Light Spectrometer from a "battery powered" head inspection lamp

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At the end of my light sensing instructable where I played with the measurement of reducing sun light levels as well as making it into a laser game , I mentioned that I wanted to have a go at making a full spectrometer. This instructable is a step closer.

In the instructable I said I wanted to add a diode array, unfortunately I have not been able to get hold of one, so this will be a scanning spectrometer.

Diode array spectrometers

These work generally by having no moving parts. The light passes through the sample and is split into its separate spectral components using a prism or similar device. The separate wavelengths then fall onto the detector where each absorbance value is measured at the same time.

Scanning spectrometers

These work slightly differently in that the light is first split by the prism, it the passes through a narrow slit which helps to reduce the number of wavelengths of light that can fall on the sensor after it has passed through the sample.

In this type of system there is a single detector that has to measure each value in turn and because of this the light splitting device has to rotate to direct the sequence of wavelengths onto the sample and so onto the detector.

The spectrometer in this guide will use a diffraction grating a hologram printed onto a price of glass, I got this from work (a spare from a service visit).

If you don't have access to a gratting, you can make one by cutting a piece of a DVD.

HOW Usefull is this instrument

The usefullness of this instrument is limited only by your imagination examples are as follows:

  1. Analysis of the light spectrum (measuring the amount and colour of available light
  2. Measurement of spectral reflectance transmitance and absorbance of materials
  3. Food colourants analysis
  4. Environmental analysis - pollution in water - its inexpensive and connect to a laptop and not a big issue if it gets field damaged
  5. Training tool (educational) A great way to get your children into progamming and developing imaginative solutions.

The above and many more would of course all require calibration of the equipment

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RaahishK1 month ago

Hey! I really like your design and am planning to make it. I am doing a research on the impurities present in hard water in different areas in my town. I am planning to use this spectrometer to find out the impurities (from the wavelengths) and the concentrations of the ions. How can I do it using this instrument? You mentioned in the beginning of the instructable that this is possible. Please help me out here! Thanks!

HUKBMBEAR (author)  RaahishK1 month ago

Hi thanks for the comments and will be create if you do make it. As for concentration analysis. Keep in mind that accuracy will depend upon the greating. You will need to know the type of contaminants you are looking for and will need to accurately make up standard solutions. Example say your looking for chloride make up a set of solutions at a range of concentrations. Run each on the instrument several times , pick a point on the traces and plot the average for each solution against concentration. Then measure you unknown and read off the concentration from the average reading and the chart. Keep in mind that you do not have a database of responses so will not be able to use this to identify only measure.

Have fun and let us know how it goes

HUKBMBEAR (author) 4 months ago

Hi thanks for comment.I think that the type of motor is an important factor in accuracy of the selection of the part of the spectrum being directed to the sensor, however its dependant upon the the quality of the detector. At present its only an ldr so its sufficient to use the servo and take an average of the readings. I am still trying to identify a better quality sensor then I think it will be good to have the stepper, not only for accuracy, but also repeatability.

Thanks again

MartinStrydom4 months ago

Hi sir.

I love this project that you are doing. I am just curious about the precision of a servo motor. Wont a stepper motor work better for this part?

bomb maker16 months ago

can you please please please give me the exact values for the LDR and the POT. please. at least tell me in what range there values are. thx for the great project.

HUKBMBEAR (author)  bomb maker16 months ago

Hi have had a look at the LDR there are no values to be seen, it was one left over from a cybot, but it is not of any great resistance - a low value one (cheap is best) one will do - the higher the resistance the less responsive the spectrometer will be

thx, really helpful. But can you at least tell me in what range the LDRs resistance is? Like from Xohms to Y ohm. Or at least tell me if its in the range of kiloohm or ohm. Really really helpful tanks for the great project

HUKBMBEAR (author)  bomb maker16 months ago

Hi back from holidays now and have had a chance to put a meter on the LDR if I am reading correctly the LDR goes from 10 K Ohm to 130 K Ohm approximate readings only as you will get a bit of variation depending on the light levels you measure in.

HUKBMBEAR (author)  HUKBMBEAR6 months ago

Just an additional comment - its the POT that will give you the range as it adjusts the overall resistance of the LDR and so the speed of the signal measured, You could try other detectors e.g. the sensor from a TV remote - Try it as is then try cutting the crystal bulb off an see if it changes sensitivity.

HUKBMBEAR (author)  bomb maker16 months ago

Hi will take a look and post info shortly - just back from holiday so a bit of a mess at present.

Just looked at pot - LIN 220 Kohm let you know the LDR shortly

Crtek7 months ago

Only one question...Could/should a prism be put on the motor insted? Would it be easier to regulate the wavelengths?

Very nice instructable. One of the best i've seen. I'm considering building one like yours.

HUKBMBEAR (author)  Crtek7 months ago

Hi I tried to reply last night but the message did not send for some reason.

A Prism will work, but depending on quality /size etc you will only get a narrow split of the spectrum, in this case you could try to narrow the slit gap to be more selective on which part to the spectrum you are sampling at any one time.

Alternatively do a search for CD spectroscope - you can use a good quality CD/dvd and cut a section out - mount that on the motor it should work fine (listed in the instructable).

here's a link that might help:

The grating I used was a part replace during the service of a piece of lab equipment, if you want one like that you could try to email the service department at Perkin Elmer or possibly Schimadzu and ask if they have any old parts - they are of no further use in a laboratory environment once swapped out during a service and are usually just binned.

Hope this helps

Sibeiko7 months ago

As a fellow scientist, I've got to hand it to you, this seems really well thought out and made - I've seen lab equipment comparable to this (though arguably more expensive!).

I am curious though, I saw where you were working with different light sources. I'm willing to be the household incandescent gave issues due to it's size (the whole point source thing and all). I am wondering if you compared the 'empty' runs (no and/or empty cuvet) to a blackbody curve - this could help with looking at other materials, as well as other light sources (such as LEDs). Unless your headlamp was using those nice fancy white leds, in which case I'd be willing to bet that one of the older flashlight incandescents (or a non-LED uncolored Christmas light bulb) would give some good relatively unbiased runs that you could subtract out with said bb curve. In any case, I'd guess that this'd be something I think many readers here would also be interested in.

Again, I've got to say, pretty nice setup - I'm impressed!

HUKBMBEAR (author)  Sibeiko7 months ago

Thanks for the great suggestions on comparison /subtraction. Yes I did try incandescent - but for the size of body I could not at the time finds a suitable lamp. The light sight source used is the LED sort - the head lamp had a cluster of 6 which I was able to fit inside the turned tube to help direct the light and shield the detector fro direct light.

I think the secret will be to get better signal to noise ratio which you can see when I reduced the rotation speed- this gave the detector a fighting chance to take a reading. If I can add to this better slits the readings will be more specific to the wavelength being transmitted - I have a couple of vernier gauges that I will be trying to make a set of adjustable slits to test this at differing settings.

I think that once I can doe this it will make testing different light sources more meaningful and allow for better calibration of the setup.

Its been nice to talk - but have to go now - thanks again for the comments and if anyone else has suggestions please add them on to the responses.

Naren_Murali8 months ago

great instructable rare to see elec tonics and physics in instructables

HUKBMBEAR (author)  Naren_Murali8 months ago

Thanks I work a bit with Lab equipment and from an interest created the light sensor and arduino combination that developed into this one. I hope to take it a bit further and use a diode array to get better descrimination and signal strength.

Thanks again glad you liked

Sounds Interesting and well written !

HUKBMBEAR (author)  Umamaheswari Anandane9 months ago

Thank you glad you liked it

craftclarity9 months ago

Wow, heckuva build....!

HUKBMBEAR (author)  craftclarity9 months ago

Thanks, it is an interesting one and I had fun making it. I think the hardest part was getting the software to run the servo without interrupting the operation of the led timers. Thanks for your comment. If you like this did you have a look at the other light sensor that this started from.