My experiments with the PLab 3.0 spectrometer and PLab oil-test kit prototypes demonstrated the requirement to build mechanically stable devices. While it can be educational to observe a working webcam+DVD spectrometer and recover colorful plots, performing real measurements requires repeatability which means stability.
I'd like to propose some design guidelines for these PLab kits:
1 - The optical path from slit to DVD to camera must be a) rigid and b) allow for precise alignment.
2 - Black paper is good for low reflections and can provide a low cost light-tight volume space, but is useless in terms of mechanical rigidity.
3 - Given the slit-camera distance, with a narrow photo-image slit, the slit material must be held fixed relative to the DVD/Camera to reduce spectral noise (for better resolution).
4 - Any "add-on", such as the oil-test-kit module, must be mechanically tied to the spectrometer "frame" (which provides the spectrometer optical path stability) -- a paper interface is not enough.
5 - The UV laser illumination of an oil sample (to be viewed by the spectrometer) must also be rigid so as to eliminate amplitude variations in the spectral signal. (Remember, if you are comparing signatures of samples where the major difference is amplitude of a broad peak, amplitude variations can easily create measurement noise that is a large fraction of the signal you want to measure.
6 - Mounting of oil-test-kit components must be rigid, relative to each other and relative to the spectrometer because any variation or vibration can cause poor measurement repeatability and sample-to-sample repeatability.
7 - Reflections. Light from the UV laser can easily reflect off the black paper and cause intensity variations. Black paper is good, but reflections from it, and from distortions in the cuvettes, can cause significant variations.
8 - At such "close range", the alignment (and alignment stability) of the spectrometer optical path through the slit becomes critical.
A measure of stability of such a system could be verified by repeat set-up, calibration and measurement of multiple samples and comparing the resulting data to calculate the measurement noise error.
Comments are welcome.
For #4-6, I'm not sure I agree. The rigidity of the attachment of the Oil Testing Kit to the spectrometer itself only has an effect on amplitude, as you point out. But we already use an attenuator based on a design you proposed -- printed on acetate:
And in our analysis, as I went through on Monday, we use macros in Spectral Workbench to equalize the height for comparison. And we haven't fully explored the differing total amount of fluorescence produced by different oils or different concentrations. Not to mention possible variability in laser intensity. (Note the links in my Monday post to our work on equalizing curve height and area)
So with several different ways that amplitude is already varied, and that we work to equalize amplitude in software, I'm just not persuaded the rigidity of the attachment itself is having a critical effect on the kinds of oil comparisons we're doing. How would you propose to test that out?
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Thanks - Lots of great feedback.
- First, remember that we are dealing with light with extremely short wavelengths -- which is why most optical experiments are mounted on granite slabs.
I found that with velcro it was impossible to maintain a fixed alignment of the parts.
I found that even the double-stick tape holding the camera was marginal for stability -- remember that if the camera angle moves, the position of the spectrum in the display moves (not the relative wavelength cal) which means the pixel data is subject to this noise -- i.e. location of peaks can change during your measurements.
The Spec is much less sensitive to the slit moving sideways but a lot more sensitive in the other direction. If the mechanics locating the slit relative to the camera is not stable, then then that will translate into measurement noise.
[ By noise, I include anything from random noise to jitter to low-frequency shifting. This means that if the housing "bends" a tiny bit between sample measurements you will no longer have a single reference point.]
Not quite: Note in the plot the difference between the V3.0 and V3.1 (ignore the 1600 pix plot for now). The effective double peak resolution bandwidth was significantly improved because of the improved mechanical stability. No, the camera, DVD and slit did not change, so it was purely a result of lower noise. If the spectrometer stops bouncing around, the rest of the system is able to perform to it's measurement limits. Going to 1600 pix was not much of an additional improvement -- largely because of other system limits which mask the effect of more pixels.
While a piece of the black paper, on a flat surface, is dimensionally quite stable ( in 2D) a U-shape or even a double-U-shape as a folded enclosure is NOT dimensionally stable (3D). Yes, the board in 3.0 was a good idea, but then the DVD was mounted on folded paper (wobbly), stuck on with velcro (still more wobbly) and then the slit was held between paper at the end of the paper cover -- which was NOT attached to the board (yet more wobbly). Not a stable, rigid design for an optical measurement system.
Yes, the black paper is good for low reflections and for blocking out ambient light -- in fact, I found the 3.0 to be generally very good at keeping ambient light noise out of the spectral signal. The problem is using the black paper for mechanical stability of the optical path. That is why I experimented with wood and glue to create a rigid optical path and then let that assembly "float" inside the black paper cover.
Given the above, when coupling to another module (i.e. the oil-test-kit) the concept is to mechanically couple the V3.1 wood platform to an oil-kit wood platform and let the black paper covers handle the ambient light and internal reflections.
The amplitude effects are important if you are expecting to compare sample spectra amplitudes and amplitude stability is important if you want to compare measurements between setups or days or measurement devices. Remember, as of yet there is still no process for users to do any form of amplitude calibration or even amplitude reference for measurements.
Yes, the attenuator is a good idea and, since it is filtering the light source, it's sensitivity is lower. From a user point of view however, there is a need to ensure and assure the user can select a single attenuation step, not a partial step, just for repeatability. A continuously variable attenuator could also work (and maybe eliminate this issue) but it is still best to ensure the setting, whatever it is, remains unchanged over the life of the measurements and sample changes during measurement comparison.
When trying to measure the oil spectra, I was simply unable to keep the paper-to-paper alignment stable enough to reduce the amplitude noise to below ~10% while holding all the parts -- and the whole assembly would shift while sitting on the desk. This is not a recipe for a stable measurement instrument even for PLab.
Yes, the issue of kit cost is real, but at least the material cost in my 3.1 prototype was very cheap -- leading me to believe it is possible to keep the kit cost very low by scaling the design of manufacturing the component parts and being creative about the design.
What "kind of oil comparisons" are more important? Any broadband spectral comparison will require a stable amplitude -- which so far, there is little control. If you compare 5 oils and you get a nice spread of broad peaks, then you know the measurement is sensitive to a change in oils. If you always did these 5 oils (and they and the system never change) then that might start to be some form of reference. But if the system is not sufficiently stable, then you don't have a reference. I think it is possible but I just don't see the component design is, as yet, sufficient to send out measurement kits where 95% of users can get it right the first time.
I do believe that PLab can, in fact, produce kits which are stable, can be calibrated and will be repeatable from user to user such that results can be rationally compared; the components are just not there yet. I may be alone in this, but I believe the future value of the kits is very dependent on the measurement stability, repeatability and ability to verify measurement specifications of the first official devices to go to users. I believe it can happen.
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Sorry, I've been coding. Some more thoughts:
In the graph I posted of the basic, upgraded, and hi-res spectra, I realize it's hard to say what's more consistent without some absolute reading to compare to -- what we can do, however is see how much movement or crushing/moving of the Spec v3's case is needed to cause wavelength shift. Note the shif in this spectrum, for which I don't know what instrument was used, but which demonstrates wavelength drift:
If we can demonstrate that a) the amount of rigidity doesn't prevent such shifts with reasonable handling of the device, and b) the shifts are large enough to substantively affect the outcome of tests we wish to do, then we should definitely work on rigidity. I haven't had trouble with wavelength shift, but perhaps I'm not pushing the limits of my spectrometer's durability as much as the average user.
For kit costs, the wood blocks you proposed are indeed cheap as materials (and well thought through!), but I'd like to push for even fewer unique parts and easier manufacturing. Any opportunity to use the same part multiple times, or to reduce the need for more steps to manufacture it (hole drilling, angle cutting, etc) is very helpful to keep the number of manufacturing steps down. I'm also not convinced we can't create a more stable paper design for the grating and slit -- assuming that we demonstrate that more rigidity is needed.
To be clear -- at the moment, we are discarding amplitude information in our procedure in the Oil Testing Kit (read more here), but I definitely agree that we should make it easy for users to enter -- as tag metadata, perhaps -- what attenuation level they were using. This would be important for any quantitative work especially. There are many reasons to more precisely -- and absolutely -- control amplitude, but at the moment we haven't staked that out as a necessity.