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Sulaiman
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My head hurts 
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Folowing Twospoons coment above, it seems that a diffraction grating is only really useful over one octave of wavelengths
A simple version of the diffraction equation is sin( Θ ) = m.λ/d
sin(angle of diffraction) = m.(wavelength)/(distance between lines)
m = mode = 0,1,2,3.. etc.
so for example, (mode 1 900nm) will be at the same angle as (mode 2 450nm) also (mode 3 300nm) etc..
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Ubya
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Quote: Originally posted by Sulaiman  |
My head hurts
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Folowing Twospoons coment above, it seems that a diffraction grating is only really useful over one octave of wavelengths
A simple version of the diffraction equation is sin( Θ ) = m.λ/d
sin(angle of diffraction) = m.(wavelength)/(distance between lines)
m = mode = 0,1,2,3.. etc.
so for example, (mode 1 900nm) will be at the same angle as (mode 2 450nm) also (mode 3 300nm) etc.. |
Yea overlapping patterns are a common problem, real spectrophotometers have filters after the diffraction grating, so if you want to see 900nm in the
first diffraction order, you put a filter that lets pass only light over 800nm for example, this way you block the other orders. Or better they use
spacial diffraction gratings that have triangular grooves(blazed grating). But this is relevant only for precise measurings, the second order of
diffraction has pretty weak intensity, before using the DVD I tried to just tilt the webcam and use the second order to get a bigger resolving angle
between the lines, but I could barely see any distinguishable peaks over the background noise. So yea if we build a real spectrophotometer of course
we need more things, it's not a coincidence their high price.
[Edited on 6-11-2019 by Ubya]
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Sulaiman
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before I get too involved in yet another significant project i'd like some advice;
IF I made a spectrophotometer for visible light (e.g. 380nm to 760 nm),
with 1nm resolution and 30dB to 40dB SNR,
would it be useful for my general chemistry hobby ?
e.g. could such an instrument analyse the products of sugar:yeast fermentation ?
EDIT:________________________________________________________________
D'oh ! ... the answers are obvious (after a little more googling)
only chemicals/pigments/dyes that can be seen by eye are going to be 'seen' by a VIS spectroscope,
so the most I could do is Beer–Lambert law type concentration measurements ... boring.
It seems that parts of the UV spectrum have some use in chemical spectroscopy,
but all of the interesting stuff is in th IR (micrometers) range
[Edited on 8-11-2019 by Sulaiman]
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Twospoons
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I see what you mean. So the answer there is a monochromator using a ZnSe prism for dispersion, and hacking the sensor out of a PIR ( they work out to
10um or more)
Helicopter: "helico" -> spiral, "pter" -> with wings
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Sulaiman
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I have more than enough ongoing major projects,
so I'll give diy spectrometry a miss.
I'll probably just use the grating for amusement,
and possibly for a little astronomy.
At least I now see why a couple of members were stimulated by the Raman spectrometer thread.
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phlogiston
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Another application may be atomic emission spectroscopy. Spray a sample into a flame, or excite it in some other way (eg. sparking between metals, low
pressure gas with high voltage)
It would be interesting to see to what extent you can do elemental analysis with a simple setup like this.
Being able to do qualitative analysis (is element X present or not?) would be pretty useful.
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Tdep
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Ok I've been convinced that this is cool. I'v invested $12 total to get a webcam and a 1000 line diffraction grating, and we'll see what happens when
they arrive
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wg48temp9
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I think to do any analysis the system needs to be developed to the point that lines are visible. From a brief play with the software it does not have
any calibration function and the zero level is not zero. I suspect that's due to the image not being black clamped meaning black (zero signal) is not
at a specific level.
I am wg48 but not on my usual pc hence the temp handle.
Thank goodness for Fleming and the fungi.
Old codger' lives matters, wear a mask and help save them.
Be aware of demagoguery, keep your frontal lobes fully engaged.
I don't know who invented mRNA vaccines but they should get a fancy medal and I hope they made a shed load of money from it.
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Ubya
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| Quote: Originally posted by wg48temp9 | | I think to do any analysis the system needs to be developed to the point that lines are visible. From a brief play with the software it does not have
any calibration function and the zero level is not zero. I suspect that's due to the image not being black clamped meaning black (zero signal) is not
at a specific level. |
Yeah the intensity scale it's not fixed, it varies with the maximum peak, even pure darkness won't show a flat line as the sensor noise will just be
amplified in the scale.
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wg48temp9
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| Quote: Originally posted by Ubya |
Yeah the intensity scale it's not fixed, it varies with the maximum peak, even pure darkness won't show a flat line as the sensor noise will just be
amplified in the scale. |
Yes exposure/integration time, video gain and color balance need to be fixed on the webcam and not set to auto but not all webcams have the option to
switch off the auto control of them.
The software apparently integrates many frames from the webcam and may have its own auto functions for some reason..
Assuming the auto functions can be turned off, a black area could be added to one end of the spectrum so the software could then adjust the offset of
the whole image so that that area is zero in the spectrum.
I do not recall if the software is open source or not. But if we were going to that much trouble it would probably be best to use a camera board for a
arduino to replace the webcam.
In a thread about GC, free software was mentioned but I don't recall name. and can not find the thread. That software may be a more sophisticated
alternative.
Found it. Its called Openchrom see https://lablicate.com/platform/openchrom
[Edited on 11/14/2019 by wg48temp9]
I am wg48 but not on my usual pc hence the temp handle.
Thank goodness for Fleming and the fungi.
Old codger' lives matters, wear a mask and help save them.
Be aware of demagoguery, keep your frontal lobes fully engaged.
I don't know who invented mRNA vaccines but they should get a fancy medal and I hope they made a shed load of money from it.
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Sulaiman
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From what i read it looks like that software is for analysing various file formats,
not for live camera interface/control, display and interpretation.
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Ubya
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| Quote: Originally posted by Sulaiman | From what i read it looks like that software is for analysing various file formats,
not for live camera interface/control, display and interpretation. |
the theremino software should be able to export the spectrum data (not just the image as the few i posted here), but i don't know if the formats are
compatible
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wg48temp9
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| Quote: Originally posted by Sulaiman | From what i read it looks like that software is for analysing various file formats,
not for live camera interface/control, display and interpretation. |
Yes that's correct. It takes a series of values stored in a file and processes it and can then display as various types of spectrums (graphs), compare
to references, perform calibrations and area under peaks and much more.
I should add I down loaded the software from the link I gave. Its huge almost 1/2 Gbyte but I could not get it to install due to problems with the
length of paths of some files (maybe a problems on this PC) and problems with my java.
On a previous PC I was able to load a version of the software and was able play with it.
If I remember correctly there are plugins that are available that allows it to be used in real time but I doubt there is one for a webcam.
Apparently there are also other open source versions of this type of software that appeared to be free but I have not confirm that because I haven't
used them.
I am wg48 but not on my usual pc hence the temp handle.
Thank goodness for Fleming and the fungi.
Old codger' lives matters, wear a mask and help save them.
Be aware of demagoguery, keep your frontal lobes fully engaged.
I don't know who invented mRNA vaccines but they should get a fancy medal and I hope they made a shed load of money from it.
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Ubya
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from this spectrum i noticed that the near IR lines were much more resolved vs the lines on the left. at first i thought it was a lens problem, but
then i tried to rotate the webcam so i would have the lines shifted on the right side of the sensor/grating.
i guessed right.
apparently the dvd diffraction grating is not uniform, the resolution is better on the right of the viewing field. now i can get a bit better
resolution but i can't get a full spectrum.
another thing i did i tried to measure the transmittance spectrum of a dilute copper acetate solution
i used an incandescent tube as a light source, i can't recognize any specific lines, just a general absorbtion in the red/near infrared spectrum
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G-Coupled
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Really interesting stuff - wouldn't different CD/DVDs have different grating patterns on them?
Have you tried using different discs?
Maybe burning a CD/DVD with a particular bit pattern on it (0,1,0,1 etc.) provide a more predictable, structured pattern?
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Ubya
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| Quote: Originally posted by G-Coupled | Really interesting stuff - wouldn't different CD/DVDs have different grating patterns on them?
Have you tried using different discs?
Maybe burning a CD/DVD with a particular bit pattern on it (0,1,0,1 etc.) provide a more predictable, structured pattern? |
honestly i didn't try that, i used virgin DVDs and CDs. IMO they are good only if you don't have other options, if you need to find and burn a
particular bit pattern you are better just buying a diffraction grating on ebay, even the plastic cheap ones are better than the better tweaked
diffraction gratings made from DVDs or CDs.
diffraction gratings from discs have an intrinsic curvature, i always used the outer edge of the disc where the radius is biggest, but still, you
could see the curved spectral lines.
for christmas my girlfriend gave me a diffraction grating as a gift (10x30cm PET diffraction grating 1000 lines/mm for like $6.90), and i used right
now less than 1x1cm, but the results are clearly visible.
 
on the left my best cold CFL spectrum using a dvd grating, on the right the first spectrum made with my new grating. as you can see the lines are much
thinner, and some bands are more resolved
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Twospoons
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You getting a diffraction grating for christmas reminds me of my poor mum going to radioshack with my list in hand when I was a kid. If not for the
helpful sales people she would have been completely lost.
Its an interesting comparison though, between the DVD and the grating.
The grating is clearly better, but the DVD isn't actually that bad really.
Helicopter: "helico" -> spiral, "pter" -> with wings
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Ubya
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| Quote: Originally posted by Twospoons |
You getting a diffraction grating for christmas reminds me of my poor mum going to radioshack with my list in hand when I was a kid. If not for the
helpful sales people she would have been completely lost.
Its an interesting comparison though, between the DVD and the grating.
The grating is clearly better, but the DVD isn't actually that bad really. |
it's been years since i got a "normal" gift  . for my graduation my collegues
gave me a 60cm chromatography column, i still need to try it.
anyway, yea the dvd is not bad, but it bugged me a lot that the resolution would change so much going to the outer edge of the disk, you can still
work with it though, you just need to cut your spectrum in multiple pieces and register each piece at the outer most edge of the grating (remembering
to calibrate each time), it can be done, but honestly i prefer to spend $7 and have pretty much a life supply of diffraction gratings (for this
spectrophotometer at least).
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Texium
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Thread Moved
29-11-2023 at 12:04 |
Ubya
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well it's been a while!
i've been playing on and off with my spectrophotometer, and i recently got some way better results by changing the webcam lens.
if you can find a webcam that already has a 12mm lens holder, great, otherwise you'll have to do like me and come up with some janky adapter.
From this
to this
650nm red laser
Hot CFL
UV Blacklight (fluorescent tube type)
UVC mercury lamp (very weak)
the peaks are much more resolved and the sensitivity is quite a bit higher. with the older lens I couldn't distinguish at all from the background
noise the mercury peaks of my UVC lamp, now I can at least see some.
while I'm very pleased there are still a few things I need to work on. the focusing is odd, I can't get the whole spectrum focused at the same time.
It's not a professional lens, but I don't think this is chromatic aberration from the lens design.
I also played a bit with the webcam and grating positions/orientations. this is what i ended up with. probably the easiest configuration, but it has
its issues. I could try to move away the grating to get more of the spectrum in the view field.
I'm also screwing around and learning with random lenses at home and a simulator online (https://phydemo.app/ray-optics/). The resolution of the spectrophotometer is also influenced by the number of illuminated slits in the grating, so
I want to add a collimator lens to illuminate a width equivalent to the sensor lens and get the best out of this simple setup.
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