Public Lab Research note

Help!! Problems with spectrophotometer spectra trying to build "countertop" set

by dckim | | 3,896 views | 8 comments |

Read more:







I was so happy with getting spectra using standard desktop package that I wanted to go all in and build a desktop "countertop" spectrophotometer similar to what Warren built. However, after spending a LOT of time putting things together, I find that I am not able to get any spectra with the new setup. Please help because I'm really need the help! I have posted some pictures and the spectra that I see...any comments would be appreciated. Please help!!

UPDATE: I just so happen to just place a paper between the light source and the spectrophotometer and I did see a spectra of different colors although they were very very broad. At least it's better than just a one color spectra of purple as shown below but again, the spectra is very very broad. I'm thinking that the light is somehow passing through the black paper with the camera inside picking up light everywhere. I tried dimming the light without the paper in between but all I got was the purple spectra (albeit at a lower intensity). I tried tilting the spectrophotometer at an angle to the light source and I was able to get some different colors but no matter what, I am not able to get a sharp spectra. Is it possible that the DVD film used to scatter the light is somewhat dirty? I did fiddle around a lot inside trying to get the wooden block to stick. Any advice or suggestions would be greatly appreciated!

Spectra :( pic1.jpg

Setup lying on the side CameraAwesomePhoto.jpg

Insides (while the light is on) CameraAwesomePhoto1.jpg

Insides CameraAwesomePhoto2.jpg

Closeup of the insides CameraAwesomePhoto3.jpg


Past Spectras using just the kit that was ordered online (without the desktop construction using PVC) was somewhat successful. However, the above desktop setup definitely did not work as one can see from the very top spectra!



I have attached two spectra (one looking at a fluorescent T8 bulb and another one using a fluorescent T8 bulb advertised as a "Grow" lamp). What do people think with regard to the spectra (ie, is this what people typically get and is there ways to make a clearer separation between the colors?)





I attached the picture of the slit. It's from the "standard" desktop spectrophotometer kit that was purchased from here. Any tips with regard to getting it sharper would be greatly appreciated! Also, is there any way to download the spectra (with actual data points) where it could be manipulated (ie, subtract spectra from one another, manipulate, etc using something like Excel)? Thanks so much warren!

Is this a question? Click here to post it to the Questions page.

Yes, you can download in the left-hand column on the spectrum page, it's a small grey bar with XML,JSON, CSV download links.

So you think your brightness issues are solved?

For sharpness, you might try refocusing the webcam. I'm not sure which version you have -- was there a wood block in the kit? If so, perhaps moving the camera further from the slit would help.

Is this a question? Click here to post it to the Questions page.

Hi dckim. One may get sharper and bright images by reducing the slit width (by cutting a narrower slit in a new black card) but then you loose brightness rapidly if the light source is not right in front of the spectrometer slit. Theoretically the diffraction signal -proportional to the entering intensity times sine(width)/width to the square- will reach a peak in sharpness reducing the slit width but the light entering the slit reduces and the overall spectrum intensity drops, the area of the slit is reduced so the amount of light passing through it.

Hi, Dckim - I just got your email. Can you upload a screenshot of the "Configure" panel in the Capture interface? I think you may need to adjust where you are taking your sample row. Also, if you can dim the light a lot, I think that will also help.

Is this a question? Click here to post it to the Questions page.

Hello Warren! I have attached a screenshot of the "Configure" panel in the Capture interface using four different light intensities (at the very top of this thread). They are labeled "pic10", "pic11", "pic12", "pic13" with the last file being at the lowest light intensity I can produce on my dimmer.

I also attached pictures (at the very top) of my "countertop" spectrophotometer in case there is something wrong in the way I built the thing. Any comments or help please????

Is this a question? Click here to post it to the Questions page.

Hi - it doesn't seem like it's lined up with the light bulb; but you could try to diffuse the light by placing a piece of semi-transparent (milky) plastic in front of the imaging slit? Or a thin piece of paper (tracing paper...), although paper usually has UV dyes in it so it'll fluoresce a bit.

Is this a question? Click here to post it to the Questions page.

Finally, I got some interesting data.....BUT the results confuse me. I am looking at algae in a petri dish placed between the light source and the spectrophotometer. I then compare the results with water in a petri dish placed between the light source and spectrophotometer. I expected the intensity of the algae at certain wavelength (like 400-450 and 650-700 as those are the wavelengths for chorophyll) to be reduced versus that of just plain water since I thought that's how adsorbtion spectroscopy works. Increase in chorophyll and algae should correspond to lower intensity at those wavelengths associated with chorophyll, right? However, I am seeing higher intensity in the spectra for the algae vs that of water. What am I missing? Any advice or help would be appreciated.

Is this a question? Click here to post it to the Questions page.

Can you post links to some of your spectra demonstrating this? I think it might be best to just post a research note with some photos of your setup and links to your data, so we don't get too off-topic on this particular thread.

Is this a question? Click here to post it to the Questions page.

Login to comment.

Public Lab is open for anyone and will always be free. By signing up you'll join a diverse group of community researchers and tap into a lot of grassroots expertise.

Sign up