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Forum 2005-2010 (read only) => Hardware => Interfacing => Topic started by: darudude on Sep 07, 2009, 02:26 pm

Title: Sensing wavelength of lights
Post by: darudude on Sep 07, 2009, 02:26 pm
Hello all,

I haven't posted here for some time cause I've been busy with work, but I just thought of a great project for my Arduino and was looking to see if it was feasible.

The question is:

Is there a way to tell the intensity of a particular wavelength of light?

I've looked into photodiodes and phototransistors and haven't found something I could use. I need to be able to tell the intensity of various wavelengths of light and not just overall light intensity. I know a spectrometer would do this for me, but a good digital one is $10000, I'm looking to spend a couple bux lol

Any help would be appreciated. I'm looking for any feedback so don't hesitate to write your thoughts or ideas.
Title: Re: Sensing wavelength of lights
Post by: Reggie on Sep 07, 2009, 02:33 pm
I haven't tried it but this looks interesting:

http://www.cs.cmu.edu/~zhuxj/astro/html/spectrometer.html
Title: Re: Sensing wavelength of lights
Post by: darudude on Sep 07, 2009, 02:44 pm
Yeah I looked into it.

Couple things though. I wanted a more autonomous solution. To make this solution autonomous I would have to add a digital camera and do some software photo analysis, setting thresholds, etc. This solution would be more expensive and way too big for what I have in mind.

I was thinking of having an array of narrow wavelength photodiodes. As in one that detects 400nm, another that detects 450, another for 500, etc. Than each photodiode would only give me then intensity for that very narrow range of wavelength. However, I can't find photodiodes that have narrow wavelengths, they all seem to be made for a broad spectrum.
Title: Re: Sensing wavelength of lights
Post by: Grumpy_Mike on Sep 07, 2009, 02:56 pm
Quote
However, I can't find photodiodes that have narrow wavelengths, they all seem to be made for a broad spectrum.


That is true due to the way photo electric effect works.

How about making your own spectrometer with a CD as a reflective diffraction grating. Then use a liner CCD array to detect the wavelength. I used a liner CCD array to count coins years ago so you could use the same type of circuit.

The schematic is here.

(http://farm3.static.flickr.com/2450/3851893046_84f7e072b7.jpg)
Title: Re: Sensing wavelength of lights
Post by: Reggie on Sep 07, 2009, 03:14 pm
You could also look at optical filters to display the bands that you want. Ha, Hb, OIII, etc. and/or block the ones you don't want UV, IR etc.
Title: Re: Sensing wavelength of lights
Post by: darudude on Sep 07, 2009, 03:31 pm
Hmm... I'm not exactly sure how a CCD works, I can't seem to find much literature on it. But how does it detect wavelength? I though its more of a size detector? Like isn't it an array of capacitors that get charged when photons hit it? if you block certain surface area, teh capacitors that don't get hit by light don't charge up? How does this help detect wavelength?
Title: Re: Sensing wavelength of lights
Post by: RanTalbott on Sep 07, 2009, 03:53 pm
That's a really clever idea, Mike.

How flat is the spectral response curve on the typical CCD?  Would you need to add a compensation table to adjust for its shape?

Ran
Title: Re: Sensing wavelength of lights
Post by: Grumpy_Mike on Sep 07, 2009, 04:04 pm
Quote
I'm not exactly sure how a CCD works


It's just an array of photo capacitors.

Quote
But how does it detect wavelength?


I said:-
Quote
making your own spectrometer with a CD as a reflective diffraction grating


So the wavelength is given by liner position, a line spectra would show up as a number of separate lines. The liner CCD sensor just detects the light intensity at any one position along the length of the sensor.

Quote
I can't seem to find much literature on it.

It's a TI device or just search for liner CCD. Alternatively you could rescue one from an old scanner.

Quote
Would you need to add a compensation table to adjust for its shape?


I would have thought so.  ;)
Title: Re: Sensing wavelength of lights
Post by: oransen on Sep 08, 2009, 02:29 pm
You say:

"I'm not exactly sure how a CCD works, How does this help detect wavelength"

Imagine that the CCD is a long line of light detectors. Imagine you shine a rainbow on the CCD.  A part of the CCD will get red light, a part green light and so on. So the intensity of the different parts gives you the intensity of the different colors.

You can get your "rainbow" from a prism or, possibly, a bit of a broken CD or DVD.
Title: Re: Sensing wavelength of lights
Post by: borref on Sep 08, 2009, 02:55 pm
Quote
However, I can't find photodiodes that have narrow wavelengths, they all seem to be made for a broad spectrum.

Interesting project - I will look forward to see what you find.

One approach could be to use multiple low cost LDR's, but add colored filter caps on each one. E.g. Red, Green, Blue or whatever wavelength equivalents you would need.

I also remember seeing (could not find it again) what I recall as a low cost color sensor that would give you the relative mix of wavelengths and through this allow you to determine color. This in combination with a broad spectrum LDR (for intensity) can perhaps also be of value.

Title: Re: Sensing wavelength of lights
Post by: RanTalbott on Sep 09, 2009, 12:10 am
If memory serves,  Sparkfun sells something they call a "color sensor",  and Marlin P. Jones (mpja.com) had (has??) a surplus assembly with 3 photocells and color filters for about $5.  But it's been a few years since I saw the one at MPJA,  so it could well be sold out by now.

Ran
Title: Re: Sensing wavelength of lights
Post by: zooto68 on Sep 09, 2009, 12:13 am
Just use filters and a motorised filter wheel (as in astrophotography)
Title: Re: Sensing wavelength of lights
Post by: Grumpy_Mike on Sep 09, 2009, 11:19 am
The trouble with the colour sensor is that it does not distinguish between different wavelengths in a line spectrum. These give you the RGB components in a continuous spectrum.

With the filter wheel, you need to use narrow band interference filters if there are more than two spectral lines viable at the same time.
Title: Re: Sensing wavelength of lights
Post by: ungravitational on May 03, 2010, 02:38 pm
What a great thread! I'll try the CD option and post any results.
:)