> calculated absorption spectra as a function of species mix, background
>> gas, temperature, and pressure. It’s pretty slick.
>>
>> There are two main types of measurement for this: LEDs + fancy
>> interference filters, where you find a spectral region with a
>> concentration of absorption lines; or tunable diode lasers, where you
>> pick one line and dither back and forth across it.
>>
>> Cheers
>>
>> Phil Hobbs
>
> To be honest, the absorption theory itself is the most trivial and
> probably least interesting part here. The gas I am after is yellow, has a
> strong absorption maximum around 400 nm, and like any respectable gas has
> absorption bands whose widths are matched rather to incascendent bulb than
> single-mode laser. I even do not see a strong need for any filters at this
> point.
> Absorbance probably would be in range of 1-5%.
>
> Sure, one could argue about the detailed structure of those absorption
> bands
> and do something more sophisticated, like scanning a tunable laser (as is
> done for iodine molecules, for example). Maybe. But that is probably a
> different project. Maybe better...
>
> In principle, the same approach could also be used to measure ozone, which
> absorbs red light.
>
> Yes, at very low concentrations some things might start to break down, but
> that is
> part of what I would like to measure rather than assume.
>
> What really interests me is whether absorbance can be measured over a very
> long open path using a "single-ended" instrument, with only a
> retroreflector on the far side, and in potentially rough air.
>
> One can hope that if the light comes back along essentially the same path
> as it left, then most disturbances affect both wavelengths in a similar
> way. If the two channels are strictly coaxial and measured very quickly
> one after the other, a relative (differential) absorbance measurement
> might actually be quite accurate.
>
> The output signal I have in mind is simply the difference in absorbance
> between the two channels, possibly high-pass filtered with a time constant
> of minutes, to emphasize moderately slow changes (for example some cars
> passing through the beam).
>
> What I cannot really find is any simple (or serious!) analysis of the case
> where spectrophotometry is done with a retroreflector rather than a
> separate receiver. Or in situation, when absorbing medium is always
> present, which makes a true reference channel hard to make.
>
> Cheers,
> Michal Wlodarczyk
>
The absorbance of small molecules is concentrated in many narrow lines. At
low concentrations, the broadband absorption is linear in the
concentration, but at higher concentrations the lines start to saturate
—there’s no more light to absorb in the middle of the stronger lines.
The absorption thus becomes much less sensitive to the concentration, since
only the weak lines and the wings of the strong ones contribute. If the
average absorbance is getting up into the percents, it’s likely that this
is happening.
Hitran is your friend if you want an accurate result, but of course it’s
your project.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
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