DR. HANNEMAN'S MINERAL COLLECTION WILL BE AVAILABLE FOR SALE IN THE NEAR FUTURE
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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Tue Jul 12, 2011 3:59 pm 
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Gene, you were right as always. I lost the ruby doublet with the new grating but the range is now 380-960nm. I'm still waiting for solux bulbs in hope to get more clear line on both ends. I tried cheap chinese daylight filter but it was not enough. I solved the mystery of the unplugged thermo cooler fan. The supplier had unplugged it because it seemed to cause spectral noise. Well it does first but if you let it to run for an hour the dark signal is practically zero. There is couple of hot areas in CCD. I'm saying areas because they are not hot pixels but groups of adjacent 6-10 pixels showing higher dark signal. I guess nothing can be done in hardware.

Here is my PL setup using Thorlabs parts. It has 20mW 405nm laser, a shutter and "slotted tube" sample chamber. The outer shield of the tube is rotatable and ensures unwanted light is kept out. The shutter is there for not having to close light source during dark signal measurements. It also makes it possible to perform the whole analysis safely without even seeing the laser light. The probe is "almost bare fiber". I have wrapped 0.2mm thick plumber's teflon tape on the front of it. The fiber end sees a bit more light keeping the integration times smaller.

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This is normalized 405nm PL spectrum of natural unheated red spinel vs. russian flux red spinel. As expected the highest peak of the synthetic stone has shifted couple of nanometers because of the heat. Not bad result for an 199$ "toy".

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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Tue Jul 12, 2011 4:29 pm 
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mikko wrote:
I lost the ruby doublet with the new grating but the range is now 380-960nm.

Yes, all the spectrometers covering that sort of range lose the ruby doublet.

mikko wrote:
Here is my PL setup using Thorlabs parts.

Nice spinel spectrum. But...

I know well from personal experience that PL measurements should be made with the excitation source beam at right angle to the spec's fiber optic. For example, if you study closely the SSEF document that you linked to, you'll see on page three that they do PL using a ring light whose beam axes will be at right angles to the spec fiber. This arrangement has some advantages over the direct illumination in your setup.

For PL, you do not want a spectrum of the source beam, only the fluorescence that results. Most of the source beam propagates along one direction. In contrast, the fluorescence that results is emitted in every direction. So by not viewing along the primary direction of the source beam, the excitation light doesn't saturate the spec's CCD pixels and bleed over into other pixels. In the right-angle measurement, because of scattering, the excitation spectrum isn't completely eliminated, but it is greatly reduced.

Another reason to use the right-angle geometry is to reduce (and nearly eliminate) unwanted fluorescence from fingerprint oil on the surface of the stone that gets into the spec. In your arrangement, the spec fiber has a direct view of the gem surfaces crossed by the excitation beam... and thus a direct view of fluorescence occurring at these surfaces. With a relatively strong gem fluorescence, you may not even notice this additional fluorescence. But as you try to measure weaker fluorescence, many narrow lines will begin to appear throughout the entire spectrum (even at wavelengths shorter than the excitation source) that are caused by oil on the surface of the gem.


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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Tue Jul 12, 2011 5:01 pm 
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Brian wrote:
I know well from personal experience that PL measurements should be made with the excitation source beam at right angle to the spec's fiber optic.


Heh... :D


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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Tue Jul 12, 2011 5:10 pm 
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Another reason to use the right-angle geometry is to reduce (and nearly eliminate) unwanted fluorescence from fingerprint oil on the surface of the stone that gets into the spec.


Yes, I noticed that too but was unsure why it happened. That's why I asked about teflon fluorescence. It was fingerprint oil, not teflon.

Okay, I will need to modify the PL source to hit the sample from 90 degrees. I'm waiting for 365nm led shipment too and was planning to make a "ring" inside the tube. That will fix the problem for UV fluorescence measurements.

Raman probes have high pass edge filters to remove excitation wavelength. That could help with the CCD saturation in PL spectroscopy too, right?

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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Tue Jul 12, 2011 9:17 pm 
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Yes mikko, now you know that straight-through PL spectra is very, very bad. Even just the thought of how I learned this lesson makes Tim giggle. :wink:

Speaking of LEDs and high-pass filters... The long-wavelength tail of a UV LED can extend over quite a long range and even exhibit further structure (if you look carefully enough). So a high-pass filter is very useful for cutting off that long-wavelength tail. Can get a bit pricey, though.


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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Thu Jul 14, 2011 1:19 pm 
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Why would this GIA PL setup be any better than straight-through method? They are using reflectance probe.

I found the patch cable supplied with the unit attenuates almost all the light > 800nm. If anybody has an extra SMA cable good enough in 400-950nm range please sent me pm.

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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Fri Jul 15, 2011 1:20 pm 
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Because the excitation energy is going in one direction and the fluorescence energy is returned.
There IS some returned reflected excitation just as there would be in the right angle method caused by things in the stone which scatter energy.

But even in analytical fluorescence spectroscopy where transparent liquids are examined in quartz cuvettes the instruments often have a reflected or Face Front mode.


Last edited by G4Lab on Fri Jul 15, 2011 5:23 pm, edited 1 time in total.

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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Fri Jul 15, 2011 4:26 pm 
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Gene, you are spot on.


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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Sat Jul 16, 2011 5:29 am 
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mikko wrote:
I'm waiting for 365nm led shipment too


which leds did you purchased?

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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Sat Jul 16, 2011 7:49 am 
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These

They were practically free so I'm expecting they have long tail towards the blue. I'll take spectra as soon I get them.

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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Sat Jul 23, 2011 11:00 am 
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271nm / 12nm. Sounds quite near but these are unusable. As distribution is not gaussian
these statistics are deceptive. Most of the intensity is lost if 365nm filter is used.
Is this uneven distribution typical to all UV leds?

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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Sat Jul 23, 2011 11:46 am 
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hey Mikko

mikko wrote:
271nm / 12nm.


a Typo that made me jump till i saw the graph below.. :lol:

Quote:
Is this uneven distribution typical to all UV leds?


dunno, maybe has something to do with leds cheapness...... :wink:

ciao
albé

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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Sat Jul 23, 2011 1:46 pm 
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The emission wavelength of LEDs depends on quite a number of things and is one of the reasons they are not suitable to use as wavelength standards.

In contrast to atomic emissions from Sodium or Mercury lamps which are stable many decimal points to the right of what we need. If you discover THOSE things varying then it means there is a black hole drifting nearby distorting space and time. :shock: :shock:

For the same reason , where diode lasers have been substituted for Helium Neon Gas lasers the wavelength cannot be used as a standard unless extreme measures are taken which cost much much more than a HeNe laser does.

When a manufacturer would want to make an LED 365 source he would order the LEDs "binned" by the manufacturer. That means they would be sorted by peak wavelength and all the ones that were peaked on 365 would go into a bin which would get delivered on that order. The ones that emit at 371nm would end up sold to an electronic surplus place and eventually end up on ebay.

It still applies that the shorter the emission wavelength the more expensive LEDs are.


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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Sat Jul 23, 2011 5:35 pm 
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mikko wrote:
Most of the intensity is lost if 365nm filter is used.
Is this uneven distribution typical to all UV leds?


Very typical. Always a long extension in the long-wavelength tail.

And the behavior of the filter is also very typical.


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 Post subject: Re: Spectrometer DIY Kit
PostPosted: Sat Jul 23, 2011 8:38 pm 
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And UV interference filters have crappier performance than VIS. Less transmittance even at the peak.


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