Design and construction of scintillometer for evapotranspiration

[c_mitra]

You can well detect it with a simple instrument when looking to the sky. To measure the absorption for path length of a few meters, you need tuned mono-mode lasers.

Most commercial NIR spectrophotometers use a path length 10-20cm and use a simple source of IR (a hot nichrome wire or a SiC rod) with a simple grating monochromator. The detectors vary but all can detect both CO2 and H2O in air to a decent accuracy (better than 1% accuracy). The few I have seen use a chopper stabilized low noise amplifier to get a decent signal.

Around 1.4um the absorption is quite strong and and the transmittance is low and the attenuation is very high. This is very interesting for astronomers but I do not have exact details.

 

[adwnis123]

Most commercial NIR spectrophotometers use a path length 10-20cm and use a simple source of IR (a hot nichrome wire or a SiC rod) with a simple grating monochromator. The detectors vary but all can detect both CO2 and H2O in air to a decent accuracy (better than 1% accuracy). The few I have seen use a chopper stabilized low noise amplifier to get a decent signal.

Around 1.4um the absorption is quite strong and and the transmittance is low and the attenuation is very high. This is very interesting for astronomers but I do not have exact details.

Thank you, I am looking on it.

Here:

https://tel.archives-ouvertes.fr/tel-01285049/document

on page 26 it says:

For example, when excited with a 355 nm radiation, the nitrogen molecule emits light at 387 nm as a result of the
Raman scattering while the water vapor emits at 407 nm [Whiteman et al. 2011].

Supposing I find a laser emiting 355 nm, the problem is how do I sense 407 nm on a phototransistor(?) or any other sensor(?)

Thank you...

 

[BradtheRad]

Consider putting a filter in front of the photodetector.

Website listing a narrowband 405 nm filter. $95.

https://www.dynasil.com/catalog/optical-filters/405-nm-bandpass-filter-10-nm-fwhm-25-0-mm-dia/

Or if you're very lucky you can use an inexpensive color filter. 407 nm is at the border between visible and ultraviolet. The sort of thing you might find at a camera shop, or with the help of a friendly photographer, or club.

Suppose you have a dark room, and you send only one wavelength? Then you might detect 407nm with an easier method, with less expensive componsnts. Such as a wider band filter. Perhaps a simple blue-ultraviolet filter.

 

[c_mitra]

Supposing I find a laser emiting 355 nm, the problem is how do I sense 407 nm on a phototransistor(?) or any other sensor(?)...

For Raman scattering, the excitation wavelength is not at all critical. For example, you can use any UV laser to study Raman scattering. The scattered light will have a longer wavelength. What you need is an excellent filter to remove the excitation frequency so that you can see the scattered wavelength.

The problem is that the Raman scattering cross section is normally small (often much smaller than the absorption cross section) and you must be careful so that that you can detect small shifts in presence of a large number of unshifted photons.

 

[c_mitra]

The scattered light will have a longer wavelength.

To be more specific, the frequency difference between the incident light and the Raman scattered light will remain the same (that actually corresponds to vibrational energy). Raman scattering can take place at all wavelengths of incident light but the proportion of Raman scattered light gets lesser as the wavelength increases. That is the reason commercial Raman spectrophotometers use high intensity UV lasers for excitations.

 

[adwnis123]

From a search in google, 355nm laser are a bit expensive for me. I searched on ebay, I found this:

**broken link removed**

and this:

https://www.ebay.com/itm/405nm-500m...367592?hash=item4af35a6568:g:9V8AAOSwLwBaeUOX

and laser pointers 5mW 405 nm.

Can I use something of the above.
Ok, for 405 nm laser emitting, what scattered wavelength should I wait to get on the receiver, in order to decide what filter (nm) should I use?

Thank you..

 

[c_mitra]

when excited with a 355 nm radiation, the nitrogen molecule emits light at 387 nm as a result of the Raman scattering...

The calculation is simple: The vibrational energy corresponds to (1/387nm -1/355nm); this is the energy needed to excite the nitrogen molecule.

If the excitation wavelength is 405, the scattered wavelength can be calculated as: (1/x -1/405)=1/387-1/355; OR 1/x=1/405+1/387-1/355=1/447.

You see that the scatter shift is not equal on the wavelength scale but is same on the frequency scale.

 

[adwnis123]

The calculation is simple: The vibrational energy corresponds to (1/387nm -1/355nm); this is the energy needed to excite the nitrogen molecule.

If the excitation wavelength is 405, the scattered wavelength can be calculated as: (1/x -1/405)=1/387-1/355; OR 1/x=1/405+1/387-1/355=1/447.

You see that the scatter shift is not equal on the wavelength scale but is same on the frequency scale.

So, I send 405nm wavelength laser, I get 447nm in the phototransistor (with the bandpass filter in front to sense the light at the certain wavelength), and I calculate the time?

I focus on water vapour (H₂O) by the way.

Thank you...

 

[adwnis123]

Also, If I want to check for other elements like Nitrogen? What wavelength should I get? with the same laser emitting at 405 nm...

The laser light I should send is "raw" laser light and measure the reflected beam's wavelength or should I send a coded message like: 10001101 and measure the returned message's wavelegth?

Thank you

 

[c_mitra]

The laser light I should send is "raw" laser light and measure the reflected beam's wavelength or should I send a coded message like: 10001101 and measure the returned message's wavelegth?

The basic principle is simple.

To use the stated method, you need to focus on one of the strong absorption band. N2 does not absorb in the IR but there may be Raman scattering possible (you need to check that).

You should make distinction between fluorescence and Raman scattering.

It may be better to open a new thread with a new description of the problem.

 

[adwnis123]

The calculation is simple: The vibrational energy corresponds to (1/387nm -1/355nm); this is the energy needed to excite the nitrogen molecule.

If the excitation wavelength is 405, the scattered wavelength can be calculated as: (1/x -1/405)=1/387-1/355; OR 1/x=1/405+1/387-1/355=1/447.

You see that the scatter shift is not equal on the wavelength scale but is same on the frequency scale.

Ok, supposing I measure the scattered wavelength, then I calculate the time duration between when I send the pulse and when I receive it with the different wavelength?