Recordable Blu-Ray as Diffraction Gradient?

[Jason]

Has any tried out or had any luck with using Bluray-R discs in place of DVD-R discs as a diffraction gradient? From what I've been able to dig up online the spectral resolution is significantly greater. The store's closed here right now but I'm thinking about picking a couple up tomorrow to test with seeing as they're relatively inexpensive now. 

[ToF]

Greetings!

The defraction grating density of a Blu-Ray Disc is 3125 lines/mm
of a DVD 1350 lines/mm
and of a CD 625 lines/mm

BUT there are other limitations.

By calculating the angles of first few diffraction orders by using Angle α of the grating to the light beam, Grating density  and Wavelength, you get 1st order, 2nd order and 3rd order intensities.
With 3125 lines/mm you will find yourself quickly out of range for Wavelenghts useable with most avaiable CMOS or CCDs.

Maybe playing around a bit with this small and simple calculation tool at http://www.calctool.org/CALC/phys/optics/grating will show you the problem.
I by myself would limit my setup to around 1000 or maybe 1400 lines/mm for these reasons or have different gratings and/or changing angles for different wavelenghts.

Cya

ToF

[Jason Barresi]

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Vielen Dank, ToF! That information is very useful and the calculation tool is amazing! This should give me plenty to play around with over the weekend haha

J

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[Jason Barresi]

I wonder; do you know of any site that references what wavelengths that various CCD/CMOS sensors will pick up?

[ToF]

You are welcome!

By the way - if you want to build yourself an Excel-Worksheet for calculations and better comparing this is what you could need:

(simplified) Grating Equation:  [ (greek letters) in brackets ]


d=n*(lamda)=g*sin(phi) for n Element of Z

with:
(lamda) := Wavelenght in nm e.g. 650nm
g := Lattice Constant - e.g. 1000 for 1000 lines/mm
(phi) := angle of deflection in degree
n := order of diffraction maxima eg. 1 or 2 or 3...
d := optical path difference

so (lambda)=(g*sin(phi))/n  or  sin(phi)=(n*(lambda))/g   respectively  (phi)= sin⁻1{(n*(lambda))/g}


Maybe you can visualise the results in a chart and share it with the board?

So long!

ToF

[Adam Griffith]

I see what you mean, ToF.  The results are NaN for all orders when the lines per mm is too high (Blu-ray disc).  At 1350 lines per mm (DVD), however, we only get one order of diffraction which is perfect given our little spectrometer.

Thanks!

Adam

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[Jason Barresi]

This is great! I'm a soldier and computer programmer by trade with limited education in the area of physics so I'm actually learning a lot of this as I go. Being a programmer though, I am very good with math and love equations, graphs and sketches :-) 

I'm working on getting all of my stuff together and planning on posting a bunch of research notes and sketches this weekend. Currently I've been working with 4 prototype designs that I fabricated in my basement wood shop. As I'm gaining more understanding of how the spectrometer actually works and the physical/angular requirements, I think I'll be able to come up with some pretty good designs for mobile and desktop models... I'm still tossing around ideas for a portable/backpack model though.

J

On Wed, Sep 12, 2012 at 7:48 AM, ToF <thomas.j...@gmail.com> wrote:

[Thomas Fichtner]

If you use something below ~300 nm it will work - but sensing that kind of UV with a digicam? Would be a challenging task, including a band pass filter, special kinds of glass, a different RAW-interpretation and so on.
Most IR-range would be easier.

By the way - the resolution of this kind of spectrometer we use is not constant but it is okay so far - you can easily compensate for this - if necessary at all.

And most of us get second order of diffraction with it. Well at least the ones using the grating at an angle of 30, 45 or 60 degree depending on the wavelenght - but this has it own pros and cons.

2012/9/12 Adam Griffith <adamdg...@gmail.com>

[Jeffrey Warren]

Hi all -- it's so great to see so much activity on the list on spectrometry. There didn't used to be as many spectrometry folks on here -- I guess we can thank Kickstarter.

Anyhow just a few comments and additions:

Re: blu-ray -- i'm very glad to see some MATH to back up what we basically determined empirically by just testing lots of different discs :-P. I'm not as good at the theory, and like to just try lots of things out -- but it's so great that we now know *why*...

I wonder; do you know of any site that references what wavelengths that various CCD/CMOS sensors will pick up?

Jason - very good question. Some manufacturers post data, but it's usually smoothed, marketing-style data, not real calibration data. Example: http://publiclaboratory.org/notes/warren/12-12-2010/can-we-use-back-illuminated-cmos-video-cameras-uv-imaging

I think we're going to have to do our own radiometric calibration using a known light source-- maybe a 'black body' source, i.e. just a very hot glowing object. Or, a light bulb we mail around :-). Those have very easy to model spectra, and it seems like we should be able to figure out what our cameras are seeing. 

If data is very reliable between cameras of the same model (in the Kickstarter we'll be buying a batch of thousands) then we'll only really have to do this once per model. Or if light bulbs are very consistent, we can all just use one light bulb at a given distance or something. That would be nice -- keep in mind that ALL data on the SpectralWorkbench website is currently not radiometrically calibrated-- so if a given webcam just sees reds more brightly, we don't really know... yet!

Also -- the graph i linked to has a clue to one other question I saw -- UV. Look again: http://publiclaboratory.org/notes/warren/12-12-2010/can-we-use-back-illuminated-cmos-video-cameras-uv-imaging

that's for a "back-illuminated CMOS sensor" -- the kind in the iPhone 4 and many webcams. I think this means that if we use that type of sensor, and remove the glass lenses (maybe use a pinhole) -- we could get quite a bit of UV. Maybe even down to 200nm! But it's just an idea... we'll have to try it.

Finally -- as some of these ideas become more concrete, we should try to move them into research notes on the website because then we're creating more permanent documentation. It's hard for people to dig through the mail archive to find this stuff!

Jeff

 

[Jeffrey Warren]

One more thing -- there is an interesting link to a test of Blu-ray diffraction gratings here: 

http://publiclaboratory.org/notes/warren/8-27-2012/spectrometry-discussion-across-web

bottom link -- has some good photos too.

Jeff

[Jeffrey Warren]

Hey ToF - would you mind if I reposted your comments on gratings up above, as a wiki page? Lots of people are asking me on Kickstarter about what kinds of gratings are best, so I want to make a page where people can read up and also share what they find. I was going to make a page at http://publiclaboratory.org/wiki/diffraction-gratings

Jeff

[Thomas Fichtner]

You are welcome to do so.

Greetings.

[ToF]

Maybe you could include the considerations about prisms (BK 7 discussion with stonefisher)?
I do not get along with the page editors html like language very well or I would have done it by myself.

Gn8 (it's 23:10 here in Europe and tomorrow the new semester at the university begins...)

[sanjayk...@gmail.com]

On Wednesday, September 12, 2012 at 5:57:12 AM UTC+5:30, Jason wrote:
> Has any tried out or had any luck with using Bluray-R discs in place of DVD-R discs as a diffraction gradient? From what I've been able to dig up online the spectral resolution is significantly greater. The store's closed here right now but I'm thinking about picking a couple up tomorrow to test with seeing as they're relatively inexpensive now. 

Wait so did it work?? I need to know for my Physics Experiment.

[Jeffrey Warren]

Hmm, not sure, but you could post a question on https://publiclab.org/spectrometry -- also there is this page that could be helpful? https://publiclab.org/wiki/diffraction-gratings

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