How Should Light Be Illuminated in Spectrometer Designs for Liquid Analysis?

[Getterdog]

TL;DR

I need to know about light transmission through sample

Hi, I’ve built a classical design spectroscope for astronomy but with so many
cloudy days here,I like to adapt it to looking at absorption spectra of liquids.
i have an adjustable slit, and I’m going to buy a cuvette to hold the liquids. But my question is,
in a normal spectrometer, is collimated light shown through the sample,or is light focused with
a lens first,or does it even matter how it’s illuminated? Thanks john

 

[sophiecentaur]

@Andy Resnick should be able to help you with this one. If you can't get a cuvette at a decent price, I'm sure there will be cheap, clear plastic containers with one flat side that could do the job. If your main interest is in astronomy there's no need to go wild on temporary equipment.

If you are looking in the direction of astronomy, eventually, then your sensing equipment will cope easily with what you can get from a Halogen lamp (probably the best spectrum you can get as a source).

 

[Andy Resnick]

Summary:: I need to know about light transmission through sample

Hi, I’ve built a classical design spectroscope for astronomy but with so many
cloudy days here,I like to adapt it to looking at absorption spectra of liquids.
i have an adjustable slit, and I’m going to buy a cuvette to hold the liquids. But my question is,
in a normal spectrometer, is collimated light shown through the sample,or is light focused with
a lens first,or does it even matter how it’s illuminated? Thanks john

AFAIK, in most spectrometer/fluorometer designs the sample is illuminated with collimated-ish light to maximize system performance in terms of obtaining precise spectra, but the basic designs can accommodate arbitrary illumination conditions.

 

[Getterdog]

Thanks. Do you have any suggestions for any liquid that has relatively sharp absorption lines.
I’m using a 1200l/mm reflection grating so I’m looking for the best resolution.

 

[sophiecentaur]

Thanks. Do you have any suggestions for any liquid that has relatively sharp absorption lines.
I’m using a 1200l/mm reflection grating so I’m looking for the best resolution.

Sharp spectral lines only occur in gases. I don't know of any exceptions. Thing is, the Pauli Exclusion Principle forbids atoms in close proximity from sharing the same energy levels - so transitions will always be in broad bands of absorption. You could always look for gas absorption lines, locally. The gases wouldn't need to be particularly pure - you'd just need to spot the patterns of lines in amongst each other.

 

[hutchphd]

A fluorescent lamp will give a set of emission spectra above the phosphors. Any kind of noble gas (neon, or helium) at low pressure will give lines. I'll bet salt in a Bunsen burner would give a pretty good Sodium doublet. Reflected and attenuated laser light should be pretty sharp. If you can get an interference filter they can be very narrow band. You can also look at reflections if your optics is good.

 

[DrClaude]

Thing is, the Pauli Exclusion Principle forbids atoms in close proximity from sharing the same energy levels - so transitions will always be in broad bands of absorption.

?

 

[Andy Resnick]

Thanks. Do you have any suggestions for any liquid that has relatively sharp absorption lines.
I’m using a 1200l/mm reflection grating so I’m looking for the best resolution.

'Sharp' is in the eye of the beholder :) and absorption lines are going to be more difficult to obtain than emission lines. Not sure of a good fluid to suggest, sorry.

 

[sophiecentaur]

?

Pauli governs the spectra of condensed matter; you cannot get lines - only bands. (I was answering the question about liquids with narrow absorption lines)

 

[Getterdog]

One additional question, related to my set up. Is anyone aware of a formula or rule of thumb
relating the size of an extended source to a distant aperture whereby the light passed By the aperture could be considered collimated? Thanks john

 

[Andy Resnick]

One additional question, related to my set up. Is anyone aware of a formula or rule of thumb
relating the size of an extended source to a distant aperture whereby the light passed By the aperture could be considered collimated? Thanks john

The usual approach is to compute the transverse coherence length (alternatively, the coherence area) of the beam:

eqns 3.15 and 3.16: https://staff.aist.go.jp/arimoto-h/Thesis/node15.html

Slide #6: https://www.google.com/url?sa=t&rct...460/25/11076&usg=AOvVaw17aWdHGz6wshh-6tWBU_PS

 

[Getterdog]

Thanks ,in reading over the link and further info on wikipedia,the c-z theorem seems to apply only if the angular
source is less than a degree or so . So it looks like if my source is 10 mm tall as the largest dimension then I need to be about 57cm away . Hope this is right. great link.

 

[Andy Resnick]

Thanks ,in reading over the link and further info on wikipedia,the c-z theorem seems to apply only if the angular
source is less than a degree or so . So it looks like if my source is 10 mm tall as the largest dimension then I need to be about 57cm away . Hope this is right. great link.

If I understand you, the main issue is ensuring 'far field' conditions exist; that minimum distance is typically estimated using the Fresnel number and can indeed be a lot longer than expected. In that context, a lens simply reduces the far-field distance to something more manageable.